Other Space Activities

Glacier and Ice Sheet Monitoring

Last updated:Aug 2, 2024

Science

Measuring changes in the extent, density, and thickness of glaciers and ice sheets is valuable for monitoring the effects of climate change. Due to the size and inaccessibility of these geological features, systematically measuring how they change can be difficult. Space-based observations have proven invaluable in measuring glacial dynamics across vast scales. 1)

Figure 1: The Penny Ice Cap in Canada (Image credit: NASA / Micahel Studinger)

What is the relationship between glaciers and ice sheets?

In short, ice sheets are a type of glacier. Glaciers form when enough snow accumulates and compresses into ice crystals. The compaction process of fresh snow falling over burying snow occurs for several years before glacial ice forms (Figure 2). As the compressed snow gets denser, it is forced to recrystallise into ice crystals which grow larger over time. Old glaciers that are hundreds of years old can have ice crystals the size of a fist. 3) 4)

Figure 2: Diagram explaining the transition of snow to glacial ice (Image credit: Department of Geography and Environmental Science/Hunter College)

There are many different types of glaciers. For example:

Ice sheet: a glacier that covers land and extends more than 50,000 square kilometres. That area is equivalent to the size of Slovakia or Costa Rica.
Ice cap: a dome-shaped glacier that covers land and extends less than 50,000 square kilometres.
Icefield: a glacier that covers land and whose topography is heavily influenced by the terrain it covers.
Outlet glacier: a fast-flowing glacier that moves out of an ice sheet, ice cap, or icefield.
Iceberg: floating chunks of ice larger than 5 metres across, that have broken off from a larger glacier.
Bergy bit: floating chunks of ice smaller than 5 metres across, which have broken off from a larger glacier.
Alpine Glaciers: glaciers that form in mountains and valleys. These include:

Cirque glacier: a glacier that covers a cirque - a steep bowl-shaped hollow that occurs near the tops of mountains.
Mountain glacier: a glacier that covers a mountain slope.
Valley glacier: a glacier that fills a valley.
Hanging glacier: a glacier frozen onto a slope steeper than 45 degrees.
Piedmont glacier: a flat glacier at the base of a mountain.
Tidewater glacier: a flat glacier at the base of a mountain that flows into the ocean.

Where are glaciers and ice sheets found?

Whilst mostly near the poles, glaciers can be found on every continent on Earth (Figure 3). They are glaciers in Africa atop Mount Kenya, Kilimanjaro, and the Rwenzori Mountains. Whilst there aren’t any glaciers in Australia, there are others in the Oceanic continent across New Zealand, Indonesia, and Papua New Guinea.

There are currently only two ice sheets on Earth: the Antarctic Ice Sheet and the Greenland Ice Sheet. They are both on average 3.2 km thick and collectively contain 68% of all freshwater on Earth. The Greenland Ice Sheet spans 80% of Greenland with a size of 1.7 million square kilometres, roughly the same area as Iran. If the entire Greenland Ice Sheet melted, global sea levels would rise approximately 7.4 m. Meanwhile, the Antarctic Ice Sheet spans across the whole Antarctic continent of 14 million square kilometres, roughly the same area as the United States and Mexico combined, and if it all melted, the global sea level would rise approximately 60 m. 2)

How do glaciers and ice sheets move?

Alpine glaciers are frozen rivers of ice, slowly flowing under their own weight down mountains. But ice sheets instead of flowing in one direