EO (Earth Observation) Topics on Climate Change
Since the start of the space age, Earth observation is providing its share of evidence for a better perception and understanding of our Earth System and its response to natural or human-induced changes.
Earth is a complex, dynamic system we do not yet fully understand. The Earth system comprises diverse components that interact in complex ways. We need to understand the Earth's atmosphere, lithosphere, hydrosphere, cryosphere, and biosphere as a single connected system. Our planet is changing on all spatial and temporal scales.
Over the years, the entire Earth Observation community, the space agencies as well as other governmental bodies, and many international organizations (UN, etc.) are cooperating on a global scale to come to grips with the modeling of the Earth system, including a continuous process of re-assessment and improvement of these models. The goal is to provide scientific evidence to help guide society onto a sustainable pathway during rapid global change.
In the second decade of the 21st century, there is alarming evidence that important tipping points, leading to irreversible changes in major ecosystems and the planetary climate system, may already have been reached or passed. Ecosystems as diverse as the Amazon rainforest and the Arctic tundra, may be approaching thresholds of dramatic change through warming and drying. Mountain glaciers are in alarming retreat and the downstream effects of reduced water supply in the driest months will have repercussions that transcend generations. 1)
Table 1: Overview of some major international bodies involved in global-change research programs 2)
The UN Framework Convention on Climate Change (UNFCCC) is an intergovernmental treaty developed to address the problem of climate change. The Convention, which sets out an agreed framework for dealing with the issue, was negotiated from February 1991 to May 1992 and opened for signature at the June 1992 UN Conference on Environment and Development (UNCED) — also known as the Rio Earth Summit. The UNFCCC entered into force on 21 March 1994, ninety days after the 50th country's ratification had been received. By December 2007, the convention had been ratified by 192 countries. 3)
In the meantime, there were many UN conferences on Climate Change, starting with the UN climate conference in Kyoto, Japan, in December 1997. The Kyoto Protocol set standards for certain industrialized countries. Those targets expired in 2012.
Meanwhile, greenhouse gas emissions from both developed and developing countries have been increasing rapidly. Even today, those nations with the highest percentage of environment pollution, are not willing to enforce stricter environmental standards in their countries in order to protect their global business interests. It's a vicious cycle between these national interests and the deteriorating environment, resulting in more frequent and violent catastrophes on a global scale. All people on Earth are effected, even those who abide by their strict environmental rules.
The short descriptions in the following chapters are presented in reverse order on some topics of climate change to give the reader community an overview of research results in this wide field of global climate and environmental change.
Note: As of May 2019, the previously single large EO-Topics file has been split into four files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the period 2019
EO-Topics4 (Time frame: 2019)
Long-Term Sea Level Rise from Greenland Ice
• June 19, 2019: Greenland's melting ice sheet could generate more sea level rise than previously thought if greenhouse gas emissions continue to increase and warm the atmosphere at their current rate, according to a new modeling study. The study, which used data from NASA's Operation IceBridge airborne campaign, was published in Science Advances today. 4) 5)
Figure 1: Scientists at the University of Alaska Fairbanks' Geophysical Institute used data from NASA's Operation IceBridge to develop a more accurate model of how the Greenland Ice Sheet might respond to climate change in the future, finding that it could generate more sea level rise than previously thought. The Greenland Ice Sheet is the second-largest body of ice in the world, covering roughly 1.68 million km2 of Greenland's surface. If it melts completely, it could contribute up to 7 m of sea level rise, according to a new study using data from NASA's Operation IceBridge (video credit: NASA / Katie Jepson)
In the next 200 years, the ice sheet model shows that melting at the present rate could contribute 0.48 - 1.6 m to global sea level rise, said the team led by scientists at the Geophysical Institute at the University of Alaska Fairbanks. These numbers are at least 80 percent higher than previous estimates, which forecasted up to 0.89 m of sea level rise from Greenland's ice.
The team ran the model 500 times out to the year 3000 for each of three possible future climate scenarios, adjusting key land, ice, ocean and atmospheric variables to test their effects on ice melt rate. The three climate scenarios depend on the amount of greenhouse gas emissions in the atmosphere in coming years. In the scenario with no reduction of emissions, the study found that the entire Greenland Ice Sheet will likely melt in a millennium, causing 5.2 m to 7 m of sea level rise.
In the scenario where emissions are stabilized by the end of the century rather than continue to increase, the model shows ice loss falling to 26-57 percent of total mass by 3000. Drastically limiting emissions so they begin to decline by the end of the century could limit ice loss to 8-25 percent. This scenario would produce up to 1.8 m of sea level rise in the next millennium, according to the study.
The updated model more accurately represents the flow of outlet glaciers, the river-like bodies of ice that connect to the ocean. Outlet glaciers play a key role in how ice sheets melt, but previous models lacked the data to adequately represent their complex flow patterns. The study found that melting outlet glaciers could account for up to 40 percent of the ice mass lost from Greenland in the next 200 years.
By incorporating ice thickness data from IceBridge and identifying sources of statistical uncertainty within the model, the study creates a more accurate picture of how human-generated greenhouse gas emissions and a warming climate may affect Greenland in the future.
Figure 2: The researchers ran their model 1500 times, testing a variety of land, ice, ocean and atmospheric variables to see how they affected ice melt rate - including three possible future climate scenarios (from left to right: low, medium, and high emissions out to the year 2300), image credit: NASA / Cindy Starr
A clearer picture
Capturing the changing flow and speed of outlet glacier melt makes the updated ice sheet model more accurate than previous models, according to the authors. As ocean waters have warmed over the past 20 years, they have melted the floating ice that shielded the outlet glaciers from their rising temperatures. As a result, the outlet glaciers flow faster, melt and get thinner, with the lowering surface of the ice sheet exposing new ice to warm air and melting as well.
"Once we had access to satellite observations, we were able to capture the surface velocity of the whole Greenland Ice Sheet and see how that ice flows. We recognized that some outlet glaciers flow very fast — orders of magnitude faster than the interior of the ice sheet," said lead author Andy Aschwanden, a research associate professor at the University of Alaska Fairbanks' Geophysical Institute.
IceBridge's detailed ice thickness measurements helped the team to be the first to model these areas where outlet glaciers are affected by warmer ocean waters, as well as to model more of the complex feedbacks and processes influencing ice loss than previously possible. They examined the importance of factors like underwater melting, large ice chunks breaking off of glaciers, changing snowfall rates and rising air temperatures. They also examined factors that could slow down ice loss, like the movement of Earth's surface "bouncing back" from the weight of ice that is no longer there.
"At the end of the day, glaciers flow downhill," Aschwanden said. "That's very simplified, but if you don't know where downhill is, the model can never do a good job. So the most important contributor to understanding ice flow is knowing how thick the ice is."
Each of the three emissions scenarios used in the study produced different patterns of ice retreat across Greenland. The least severe scenario showed the ice retreating in the west and north, while the moderate scenario showed ice retreat around the island, except for in the highest elevation areas. The most severe scenario, in which emissions continue to increase at their present rate, showed more than half of the model runs losing more than 99 percent of the ice sheet by 3000.
At its thickest point, the Greenland Ice Sheet currently stands more than 3000 m above sea level. It rises high enough into the atmosphere to alter the weather around it, as mountains do. Today, this weather pattern generates almost enough snowfall to compensate for the amount of naturally melting ice each year. In the future, however, melting and flow will thin the interior, lowering it into a layer of the atmosphere that lacks the conditions necessary for sufficient replenishing snowfall.
"In the warmer climate, glaciers have lost the regions where more snow falls than melts in the summer, which is where new ice is formed," said Mark Fahnestock, research professor at the Geophysical Institute and the study's second author. "They're like lumps of ice in an open cooler that are melting away, and no one is putting any more ice into the cooler."
The team stressed that despite the need for ongoing research on exactly how glaciers will move and melt in response to warming temperatures, all of the model runs show that the next few decades will be pivotal in the ice sheet's future outcome.
"If we continue as usual, Greenland will melt," Aschwanden said. "What we are doing right now in terms of emissions, in the very near future, will have a big long-term impact on the Greenland Ice Sheet, and by extension, if it melts, to sea level and human society."
Bridging the data gap
The model runs were performed on high-performance supercomputers at NASA's Ames Research Center and the University of Alaska Fairbanks (UAF) using the Parallel Ice Sheet Model (PISM), an open-source model developed at UAF and the Potsdam Institute for Climate Impact Research. NASA also provided funding support for the study. While other ice sheet models could perform the simulations they did, the team said, PISM is unique for its high resolution and low computational cost.
NASA's Operation IceBridge is the world's largest airborne survey of polar land and sea ice. Using an array of aircraft and scientific instruments, IceBridge has collected data between the end of the first Ice, Cloud and Land Elevation Satellite (ICESat) mission in 2010 and the second, ICESat-2, which launched in 2018. It has measured the height of the ice below its flight path as well as the bedrock under the ice sheets.
"NASA's space and airborne campaigns, like IceBridge, have fundamentally transformed our ability to try and make a model mimic the changes to the ice sheet," Fahnestock said. "The technology that allows improved imaging of the glacier bed is like a better pair of glasses allowing us to see more clearly. Only NASA had an aircraft with the instruments and technology we needed and could go where we needed to go."
Earth's Freshwater Future
• June 13, 2019: NASA satellites are a prominent tool for accounting for water, as it constantly cycles from water vapor to rain and snow falling onto soils, and across and beneath the landscape. As Earth's atmosphere warms due to greenhouse gases and the satellite data record continues to get longer and more detailed, scientists are studying how climate change is affecting the distribution of water. 6)
Trends are beginning to emerge, especially at the extremes in the frequency and magnitude of floods and droughts. These trends affect everything from local weather to where crops can grow, and have consequences that will ripple through communities today and in the coming century.
When thinking about changes to the distribution of water around the planet, it's not just knowing where it rains or doesn't, but also how much, and how frequently heavy rain falls versus light rain. Rainfall amount impacts soils saturation and how high streams and rivers rise, which then changes their capacity to hold more in the event of another storm. Lack of rain stresses vegetation and supplemental water reserves, and when their frequency increases, those reserves are less likely to recover before the next dry spell.
NASA satellite data and ground measurements support research into long-term changes to water distribution. One of those efforts is the U.S. National Climate Assessment, which studies climate change and its potential impacts in each region of the country.
Figure 3: When we look into the vastness of space, our home planet stands out in many ways. One of the most crucial is the presence of abundant, accessible freshwater — as a liquid, solid and gas. Water helps make our planet habitable. The first question NASA researchers studying freshwater on Earth ask is: Where is the water? As it constantly cycles between water vapor, rain and snow, and reservoirs above and below ground, water is tracked by a fleet of NASA satellites. Heat travels with that water, as energy from the Sun drives freshwater's transformations between vapor, liquid water, and ice. As our planet warms due to greenhouse gases, scientists have a second pressing question: How is climate change affecting the distribution of water? (video credit: NASA Goddard, Published on Jun 13, 2019)
Among those changes, for example, is an observed increase in very heavy precipitation events across the United States. From 1958 to 2016 heavy rainfall events have increased in the northeastern states by 55%, Midwestern states by 42%, and southeastern states by 27%. The western states have also seen modest increases in heavy rain events that can overwhelm the local watershed's capacity to absorb excessive water.
"When you think about changing the distribution of precipitation, then you start to think that if you're getting more heavy precipitation, that might mean more flooding," said Christa Peters-Lidard a hydrologist and Deputy Director for Hydrology, Biospheres, and Geophysics at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "If we're going to see more heavy rainfall events and we're going to see them especially in areas that are not designed for those floods, that means that we need to think about how to adapt our infrastructure and rethink the way we've designed some of our bridges and drainage systems."
Peters-Lidard is no stranger to the realities of what changing patterns of heavy rainfall can do to communities built under different conditions. In the last five years, her home town of Ellicott City, Maryland, has seen two 1,000-year floods that destroyed businesses and homes. "It's been a devastating impact on the community," she said. In response to the floods and likelihood of more minor flooding events, "we're rethinking Main Street and where we should rebuild and where we should not."
Figure 4: NASA scientists used tree rings to understand past droughts and climate models incorporating soil moisture data to estimate future drought risk in the 21st century (image credit: NASA)
But while some areas are projected to get wetter, others will become much drier. Warming temperatures and changing precipitation patterns can lead to droughts, and NASA research shows that humans have been influencing global patterns of drought for nearly a century.
Kate Marvel and Ben Cook, researchers at NASA's Goddard Institute for Space Studies and Columbia University in New York City, investigated humans' influence on 20th-century drought patterns using historical weather data and drought maps calculated from tree rings. They found that a data "fingerprint" – a drying and wetting pattern predicted to occur in response to greenhouse gas emissions – was visible as far back as the early 1900's.
The "fingerprint" predicts that parts of Asia would become wetter in response to greenhouse gas emissions, while the southwestern United States, Central America and Europe would become drier. When the researchers compared this to actual data, they found that the pattern emerged beginning in the early in the 20th century. It dropped off briefly after 1950, presumably due to high levels of pollution in the atmosphere, but it re-emerged in recent decades and is getting stronger.
Demonstrating that humans influenced global drought patterns in the past is an important part of understanding how we may influence them in the future, said Cook. "Climate change is not just a future problem," he said. "This shows it's already affecting global patterns of drought, hydroclimate, trends, variability — it's happening now. And we expect these trends to continue, as long as we keep warming the world."
Demonstrating climate models' ability to accurately depict past droughts, helps to confirm their ability to model future droughts as well. Other research of Cook's shows that if greenhouse gas emissions continue to increase along current trajectories, the U.S. Southwest could see "megadroughts" lasting more than three decades. Cook and his team ran 17 different climate models, and all of them agree that there are likely to be longer and more intense droughts in the future.
The team was also the first to compare their projections to paleoclimate records of droughts in the distant past, such as the North America droughts between the years 1100 and 1300. This allowed them to examine droughts more severe than any in the modern record and see how future projected droughts compare. They found that future "megadroughts" could last as long or longer than the past droughts, and they will likely be even drier.
According to these climate forecasts, the future of freshwater will be full of extremes: Droughts will pose serious challenges to the safety, health, food and water supplies of plants, animals and humans in some regions, and floods will do the same in others. As freshwater flows around the planet, NASA science will be vital in not only predicting these extreme challenges, but in preparing to face them as well.
Antarctic glaciers named after satellites
• June 7, 2019: Dramatic changes in the shape of the Antarctic ice sheet have become emblematic of the climate crisis. And, in deference to the critical role that satellites play in measuring and monitoring Antarctic glaciology, seven areas of fast-flowing ice on the Antarctic Peninsula have been named after Earth observation satellites. 7)
Reports of iceberg calving, changes in ice-sheet speed, thickness and mass have informed the climate change debate. These reports are thanks largely to routine monitoring by an international fleet of Earth observation satellites.
Recognizing the importance of observations from space, the UK Antarctic Place-names Committee has approved seven new names for international use. The decision follows a request by Anna Hogg from the CPOM (Centre for Polar Observation and Modelling) at the University of Leeds, UK, who identified that the major glaciers flowing westwards from the Dyer Plateau are thinning and flowing at rates of more than 1.5 meters a day.
To describe them in scientific papers, Dr Hogg requested that seven outlet glaciers be named. "Naming the glaciers after the Earth observation satellites we use to measure them is a great way to celebrate the international collaboration in space, and on big science questions. It's fantastic news that the UK Foreign Office have formally approved these new place names, which will be on the record forever more," said Dr Hogg.
Figure 5: Reports of iceberg calving, changes in ice-sheet speed, thickness and mass have informed the climate change debate. These reports are thanks largely to routine monitoring by an international fleet of Earth observation satellites. And, in respect for the critical role that satellites play in measuring and monitoring glaciology, the UK Antarctic Place-names Committee has approved seven new names for areas of fast-flowing ice on the Antarctic Peninsula (image credit: CPOM)
The Ers Ice Stream that flows west between Jensen Nunataks and Gunn Peaks was named after the two ESA satellites – ERS-1 and ERS-2 – that operated between 1991 and 2011. They provided the first high-resolution, wide-swath and day-and-night images that were used to calculate the speed and direction of the flow of glacier ice.
The Envisat Ice Stream lies further to the west and commemorates ESA's largest Earth observation satellite, which was launched in 2002 and operated until 2012. It carried 10 instruments that extended the datasets generated by ERS-1 and ERS-2.
The Cryosat Ice Stream flows further west and is named after the ESA Earth Explorer satellite launched in 2010. CryoSat was designed specifically to detect changes in the height of polar ice using a sophisticated instrument that provides high-accuracy elevation measurements over the rugged ice-sheet margins and for sea ice in polar waters.
Still further west lies the Grace Ice Stream, which commemorates the joint Gravity Recovery and Climate Change Experiment (GRACE) mission run by NASA and the German Aerospace Center. Between 2002 and 2017, the mission mapped, for the first time, Earth's time-varying gravitational field, detecting Antarctic ice-sheet mass changes with unprecedented accuracy.
The Sentinel Ice Steam is named after the more recent series of satellites that ESA develops for the EU's Copernicus program to the environment and climate change. This program provides open access to images, allowing the public to easily view and witness ongoing, year-round changes in Antarctica and the rest of the world.
The ALOS Ice Rumples are named after a Japan Aerospace Exploration Agency mission. Its optical and radar image data acquired between 2006 and 2011 have been used to map ice in the polar regions, with dedicated imaging campaigns to capture Antarctic ice-sheet surface changes during the International Polar Year campaign that ran between 2007 and 2009.
Finally, the Landsat ice stream is the most westerly of the newly named glaciers. It is named after the joint NASA/US Geological Survey series of Landsat Earth observation satellites that have been operating since 1972. Landsat has been one of the primary systems used in Antarctic studies, providing over 40 years of uninterrupted mapping of the continent for climate and environment studies.
Fifteen space agencies currently collaborate on coordinating Antarctic data collection from a wide range of satellites, and on the planning of data acquisition and products to address the needs of the scientific community, under the banner of the World Meteorological Organization's Polar Space Task Group.
Mark Drinkwater, ESA chair of the task group, said, "Interagency planning is paying dividends for polar science, with more comprehensive multi-satellite, multi-instrument datasets and better coverage than previously possible, which enables the science community to address today's key climate research challenges."
This gesture of naming Antarctic glaciers after these ground-breaking satellites is a mark of recognition of the importance of Earth observation data in addressing the climate crisis.
Figure 6: This image from Copernicus Sentinel-3 on 28 February 2017 captures the Antarctic Peninsula (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA)
Figure 7: Sentinel Ice Stream. In respect for the critical role that satellites play in measuring and monitoring glaciology, the UK Antarctic Place-names Committee has approved seven new names for areas of fast-flowing ice on the Antarctic Peninsula. One of the ice streams, captured in this image from the Copernicus Sentinel-1 mission on 30 May 2019, is now called the Sentinel Ice Stream (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), modified by ESA)
Figure 8: The view from the window of a BAS Twin Otter plane, looking out over George VI Ice Shelf towards Western Palmer Land on the Antarctic Peninsula. Bright blue melt ponds can be seen on the ice shelf surface, and these now form every year in the warmer summer months. The photo was taken as the CPOM land ice team flew out to a field site in Western Palmer Land while taking part in the January 2018 ESA CryoVex/KAREN field campaign, where ice cores were collected to help validate satellite measurements (image credit: CPOM, Anna Hogg)
Satellites yield insight into not so permanent permafrost
• May 17, 2019: Ice is without doubt one of the first casualties of climate change, but the effects of our warming world are not only limited to ice melting on Earth's surface. Ground that has been frozen for thousands of years is also thawing, adding to the climate crisis and causing immediate problems for local communities. 8)
In Earth's cold regions, much of the sub-surface ground is frozen. Permafrost is frozen soil, rock or sediment – sometimes hundreds of metres thick. To be classified as permafrost, the ground has to have been frozen for at least two years, but much of the sub-surface ground in the polar regions has remained frozen since the last ice age.
Permafrost holds carbon-based remains of vegetation and animals that froze before decomposition could set in. Scientists estimate that the world's permafrost holds almost double the amount of carbon than is currently in the atmosphere.
When permafrost warms and thaws, it releases methane and carbon dioxide, adding these greenhouse gases to the atmosphere and making global warming even worse. With permafrost covering about a quarter of the northern hemisphere, extensive thawing could trigger a feedback loop that could potentially turn the Arctic from a carbon sink into a carbon source.
Figure 9: Ponds resulting from thawing permafrost in the Yamal Peninsula in northwest Siberia captured by the Copernicus Sentinel-2 mission on 27 August 2018. In Earth's cold regions, much of the sub-surface ground is frozen. Permafrost is frozen soil, rock or sediment – sometimes hundreds of meters thick. To be classified as permafrost, the ground has to have been frozen for at least two years, but much of the sub-surface ground in the polar regions has been frozen since the last ice age. Permafrost holds carbon-based remains of vegetation and animals that froze before they could decompose. Scientists estimate that the world's permafrost holds almost double the amount of carbon that is currently in the atmosphere. When permafrost warms and thaws, it releases methane and carbon dioxide, adding these greenhouse gases to the atmosphere and making global warming even worse (image credit: ESA, the image contains modified Copernicus data (2018), processed by ESA, CC BY-SA 3.0 IGO
Thawing permafrost isn't just releasing more greenhouse gases into air – it's also changing the landscape and destabilizing the ground, and so causing real practical problems for society.
Figure 10: Land-cover map of the Arctic covering 1500 km north to south to understand how thawing permafrost is changing the landscape. The map has been generated using information from the Copernicus Sentinel-1 and Sentinel-2 missions. Tones of yellow and beige show sparse vegetation, greens show tundra, purples show forest and reds show areas that have been disturbed by flooding or forest fires. Vegetation patterns alter snow re-distribution and therefore heat transfer in winter and fires can trigger permafrost thaw (image credit: ESA, the image contains modified Copernicus Sentinel data, processed by ZAMG)
Over 30 million people live in the permafrost zone, in towns that were built on firm ground. As the ground softens, the infrastructure that Arctic communities rely on is becoming increasingly unstable.
Determined by temperature, permafrost is an essential climate variable. Through ESA's Climate Change Initiative (CCI), temperature data that have been collected over years are gathered to determine trends and to understand more about how permafrost fits into the climate system.
Discussed in ESA's Living Planet Symposium (13-17 May 2019), satellites are an important part of monitoring permafrost, albeit indirectly, from space.
Annett Bartsch, the founder and managing director of b.geos, explained, "We can't monitor permafrost as such from space. Although it's a bit complicated, we can, however, use a lot of different types of satellite data along with in situ measurements and modelling to put together a picture of what is happening.
"Through an ESA project called Glob Permafrost, we use images captured by the Copernicus Sentinel-2 mission, for example, which give us a camera-like view of how the land surface is slumping and eroding because of thawing permafrost. The Copernicus Sentinel-1 radar mission, on the other hand gives us valuable information on widespread changes in topography. -Missions carrying thermal sensors such as Copernicus Sentinel-3 can provide information about the changes in the temperature of Earth's surface. - And, we can use information on snow conditions and land cover as a proxy for soil properties. Both snow and soil regulate heat transfer, so they determine the actual impact of increasing air temperature on the frozen soil beneath."
Research that uses satellite data has resulted in the first global map of permafrost at a spatial resolution of 1 km, and was published recently in Earth Science Reviews. The research team employed an equilibrium state model for the temperature at the top of the permafrost (TTOP model) for the 2000–2016 period, to estimate permafrost distribution at a hemispheric scale (Figure 11). 9)
The dataset contains information on the average mean annual ground temperature for 2000 to 2016. Dr Bartsch, noted, "We are now also looking at changes over time as part of ESA's Climate Change Initiative."
Other advances are also being made through the GlobPermafrost project; notably the Permafrost Information System, which consists of a database as well as a visualization platform of satellite-derived information relevant for permafrost monitoring. It includes the permafrost map and land-cover change information, ground subsidence, rock glaciers and information on lake properties for key regions.
With the damage that thawing permafrost can unleash on the climate system and on the local environments, it is no surprise that this is a hot topic. Scientists are drawing on all resources at hand to understand and monitor the situation, which, in turn, arms decision-makers with the information they need to take action.
Figure 11: Ground temperature 2000–2016. Modelled mean annual ground temperatures at the top of the permafrost for the northern hemisphere at 1 km spatial resolution derived from MODIS Land surface temperature, ESA CCI Land cover, and ERA Interim climate reanalyzes data (image credit: University of Oslo)
Note: ERA (ECMWF Re-Analysis). ERA refers to a series of research projects at ECMWF which produced various datasets (ERA-Interim, ERA-40, etc.). ERA-Interim is a dataset, showing the results of a global climate reanalysis from 1979 to date (2019).
Antarctic ice-loss and contribution to sea level rise
• May 16, 2019: By combining 25 years of ESA satellite data, scientists have discovered that warming ocean waters have caused the ice to thin so rapidly that 24% of the glacier ice in West Antarctica is now affected. 10)
A paper published in Geophysical Research Letters describes how the UK Centre for Polar Observation and Modelling (CPOM) used over 800 million measurements of Antarctic ice sheet height recorded by radar altimeter instruments on ESA's ERS-1, ERS-2, Envisat and CryoSat-2 satellite missions between 1992 and 2017. 11)
The study also used simulations of snowfall over the same period produced by the RACMO regional climate model. Together, these measurements allow changes in ice-sheet height to be separated into those caused by meteorological events, which affect snow, and those caused by longer-term changes in climate, which affect ice.
The ice sheet has thinned by up to 122 meters in places, with the most rapid changes occurring in West Antarctica where ocean melting has triggered glacier imbalance. CPOM Director, Andy Shepherd, explained, "Parts of Antarctica have thinned by extraordinary amounts. So we set out to show how much was down to changes in climate and how much was instead due to weather."
Figure 12: Antarctic ice loss 1992–2019 and contribution to sea level rise (image credit: CPOM)
To do this, the team compared measurements of surface-height change with the simulated changes in snowfall. Where the signal was greater they attributed its origin to glacier imbalance.
They found that fluctuations in snowfall tend to drive small changes in height over large areas for a few years at a time, whereas the most pronounced changes in ice thickness coincide with signals of glacier imbalance that have persisted for decades.
Prof. Shepherd added, "Knowing how much snow has fallen has really helped us to isolate the glacier imbalance within the satellite record. We can see clearly now that a wave of thinning has spread rapidly across some of Antarctica's most vulnerable glaciers, and their losses are driving up sea levels around the planet. After 25 years, the pattern of glacier thinning has spread across 24% of West Antarctica, and its largest ice streams – the Pine Island and Thwaites Glaciers – are now losing ice five times faster than they were in the 1990s. Altogether, ice losses from East and West Antarctica have added 4.6 mm of water to global sea level since 1992."
ESA's Marcus Engdahl, noted, "This is a fantastic demonstration of how satellite missions can help us to understand how our planet is changing. The polar regions are hostile environments and are extremely difficult to access from the ground. Because of this, the view from space is an essential tool for tracking the effects of climate change."
Scientific results such as this are key to understanding how our planet works and how natural processes are being affected by climate change – and ice is a hot topic at ESA's Living Planet Symposium, which is currently in full swing in Milan. This study demonstrates that the changing climate is causing real changes in the far reaches of the Antarctic.
Jakobshavn Isbrae Glacier bucks the trend
• May 14, 2019: Our planet works in mysterious ways. We are all used to hearing about the world's ice being the first casualty of climate change and, indeed, it is declining fast. However, recent findings show that one glacier is not conforming to the norm – it's actually been flowing more slowly and getting thicker. 12)
In recent years, Greenland has been losing more ice through the Jakobshavn Isbrae Glacier than from anywhere else on this huge ice sheet.
Various types of satellite data have been used to understand and monitor the glacier's flow over the last 20 years, in particular, through ESA's Climate Change Initiative. This revealed that the glacier was flowing at its fastest and losing the most ice in 2012–13. In places, the main trunk of the glacier was deflating by 10 m a year as it adjusted dynamically to ice loss and melting.
Figure 13: Jakobshavn Glacier in west Greenland viewed by the Copernicus Sentinel-2 mission on 29 April 2019. In recent years, Greenland has been losing more ice through this glacier than from anywhere else on this huge ice sheet. Various types of satellite data have been used to understand and monitor the glacier's flow over the last 20 years. This revealed that the glacier was flowing at its fastest and losing the most ice in 2012–13. In places, the main trunk of the glacier was deflating by 10 m a year as it adjusted dynamically to ice loss and melting. However, information from satellites such as ESA's CryoSat-2 and the Copernicus Sentinel-1 mission show that between 2013 and 2017, the region drained by the glacier stopped shrinking in height and started to thicken. The overall effect is that Jakobshavn is now flowing more slowly, thickening, and advancing toward the ocean instead of retreating farther inland (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA)
Complementary information from the European Commission's Copernicus Sentinel-1 radar mission and Sentinel-2 optical mission along with ESA's CryoSat-2 satellite are currently being used to keep a close eye on this critical glacier.
In particular, scientists are applying a new swath processing technique to CryoSat's altimeter data. This differs from conventional radar altimetry so that broad swaths, rather than single points, of elevations can be computed – yielding better detail on glacial change.
This new high-resolution dataset has revealed that, between 2013 and 2017, the ice at terminus of the glacier stopped decreasing in height, and started to thicken. The overall effect is that Jakobshavn Isbrae is now flowing more slowly, thickening, and advancing toward the ocean instead of retreating farther inland.
Figure 14: Changing height of Jakobshavn. Ice surface-elevation change on Jakobshavn Isbrae Glacier in west Greenland, measured using ‘swath mode' processing of CryoSat-2 data, generated through the ESA Science for Society CryoTop project. The plot on the left shows high levels of thinning ice (red) measured between 2010 and 2013, and the plot on the right shows a localized pattern of ice thickening (blue) on the faster flowing central trunk, measured between 2014 and 2018 (image credit: CPOM–A. E. Hogg)
Even so, the glacier's drainage basin as a whole is still losing more ice to the ocean than it gains as snowfall, therefore still contributing to global sea-level rise, albeit at a slower rate.
Scientists are discussing this phenomenon at this week's Living Planet Symposium in Milan. Anna Hogg, researcher in the Centre for Polar Observation and Modelling at the University of Leeds in the UK, said, "The dynamic speedup of Jakobshavn Isbrae observed from the late 2000's to 2013 was triggered by warm ocean waters in Disko Bay, entering Jakobshavn Fjord and melting ice at the glacier terminus. - In recent years, however, temperature measurements show that ocean water in Disko Bay has experienced a series of cooler years – more than one degree lower than mean temperature previously observed. This has reduced the rate of ice melt on Jakobshavn Isbrae."
However, glaciers interact with both the ocean and the atmosphere. Following the extreme surface melt event across the whole of Greenland in 2012, the ice sheet experienced very low levels of surface melt the following year.
Research suggests that it is the complex interaction of both ocean and atmospheric forcing that have driven the changes on this glacier.
Dr Hogg added, "The key question we need to answer now is whether the slowdown of Jakobshavn Isbrae just a pause, or is it more permanent? We will use ESA satellite observations combined with models to monitor change and predict this colossal glacier's future evolution."
ESA's Mark Drinkwater noted, "The balance of the cryosphere is clearly delicate, and we see large seasonal and year-to-year variability in the dynamics of the Jakobshavn Isbrae Glacier, which can easily hide the longer-term climate trend in ice loss. Further data is needed on the regional influence of ocean temperatures on tidewater glaciers in Greenland to better understand to what extent this process influences regional ice-mass losses."
Figure 15: Jakobshavn in motion. This animation uses radar images from the Copernicus Sentinel-1 radar mission and shows the glacier's flow between July 2017 to March 2019 (video credit: ESA)
Human impact on droughts goes back 100 years
• May 1, 2019: Human-generated greenhouse gases and atmospheric particles were affecting global drought risk as far back as the early 20th century, according to a study from NASA's Goddard Institute for Space Studies (GISS) in New York City, LLNL (Lawrence Livermore National Laboratory (LLNL) and Columbia University. 13) 14)
Figure 16: For the first time, scientists at NASA GISS have linked human activities with patterns of drought around the world. Getting clues from tree ring atlases, historical rain and temperature measurements, and modern satellite-based soil moisture measurements, the researchers found the data "fingerprint" showing that greenhouse gases were influencing drought risk as far back as the early 1900's (video credit: NASA Goddard/ LK Ward)
The study, published in the journal Nature, compared predicted and real-world soil moisture data to look for human influences on global drought patterns in the 20th century. Climate models predict that a human "fingerprint" – a global pattern of regional drying and wetting characteristic of the climate response to greenhouse gases – should be visible early in the 1900s and increase over time as emissions increased. Using observational data such as precipitation and historical data reconstructed from tree rings, the researchers found that the real-world data began to align with the fingerprint within the first half of the 20th century. 15)
The team said the study is the first to provide historical evidence connecting human-generated emissions and drought at near-global scales, lending credibility to forward-looking models that predict such a connection. According to the new research, the fingerprint is likely to grow stronger over the next few decades, potentially leading to severe human consequences.
Searching for human fingerprints
The study's key drought indicator was the PDSI (Palmer Drought Severity Index). The PDSI averages soil moisture over the summer months using data such as precipitation, air temperature and runoff. While today NASA measures soil moisture from space, these measurements only date back to 1980. The PDSI provides researchers with average soil moisture over long periods of time, making it especially useful for research on climate change in the past.
The team also used drought atlases: Maps of where and when droughts happened throughout history, calculated from tree rings. Tree rings' thickness indicates wet and dry years across their lifespan, providing an ancient record to supplement written and recorded data.
"These records go back centuries," said lead author Kate Marvel, an associate research scientist at GISS and Columbia University. "We have a comprehensive picture of global drought conditions that stretch back way into history, and they are amazingly high quality."
Taken together, modern soil moisture measurements and tree ring-based records of the past create a data set that the team compared to the models. They also calibrated their data against climate models run with atmospheric conditions similar to those in 1850, before the Industrial Revolution brought increases in greenhouse gases and air pollution.
Figure 17: NASA satellite images show the dramatic loss of water storage in California's Central Valley during its most recent drought (image credit: NASA)
"We were pretty surprised that you can see this human fingerprint, this human climate change signal, emerge in the first half of the 20th century," said Ben Cook, climate scientist at GISS and Columbia University's Lamont-Doherty Earth Observatory in New York City. Cook co-led the study with Marvel.
The story changed briefly between 1950 and 1975, as the atmosphere became cooler and wetter. The team believes this was due to aerosols, or particles in the atmosphere. Before the passage of air quality legislation, industry expelled vast quantities of smoke, soot, sulfur dioxide and other particles that researchers believe blocked sunlight and counteracted greenhouse gases' warming effects during this period. Aerosols are harder to model than greenhouse gases, however, so while they are the most likely culprit, the team cautioned that further research is necessary to establish a definite link.
After 1975, as pollution declined, global drought patterns began to trend back toward the fingerprint. It does not yet match closely enough for the team to say statistically that the signal has reappeared, but they agree that the data trends in that direction.
Figure 18: Regions projected to become drier or wetter as the world warms. More intense browns mean more aridity; greens, more moisture. (Gray areas lack sufficient data so far.) The new study shows that observations going back to 1900 confirm projections are largely on target (image credit: Adapted from Marvel et al., Nature, 2019) 16)
Reaching a verdict
What made this study innovative was seeing the big picture of global drought, Marvel said. Individual regions can have significant natural variability year to year, making it difficult to tell whether a drying trend is due to human activity. Combining many regions into a global drought atlas meant there was a stronger signal if droughts happened in several places simultaneously.
"If you look at the fingerprint, you can say, ‘Is it getting dry in the areas it should be getting drier? Is it getting wetter in the areas it should be getting wetter?'" she said. "It's climate detective work, like an actual fingerprint at a crime scene is a unique pattern."
Previous assessments from national and international climate organizations have not directly linked trends in global-scale drought patterns to human activities, Cook said, mainly due to lack of data supporting that link. He suggests that, by demonstrating a human fingerprint on droughts in the past, this study provides evidence that human activities could continue to influence droughts in the future.
"Part of our motivation was to ask, with all these advances in our understanding of natural versus human caused climate changes, climate modeling and paleoclimate, have we advanced the science to where we can start to detect human impact on droughts?" Cook said. His answer: "Yes."
Models predict that droughts will become more frequent and severe as temperatures rise, potentially causing food and water shortages, human health impacts, destructive wildfires and conflicts between peoples competing for resources.
"Climate change is not just a future problem," said Cook. "This shows it's already affecting global patterns of drought, hydroclimate, trends, variability — it's happening now. And we expect these trends to continue, as long as we keep warming the world."
Figure 19: In a warming world, some regions are expected to get drier, while others will get wetter; a new study suggests this trend is already underway, and has been for more than 100 years. Here, a geologist traverses Petrified Forest National Park in southern Arizona, one of many regions expected to become more arid (image credit: Kevin Krajick/Earth Institute, Columbia University)
Antarctica's Effect on Sea Level Rise in Coming Centuries
April 25, 2019: There are two primary causes of global mean sea level rise - added water from melting ice sheets and glaciers, and the expansion of sea water as it warms. The melting of Antarctica's ice sheet is currently responsible for 20-25% of global sea level rise. — But how much of a role will it play hundreds of years in the future? 17)
Scientists rely on precise numerical models to answer questions like this one. As the models used in predicting long-term sea level rise improve, so too do the projections derived from them. Scientists at NASA's Jet Propulsion Laboratory in Pasadena, California, have discovered a way to make current models more accurate. In doing so, they have also gotten one step closer to understanding what Antarctica's ice sheet - and the sea level rise that occurs as it melts - will look like centuries from now.
"Unlike most current models, we included solid Earth processes - such as the elastic rebound of the bedrock under the ice, and the impact of changes in sea level very close to the ice sheet," said JPL's Eric Larour, first author of the study. "We also examined these models at a much higher resolution than is typically used - we zoomed in on areas of bedrock that were about 1 km instead of the usual 20 km."
The scientists found that projections for the next 100 years are within 1% of previous projections for that time period; however, further into the future, they observed some significant differences.
"We found that around the year 2250, some of these solid Earth processes started to offset the melting of the ice sheet and the consequent sea level rise," Larour said. In other words, they actually slowed the melting down.
The team noted that a hundred years even further into the future - by 2350 - this slowdown means that the melting of the ice sheet is likely to contribute 29% less to global sea level rise than previous models indicated.
"One of the main things we learned was that as grounded ice retreats inland, the bedrock under it lifts up elastically," said Erik Ivins, a co-author of the study. "It's similar to how a sofa cushion decompresses when you remove your weight from it. This process slows down the retreat of the ice sheet and ultimately the amount of melting."
Although this sounds like good news, the scientists say it's important to keep it in perspective. "It's like a truck traveling downhill that encounters speed bumps in the road," said Larour. "The truck will slow down a bit but will ultimately continue down the hill" - just as the ice sheet will continue to melt and sea level will continue to rise.
The breakthrough of this study, he added, was to "reach resolutions high enough to capture as many of these 'speed bumps' as possible and determine their effects in Antarctica while also modeling sea level rise over the entire planet."
Figure 20: This animation shows projections of ice sheet retreat in Antarctica over 500 years using the previous models (shown in green) and the new models, which take into account solid Earth processes like the elastic rebound of the Earth (shown in red). The new models show that by the year 2350, melting of the ice sheet and its corresponding contribution to sea-level rise will be about 29 percent less than what previous projections had indicated for this distant time period (image credit: NASA/JPL-Caltech)
Study of the Ocean Carbon Budget
• April 17, 2019: How exactly does the ocean — the Earth's largest carbon sink — capture and store carbon? The answer to this question will become increasingly important as the planet warms and as we try to get ahead of a runaway climate scenario. 20)
That's according to UC Santa Barbara oceanographer Dave Siegel. "The whole number is about 10 petagrams (10 x 1015 grams) of carbon per year," he said of the amount of carbon transported from the ocean surface to the deep, "which is about equal to how much carbon we spit out in fossil fuel emissions every year." - The estimate is very rough, however, and one that Siegel and his colleagues are working to refine.
"Once you start worrying about how those things might be changing as the climate changes, our precision has to increase," Siegel said. "We can't have these 20 to 25 percent uncertainties, because we won't get anywhere."
Achieving this precision is at the heart of a review co-authored by Siegel and collaborators, now published in the journal Nature. The review discusses relatively lesser-known — but no less significant — mechanisms of ocean carbon sequestration. They are known as "particle injection pumps (PIPs)" — a multidimensional approach to accounting for carbon movement in the deep ocean. 21)
"We've got to finally quantify the three-dimensional circulation processes and those pesky vertically migrating animals that inject organic carbon to the deep ocean," Siegel said.
Perhaps the best-known mechanism of ocean carbon sequestration is the biological gravitation pump (BGP), which, as the name suggests, is the sinking of biological debris vertically down the water column into the ocean's interior. Bits of zooplankton fecal matter, pieces of phytoplankton, dead microorganisms and such aggregate into clumps that become large and heavy enough to sink over a span of days to weeks, becoming food for deep water and bottom-dwelling creatures.
Another, well known and more active version of carbon transport from surface to deep water comes in the form of the diel vertical migration, or DVM, in which regular evening ascents made by zooplankton animals to the surface from 100 meters within the ocean interior are thought to be the largest migration on Earth.
"They come up to the surface at night to eat, and they go down during the day to avoid being eaten. There they respire CO2 and excrete organic carbon," Siegel explained.
However, there are other factors and processes to consider that inject organic particles to depth, including three-dimensional currents and vertical migrating carnivorous animals whose ecology remains a mystery. Collectively, they are known as particle injection pumps.
"There are migrating fish and other carnivorous animals that migrate vertically on both daily and seasonal timescales," Siegel said, particularly those in the mesopelagic zone, also known as the "twilight zone," where there is little to no light. Add to the animals' mesopelagic migration pumps a myriad of mechanisms that push particles and dissolved carbon sideways and down (subduction pump) and seasonal depth changes in the upper ocean layer (mixed-layer pump) that act over hourly to interannual timescales. Together these particle injection pumps are complex yet "may sequester as much carbon as the gravitational pump." According to the review, the processes that drive the PIPs have been known to marine scientists for years, but they cannot be sampled with the tools that have been used for decades to quantify the biological gravitational pump.
The growing body of knowledge will be essential for generating state-of-the-art models that can more accurately predict how the ocean will respond to a changing climate, according to Siegel, who is also the lead scientist in an international multidisciplinary field research effort called Export Processes in the Ocean from RemoTe Sensing (EXPORTS) headed by NASA and NSF (National Science Foundation).
"We need to understand the individual mechanisms well enough so that we can figure out how to parameterize them in computer models to predict future carbon cycle states," he said. "There's a long way to go to get this all figured out."
Glaciers lose nine trillion tons of ice in half a century
• April 8, 2019: When we think of climate change, one of the first things to come to mind is melting polar ice. However, ice loss isn't just restricted to the polar regions. According to research published today, glaciers around the world have lost well over 9000 gigatons (9 x 1012 or over nine trillion tons) of ice since 1961, raising sea level by 27 mm. 22)
An international team led by the University of Zürich in Switzerland used classical glaciological field observations combined with a wealth of information from various satellite missions to painstakingly calculate how much ice has been lost or gained by 19 different glacierised regions around the world.
A paper published in Nature describes how an international team led by the University of Zürich in Switzerland used classical glaciological field observations combined with a wealth of information from various satellite missions to painstakingly calculate how much ice has been lost from and gained by 19 different glacierised regions around the world. They reveal that 9625 gigatons of ice was lost from 1961 to 2016, raising sea level by 27 mm. 23)
Figure 21: Global glacier mass loss 1961–2016 (image credit: ESA, adapted from Zemp et al. (2019) Nature, and data courtesy of World Glacier Monitoring Service)
The largest regional losses were in Alaska, followed by glaciers around the edge of the Greenland ice sheet and from glaciers in the southern Andes. Significant amounts of ice were also lost from glaciers in the Canadian and Russian Arctic, as well as from Svalbard.
Glaciers in temperate regions such as in the European Alps and the Caucasus mountain range did not escape ice loss either, but are too small to make a significant contribution to sea level.
Interestingly, the only area to have gained ice over the 55-year period was southwest Asia (noted on the map as ASW). Here, glaciers amassed 119 gigatons of ice, but neighboring southeast Asia (ASE) lost around the same amount, 112 gigatons.
Figure 22: Glacial decline. A paper published recently in Nature Geosciences describes how a multitude of satellite images have been used to reveal that there has actually been a slowdown in the rate at which glaciers slide down the high mountains of Asia. This animation simply shows how glaciers in Sikkim in northeast India have changed between 2000 and 2018. One of the images is from the NASA/USGS Landsat-7 mission captured on 26 December 2000 and the other is from Europe's Copernicus Sentinel-2A satellite captured on 6 December 2018 [image credit: NASA/USGS/University of Edinburgh/ETH Zurich/contains modified Copernicus Sentinel data (2018)]
ESA's Climate Change Initiative – a research program focused on generating global datasets for the key components of Earth's climate, known as essential climate variables – was also key to the research. The initiative's glacier project, together with ESA's former GlobGlacier project, provided glacier outlines and information on ice mass changes for thousands of individual glaciers.
Frank Paul, co-author of the study explains, "Glacier outlines are needed to make precise calculations for the areas in question. To date, this information came largely from the US Landsat satellites, the data from which are delivered to European users under ESA's Third Party mission agreement. In the future, the Copernicus Sentinel-2 mission, in particular, will increasingly contribute to the precise monitoring of glacier change."
Digital elevation models, which provide topographic details of a region, were calculated using information from the Japan Aerospace Exploration Agency's ASTER sensor on the US Terra mission and Germany's TanDEM-X mission. Both sources were processed within the Glaciers Climate Change Initiative and other projects.
These data, along with the comprehensive glaciological database compiled by the World Glacier Monitoring Service, allowed the researchers to reconstruct ice thickness changes of 19,000 glaciers worldwide. By combining these measurement methods and having the new comprehensive dataset, the researchers could estimate how much ice was lost each year in all mountain regions since the 1960s. The glaciological measurements made in the field provided annual fluctuations, while satellite data allowed them to determine ice loss over several years or decades.
Research leader, Michael Zemp, said, "While we can now offer clear information about how much ice each region with glaciers has lost, it is also important to note that the rate of loss has increased significantly over the last 30 years. We are currently losing a total of 335 billion tons of ice a year, corresponding to a rise in sea levels of almost 1 mm per year."
Figure 23: Using 15 images from Landsat, the animation compresses 25 years of change into just 1.5 seconds to reveal the complex behavior of the surging glaciers in the Panmah region of the Karakorum mountain range in Asia. Glaciers are shown in pale blue, snow in light blue to cyan, clouds in white, water in dark blue, vegetation in green and bare terrain in pink to brown (image credit: F. Paul, The Cryosphere, 2015 & USGS/NASA)
While warming ocean water still remains the main driver for sea-level rise, melting glacier ice is the second largest contributor to rising seas. Dr Zemp added, "In other words, every single year we are losing about three times the volume of all ice stored in the European Alps, and this accounts for around 30% of the current rate of sea-level rise."
Around the world, vanishing glaciers also ultimately mean less water for millions of people, less hydroelectric power and less water for crops. While melting glaciers also result in sea-level rise, they critically increase the risk of other natural hazards such as glacier-lake outburst floods and related debris flows.
The pace at which glaciers are losing mass in the long term is very important to making informed future adaptation decisions. This kind of information, therefore, is critical for international bodies assessing climate change, such as the Intergovernmental Panel on Climate Change.
Mark Drinkwater, Senior Advisor on cryosphere and climate at ESA, added, "Bearing in mind the socioeconomic consequences, the fate of glaciers in a future climate is something ESA views seriously. It is fundamental that we build upon existing monitoring capabilities using observations from the EC's Copernicus Sentinel missions, and other ESA and Third Party Mission missions. Their data crucially allow us to build a robust climate perspective to reveal regional and year-to-year fluctuations of glaciers and other parts of the cryosphere such as snow cover, sea ice and ice sheets."
AWI study reveals extreme sea-ice melting in the Arctic
• April 2, 2019: The dramatic loss of ice in the Arctic is influencing sea-ice transport across the Arctic Ocean. As experts from the AWI (Alfred Wegener Institute), Helmholtz Center for Polar and Marine Research report in a new study, today only 20 percent of the sea ice that forms in the shallow Russian marginal seas of the Arctic Ocean actually reaches the Central Arctic, where it joins the Transpolar Drift; the remaining 80 percent of the young ice melts before it has a chance to leave its ‘nursery'. Before 2000, that number was only 50 percent. According to the researchers, this development not only takes us one step closer to an ice-free summer in the Arctic; as the sea ice dwindles, the Arctic Ocean stands to lose an important means of transporting nutrients, algae and sediments. 24)
Figure 24: The shallow Russian shelf or marginal seas of the Arctic Ocean are generally considered to be the ‘nursery' of Arctic sea ice. Strong, offshore directed winds push the pack ice in winter away from the coast, and extremely low temperatures lead to the development of new ice zones. The image, obtained by an ESA satellite on 26 March 2019, shows the process of new ice formation along the Russian coast line (the Laptev Sea). In the process, algae, sediments and nutrients are mixed near the water's surface and become trapped in the ice (Photo: ESA/DriftNoise – Satellite Services)
The shallow Russian shelf of the Arctic Ocean are broadly considered to be the ‘nursery' of Arctic sea ice: in winter, the Barents Sea, Kara Sea, Laptev Sea and East Siberian Sea constantly produce new sea ice. This is due to extremely low air temperatures down to -40 degrees Celsius, and a strong offshore wind that drives the young ice out to the open sea. In the course of the winter, the sea ice is eventually caught up in the Transpolar Drift, one of the two main currents in the Arctic Ocean. In two to three years' time, it transports the ice floes from the Siberian part of the Arctic Ocean, across the Central Arctic, and into the Fram Strait, where it finally melts. Two decades ago, roughly half the ice from Russia's shelf seas made this transarctic journey. Today only 20 percent does; the other 80 percent of the young ice melts before it can become a year old and reach the Central Arctic.
Experts from the AWI (Alfred Wegener Institute) in Bremerhaven, Germany, came to this troubling conclusion after monitoring and analyzing the sea ice's movements with the aid of satellite data from 1998 to 2017. "Our study shows extreme changes in the Arctic: the melting of sea ice in the Kara Sea, Laptev Sea and East Siberian Sea is now so rapid and widespread that we're seeing a lasting reduction in the amount of new ice for the Transpolar Drift. Now, most of the ice that still reaches the Fram Strait isn't formed in the marginal seas, but comes from the Central Arctic. What we're witnessing is a major transport current faltering, which is bringing the world one major step closer to a sea-ice-free summer in the Arctic," says first author Dr. Thomas Krumpen, a sea-ice physicist at the Alfred Wegener Institute. 25)
This trend has been confirmed by the outcomes of sea-ice thickness measurements taken in the Fram Strait, which the AWI sea-ice physicists gather on a regular basis. "The ice now leaving the Arctic through the Fram Strait is, on average, 30 percent thinner than it was 15 years ago. The reasons: on the one hand, rising winter temperatures in the Arctic and a melting season that now begins much earlier; on the other, this ice is no longer formed in the shelf seas, but much farther north. As a result, it has far less time to drift through the Arctic and grow into thicker pack ice," Thomas Krumpen explains.
Those ice floes that the Transpolar Drift still carries to the Fram Strait are for the most part formed in the open sea, i.e., in regions of the Arctic Oceans far from the coasts. Consequently, compared to ice from the shelf seas, they contain significantly fewer particles like algae, sediments and nutrients – because waves, wind and tides stir up far more particles from the seafloor in shallow coastal zones than on the high seas. In addition, rivers like the Lena and the Yenisei carry major quantities of minerals and sediments to coastal areas; when the water freezes, they become trapped in the ice.
Whereas in the past, sea ice from the shelf seas transported this mineral load to the Fram Strait, today the melting floes release it on their way to the Central Arctic; what reaches the Fram Strait is less material, and with a different composition. This finding is a result e.g. of analysis of samples obtained by means of sediment traps that AWI biologists have been conducting in the Fram Strait for about two decades. "Instead of Siberian minerals, we're now finding more remains of dead algae and microorganisms in our sediment traps," says co-author Eva-Maria Nöthig. In the long term, this altered sea-ice-based particle transport is likely to produce lasting changes in the biogeochemical cycles and ecological processes of the central Arctic Ocean.
The evolution of sea ice and the ecological processes in the Arctic Ocean are also key research questions that will be addressed during the MOSAiC expedition, which will begin this September. The German research icebreaker Polarstern will journey to the Arctic and drift with the Transpolar Drift through the Arctic Ocean towards Fram Strait for an entire year, intentionally trapped in the ice. 600 people from 17 countries will take part in the expedition, which will be regularly resupplied by aircraft and other icebreakers; moreover, many times that number of experts will use the resulting data to take climate and ecosystem research to a new level. MOSAiC, the greatest Arctic research expedition in history, will be spearheaded by the Alfred Wegener Institute.
Figure 25: Overview map of the Arctic Ocean (image credit: AWI)
Figure 26: Results from backward-tracking of sea ice starting from 6 locations in Fram Strait between 1998–2017. Tracking was initiated at a two-week interval. The ice is tracked backward in time until it reaches land or fast ice or until sea ice concentration drops below 20%. (a) shows the fraction (averaged annual frequency) of sea ice leaving Fram Strait that originates from shallow shelf areas with less than 30 m water depth. The two maps (b,c) show the formation sites of sea ice leaving Fram Strait gridded on a 62.5 x 62.5 km grid: (b) for the period between 1998–2006 and (c) between 2007–2017. Ice younger than 2 month was excluded from the analysis. In (d) the gridded density of all backward trajectories is shown. The typical course of the Transpolar Drift is emphasized by high track frequencies. (e) Provides the annual averaged origin (° longitude) of Fram Strait sea ice (image credit: AWI)
Cold Water Currently Slowing Fastest Greenland Glacier
• March 25, 2019: NASA research shows that Jakobshavn Glacier, which has been Greenland's fastest-flowing and fastest-thinning glacier for the last 20 years, has made an unexpected about-face. Jakobshavn is now flowing more slowly, thickening, and advancing toward the ocean instead of retreating farther inland. The glacier is still adding to global sea level rise - it continues to lose more ice to the ocean than it gains from snow accumulation - but at a slower rate. 26)
The researchers conclude that the slowdown of this glacier, known in the Greenlandic language as 'Sermeq Kujalleq', occurred because an ocean current that brings water to the glacier's ocean face grew much cooler in 2016. Water temperatures in the vicinity of the glacier are now colder than they have been since the mid-1980s.
In a study published today in Nature Geoscience, Ala Khazendar of NASA's Jet Propulsion Laboratory in Pasadena, California, and colleagues report the change in Jakobshavn's behavior and trace the source of the cooler water to the North Atlantic Ocean more than 600 miles (966 km) south of the glacier. The research is based on data from NASA's Oceans Melting Greenland (OMG) mission and other observations. 27)
Figure 27: NASA's Oceans Melting Greenland (OMG) mission uses ships and planes to measure how ocean temperatures affect Greenland's vast icy expanses. Jakobshavn Glacier, on Greenland's central western side, has been one of the island's largest contributor's to sea level rise, losing mass at an accelerating rate (video credit: NASA/GSFC, Kathryn Mersmann)
The scientists were so shocked to find the change, Khazendar said: "At first we didn't believe it. We had pretty much assumed that Jakobshavn would just keep going on as it had over the last 20 years." However, the OMG mission has recorded cold water near Jakobshavn for three years in a row.
The researchers suspect the cold water was set in motion by a climate pattern called the North Atlantic Oscillation (NAO), which causes the northern Atlantic Ocean to switch slowly between warm and cold every five to 20 years. The climate pattern settled into a new phase recently, cooling the Atlantic in general. This change was accompanied by some extra cooling in 2016 of the waters along Greenland's southwest coast, which flowed up the west coast, eventually reaching Jakobshavn.
When the climate pattern flips again, Jakobshavn will most likely start accelerating and thinning again.
Josh Willis of JPL, the principal investigator of OMG, explained, "Jakobshavn is getting a temporary break from this climate pattern. But in the long run, the oceans are warming. And seeing the oceans have such a huge impact on the glaciers is bad news for Greenland's ice sheet."
Water Temperature and Weather
Jakobshavn, located on Greenland's west coast, drains about 7 percent of the island's ice sheet. Because of its size and importance to sea level rise, scientists from NASA and other institutions have been observing it for many years.
Researchers hypothesized that the rapid retreat of the glacier began with the early 2000s loss of the glacier's ice shelf - a floating extension of the glacier that slows its flow. When ice shelves disintegrate, glaciers often speed up in response. Jakobshavn has been accelerating each year since losing its ice shelf, and its front (where the ice reaches the ocean) has been retreating. It lost so much ice between 2003 and 2016 that its thickness, top to bottom, shrank by 500 feet (152 m).
The research team combined earlier data on ocean temperature with data from the OMG mission, which has measured ocean temperature and salinity around the entire island for the last three summers. They found that in 2016, water in Jakobshavn's fjord cooled to temperatures not seen since the 1980s.
"Tracing the origin of the cold waters in front of Jakobshavn was a challenge," explained Ian Fenty of JPL, a co-author of the study. "There are enough observations to see the cooling but not really enough to figure out where it came from." Using an ocean model called Estimating the Circulation and Climate of the Ocean (ECCO) to help fill in the gaps, the team traced the cool water upstream (toward the south) to a current that carries water around the southern tip of Greenland and northward along its west coast. In 2016, the water in this current cooled by more than 2.7 degrees Fahrenheit (1.5 degrees Celsius).
Although the last few winters were relatively mild in Greenland itself, they were much colder and windier than usual over the North Atlantic Ocean. The cold weather coincided with the switch in the NAO climate pattern. Under the influence of this change, the Atlantic Ocean near Greenland cooled by about 0.5 degrees Fahrenheit (1 degree Celsius) between 2013 and 2016. These generally cooler conditions set the stage for the rapid cooling of the ocean current in southwest Greenland in early 2016. The cooler waters arrived near Jakobshavn that summer, at the same time that Jakobshavn slowed dramatically.
The team suspects that both the widespread Atlantic cooling and the dramatic cooling of the waters that reached the glacier were driven by the shift in the NAO. If so, the cooling is temporary and warm waters will return when the NAO shifts to a warm phase once again.
The warming climate has increased the risk of melting for all land ice worldwide, but many factors can speed or slow the rate of ice loss. "For example," Khazendar said, "the shape of the bed under a glacier is very important, but it is not destiny. We've shown that ocean temperatures can be just as important."
Tom Wagner, NASA Headquarters program scientist for the cryosphere, who was not involved in the study, said, "The OMG mission deployed new technologies that allowed us to observe a natural experiment, much as we would do in a laboratory, where variations in ocean temperatures were used to control the flow of a glacier. Their findings - especially about how quickly the ice responds - will be important to projecting sea level rise in both the near and distant future."
Long-term Mud Creek landslide observation documented with InSAR
• February 7, 2018: "Stable landslide" sounds like a contradiction in terms, but there are indeed places on Earth where land has been creeping downhill slowly, stably and harmlessly for as long as a century. But stability doesn't necessarily last forever. For the first time, researchers at NASA's Jet Propulsion Laboratory in Pasadena, California, and collaborating institutions have documented the transition of a stable, slow-moving landslide into catastrophic collapse, showing how drought and extreme rains likely destabilized the slide. 28)
The Mud Creek landslide near Big Sur, California, dumped about 6 million cubic yards (5 million cubic meters) of rock and debris across California Highway 1 on May 20, 2017. The damage took more than a year and $54 million to repair. No long-term motion had been documented at Mud Creek before this event, but workers in the state's transportation department had noticed small mudslides in the weeks before the collapse and closed the highway as a precaution.
The JPL-led team identified Mud Creek as a stable landslide using an eight-year data set from an airborne JPL instrument called the Uninhabited Airborne Vehicle Synthetic Aperture Radar (UAVSAR), processed with a technique called interferometric synthetic aperture radar processing (InSAR). They calculated that Mud Creek had been sliding at an average speed of about 7 inches (17 centimeters) per year since at least 2009. They used the European Space Agency's Sentinel-1A/B satellite data to document how the sliding area's behavior changed.
The airborne and satellite data measure changes only at the ground surface, however. "From that, we tried to infer what may have happened to the landslide's sliding surface, tens of meters underground, that allowed the Mud Creek slide to transition from stable to unstable," said the study's lead author, Alexander Handwerger, a NASA postdoctoral fellow doing research at JPL. 29) 30)
Figure 28: The Mud Creek landslide in photographic and radar images. The radar velocity map shows the pre-collapse (solid line) and post-collapse (dashed line) extent of the sliding area, with faster sliding velocities before the collapse shown in darker shades of red. The highest velocities were about 40 cm/year (image credit: Google/SIO/NOAA/U.S. Navy/NGA/GEBCO/Landsat/Copernicus)
The collapse happened after several days of heavy rainfall during one of the wettest years in over a century for this area. Before 2017, a five-year drought had produced several of California's hottest and driest years ever. Using a computer model of how water affects soil, the researchers studied what would happen as the intense rains saturated the parched ground. Water would replace air in the tiny spaces between soil particles, greatly increasing the pressure on the particles. This pressure change could have destabilized the sliding surfaces belowground and triggered the collapse.
California alone has more than 650 known stable landslides. If one began losing stability in the future, could InSAR data reveal the change? To answer that question, the team compared the Mud Creek images with images of two other stable landslides in similar types of soil and rock.
Paul's Slide, only 13 miles (21 kilometers) north of Mud Creek, went through the same weather conditions yet did not fail catastrophically. A landslide in Northern California received over 1 meter more rainfall than Mud Creek without catastrophic failure. "We thought if we compared these two cases that didn't fail to the one that did, we might find some characteristic velocity pattern that would be a warning that a slide was going to fail catastrophically," Handwerger said.
The idea paid off. Handwerger found that all three stable slides accelerated slightly after the winter rainy season started and then, as the season continued, slowed down again and stabilized. This is their usual annual pattern. But after the late-season rains, Mud Creek accelerated again, increasing in speed until its ultimate collapse. The other slides did not.
"We think that second speed-up may be a signal of interest, but we only have this one case," Handwerger said. "Since we now know that stable landslides in this region can fail catastrophically and we have good data coverage here, our plan is to monitor this whole stretch of the Pacific Coast Highway and look for these unusual velocity changes. If we get enough examples, we can start to actually figure out the mechanisms that are controlling this behavior."
Figure 29: Northern and Central California Coast Ranges. (a) Elevation and Franciscan Complex lithologic unit 1 and San Andreas fault draped over a hillshade of the topography. Black polygons show mapped inventories of slow-moving landslides. Map data: Geologic map from USGS, digital elevation models from TanDEM-X. TanDEM-X data used is under copyright by the DLR. (b,c) Google Earth images (Map data: SIO, NOAA, U.S. Navy, NGA, GEBCO; Image; Landsat/Copernicus) of the Mud Creek landslide before and after catastrophic failure. Solid black and dashed black polygons shows pre- and postcatastrophic failure landslide boundaries. We mapped the pre-catastrophic failure boundaries using InSAR, Google Earth, and a digital elevation model. We mapped the post-catastrophic failure landslide boundaries using Google Earth. Software: QGIS Geographic Information System. Open Source Geospatial Foundation Project (image credit: Mud Creek landslide study team)
Figure 30: Velocity and strain rate maps of Mud Creek landslide. (a) Average horizontal velocity between February 2009 and May 2017 with velocity vectors draped over hillshade of the topography. CA1 shown with black and white line. Reference pixel corresponds to a stable area outside the landslide. (b) Average vertical velocity with negative values indicating downward motion. (c) Strain rate map showing upslope horizontal extension (positive values) and downslope contraction (negative values). The azimuth and look direction of the Sentinel-1A/B and UAVSAR SAR instruments are shown with black and orange arrows in the legend. Map data: digital elevation models from USGS and TanDEM-X. TanDEM-X data used is under copyright by the DLR (image credit: Mud Creek landslide study team)
Figure 31: Velocity time series of Mud Creek landslide. (a) Parts of the landslide that were accelerating on May 13, 2017. (b) Parts of the landslide with velocities greater than the maximum velocities during WY2016. (c) Downslope velocity, precipitation, and modelled pore-fluid pressure time series during WY2015-WY2017 condensed into a single calendar year for a representative area (averaged over 60 x 60 m, shown by black box and star in (a). The normalized pore-fluid pressure is defined as the pore-fluid pressure divided by the maximum value over the study period. Lag time corresponds to the time between the onset of precipitation and onset of acceleration. Red rectangle highlights the divergence from the characteristic seasonal velocity pattern (image credit: Mud Creek landslide study team)
2018 Fourth Warmest Year in Continued Warming Trend
• February 6, 2019: Earth's global surface temperatures in 2018 were the fourth warmest since 1880, according to independent analyses by NASA and the National Oceanic and Atmospheric Administration (NOAA). 31)
Global temperatures in 2018 were 1.5 degrees Fahrenheit (0.83º Celsius) warmer than the 1951 to 1980 mean, according to scientists at NASA's Goddard Institute for Space Studies (GISS) in New York. Globally, 2018's temperatures rank behind those of 2016, 2017 and 2015. The past five years are, collectively, the warmest years in the modern record.
"2018 is yet again an extremely warm year on top of a long-term global warming trend," said GISS Director Gavin Schmidt.
Since the 1880s, the average global surface temperature has risen about 2º Fahrenheit (1º Celsius). This warming has been driven in large part by increased emissions into the atmosphere of carbon dioxide and other greenhouse gases caused by human activities, according to Schmidt.
Figure 32: Earth's long-term warming trend can be seen in this visualization of NASA's global temperature record, which shows how the planet's temperatures are changing over time, compared to a baseline average from 1951 to 1980. The record is shown as a running five-year average (video credit: NASA's Scientific Visualization Studio/Kathryn Mersmann)
Weather dynamics often affect regional temperatures, so not every region on Earth experienced similar amounts of warming. NOAA found the 2018 annual mean temperature for the contiguous 48 United States was the 14th warmest on record.
Warming trends are strongest in the Arctic region, where 2018 saw the continued loss of sea ice. In addition, mass loss from the Greenland and Antarctic ice sheets continued to contribute to sea level rise. Increasing temperatures can also contribute to longer fire seasons and some extreme weather events, according to Schmidt.
"The impacts of long-term global warming are already being felt — in coastal flooding, heat waves, intense precipitation and ecosystem change," said Schmidt.
NASA's temperature analyses incorporate surface temperature measurements from 6,300 weather stations, ship- and buoy-based observations of sea surface temperatures, and temperature measurements from Antarctic research stations.
Figure 33: This line plot shows yearly temperature anomalies from 1880 to 2018, with respect to the 1951-1980 mean, as recorded by NASA, NOAA, the Japan Meteorological Agency, the Berkeley Earth research group, and the Met Office Hadley Centre (UK). Though there are minor variations from year to year, all five temperature records show peaks and valleys in sync with each other. All show rapid warming in the past few decades, and all show the past decade has been the warmest (image credit: NASA's Earth Observatory)
These raw measurements are analyzed using an algorithm that considers the varied spacing of temperature stations around the globe and urban heat island effects that could skew the conclusions. These calculations produce the global average temperature deviations from the baseline period of 1951 to 1980.
Because weather station locations and measurement practices change over time, the interpretation of specific year-to-year global mean temperature differences has some uncertainties. Taking this into account, NASA estimates that 2018's global mean change is accurate to within 0.1 degree Fahrenheit, with a 95 percent certainty level.
NOAA scientists used much of the same raw temperature data, but with a different baseline period and different interpolation into the Earth's polar and other data poor regions. NOAA's analysis found 2018 global temperatures were 1.42 degrees Fahrenheit (0.79 degrees Celsius) above the 20th century average.
NASA's full 2018 surface temperature data set — and the complete methodology used to make the temperature calculation — are available at: https://data.giss.nasa.gov/gistemp
GISS is a laboratory within the Earth Sciences Division of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The laboratory is affiliated with Columbia University's Earth Institute and School of Engineering and Applied Science in New York.
NASA uses the unique vantage point of space to better understand Earth as an interconnected system. The agency also uses airborne and ground-based monitoring, and develops new ways to observe and study Earth with long-term data records and computer analysis tools to better see how our planet is changing. NASA shares this knowledge with the global community and works with institutions in the United States and around the world that contribute to understanding and protecting our home planet.
Upper-ocean warming makes waves stronger
• January 14, 2019: Sea level rise puts coastal areas at the forefront of the impacts of climate change, but new research shows they face other climate-related threats as well. Scientists found that the energy of ocean waves has been growing globally, and they found a direct association between ocean warming and the increase in wave energy. 32)
In a study published January 14 in Nature Communications, researchers report that the energy of ocean waves has been growing globally, and they found a direct association between ocean warming and the increase in wave energy. 33)
A wide range of long-term trends and projections carry the fingerprint of climate change, including rising sea levels, increasing global temperatures, and declining sea ice. Analyses of the global marine climate thus far have identified increases in wind speeds and wave heights in localized areas of the ocean in the high latitudes of both hemispheres. These increases have been larger for the most extreme values (e.g., winter waves) than for the mean conditions. However, a global signal of change and a correlation between the localized increases in wave heights and global warming had remained undetected.
The new study focused on the energy contained in ocean waves, which is transmitted from the wind and transformed into wave motion. This metric, called wave power, has been increasing in direct association with historical warming of the ocean surface. The upper ocean warming, measured as a rising trend in sea-surface temperatures, has influenced wind patterns globally, and this, in turn, is making ocean waves stronger.
"For the first time, we have identified a global signal of the effect of global warming in wave climate. In fact, wave power has increased globally by 0.4 percent per year since 1948, and this increase is correlated with the increasing sea-surface temperatures, both globally and by ocean regions," said lead author Borja G. Reguero, a researcher in the Institute of Marine Sciences at the University of California, Santa Cruz.
Climate change is modifying the oceans in different ways, including changes in ocean-atmosphere circulation and water warming, according to coauthor Inigo J. Losada, director of research at the Environmental Hydraulics Institute at the University of Cantabria (IHCantabria), where the study was developed.
"This study shows that the global wave power can be a potentially valuable indicator of global warming, similarly to carbon dioxide concentration, the global sea level rise, or the global surface atmospheric temperature," Losada said.
Understanding how the energy of ocean waves responds to oceanic warming has important implications for coastal communities, including anticipating impacts on infrastructure, coastal cities, and small island states. Ocean waves determine where people build infrastructure, such as ports and harbors, or require protection through coastal defenses such as breakwaters and levees. Indeed, wave action is one of the main drivers of coastal change and flooding, and as wave energy increases, its effects can become more profound. Sea level rise will further aggravate these effects by allowing more wave energy to reach shoreward.
While the study reveals a long-term trend of increasing wave energy, the effects of this increase are particularly apparent during the most energetic storm seasons, as occurred during the winter of 2013-14 in the North Atlantic, which impacted the west coast of Europe, or the devastating 2017 hurricane season in the Caribbean, which offered a harsh reminder of the destructive power and economic impacts of coastal storms.
The effects of climate change will be particularly noticeable at the coast, where humans and oceans meet, according to coauthor Fernando J. Méndez, associate professor at Universidad de Cantabria. "Our results indicate that risk analysis neglecting the changes in wave power and having sea level rise as the only driver may underestimate the consequences of climate change and result in insufficient or mal-adaptation," he said.
The study focus is on WP (Wave Power), which measures (over cumulative periods of time) the transport of energy that is transmitted by air-sea exchanges and employed for wave motion. WP has not been studied as a climate change indicator yet, but it can potentially characterize the long-term behavior of the global wave conditions better than wave heights. To investigate trends in WP over time and its relationship with global warming, the study team calculates and analyzes long-term global time series of WP and SST (Sea Surface Temperature). Changes in wave climate have been previously studied through four types of data: buoy measurements, observations from ships, satellite-based altimetry records, and numerical modeling. This study combines satellite altimetry and model results (validated with buoy measurements) to determine GWP (Global Wave Power) because observations from buoys and satellite altimetry do not provide continuous data over space and time.
Figure 34: Spatial mean annual Wave Power calculated globally and by ocean basin. The dashed lines represent the 10-year moving averages. The Southern Ocean is defined between latitudes of 40ºS and 80ºS. The mean regional Wave Power is calculated as the spatial average of each historical wave power time series. The solid lines indicate each time series. The dashed lines correspond to the 10-year moving average (image credit: UC Santa Cruz, IHCantabria)
The long memory of the Pacific Ocean
• January 4, 2019: The ocean has a long memory. When the water in today's deep Pacific Ocean last saw sunlight, Charlemagne was the Holy Roman Emperor, the Song Dynasty ruled China and Oxford University had just held its very first class. During that time, between the 9th and 12th centuries, the earth's climate was generally warmer before the cold of the Little Ice Age settled in around the 16th century. Now ocean surface temperatures are back on the rise but the question is, do the deepest parts of the ocean know that? 34)
Figure 35: Cold waters that sank in polar regions hundreds of years ago during the Little Ice Age are still impacting deep Pacific Ocean temperature trends. While the deep Pacific temperature trends are small, they represent a large amount of energy in the Earth system (Photo by: Larry Madin, WHOI)
Researchers from WHOI (Woods Hole Oceanographic Institution) and Harvard University have found that the deep Pacific Ocean lags a few centuries behind in terms of temperature and is still adjusting to the entry into the Little Ice Age. Whereas most of the ocean is responding to modern warming, the deep Pacific may be cooling.
"These waters are so old and haven't been near the surface in so long, they still ‘remember' what was going on hundreds of years ago when Europe experienced some of its coldest winters in history," said Jake Gebbie, a physical oceanographer at WHOI and lead author of the study published 4 January 2019, in the journal Science. 35)
"Climate varies across all timescales," adds Peter Huybers, Professor of Earth and Planetary Sciences at Harvard University and co-author of the paper. "Some regional warming and cooling patterns, like the Little Ice Age and the Medieval Warm Period, are well known. Our goal was to develop a model of how the interior properties of the ocean respond to changes in surface climate."
What that model showed was surprising.
"If the surface ocean was generally cooling for the better part of the last millennium, those parts of the ocean most isolated from modern warming may still be cooling," said Gebbie.
The model is, of course, a simplification of the actual ocean. To test the prediction, Gebbie and Huybers compared the cooling trend found in the model to ocean temperature measurements taken by scientists aboard the HMS Challenger in the 1870s and modern observations from the World Ocean Circulation Experiment of the 1990s.
The HMS Challenger, a three-masted wooden sailing ship originally designed as a British warship, was used for the first modern scientific expedition to explore the world's ocean and seafloor. During the expedition from 1872 to 1876, thermometers were lowered into the ocean depths and more than 5,000 temperature measurements were logged.
"We screened this historical data for outliers and considered a variety of corrections associated with pressure effects on the thermometer and stretching of the hemp rope used for lowering thermometers," said Huybers.
The researchers then compared the HMS Challenger data to the modern observations and found warming in most parts of the global ocean, as would be expected due to the warming planet over the 20th Century, but cooling in the deep Pacific at a depth of around two kilometers.
"The close correspondence between the predictions and observed trends gave us confidence that this is a real phenomenon," said Gebbie.
Figure 36: The HMS Challenger, a three-masted wooden sailing ship originally designed as a British warship, was used for the first modern scientific expedition to explore the world's ocean and seafloor. Gebbie and Huybers compared the cooling trend found in the model to ocean temperature measurements taken by scientists aboard the HMS Challenger in the 1870s and modern observations from the World Ocean Circulation Experiment of the 1990s (Painting of the HMS Challenger by William Frederick Mitchell originally published for the Royal Navy)
These findings imply that variations in surface climate that predate the onset of modern warming still influence how much the climate is heating up today. Previous estimates of how much heat the Earth had absorbed during the last century assumed an ocean that started out in equilibrium at the beginning of the Industrial Revolution. But Gebbie and Huybers estimate that the deep Pacific cooling trend leads to a downward revision of heat absorbed over the 20th century by about 30 percent.
"Part of the heat needed to bring the ocean into equilibrium with an atmosphere having more greenhouse gases was apparently already present in the deep Pacific," said Huybers. "These findings increase the impetus for understanding the causes of the Medieval Warm Period and Little Ice Age as a way for better understanding modern warming trends."
This research was funded by the James E. and Barbara V. Moltz Fellowship and National Science Foundation grants OCE-1357121 and OCE-1558939.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).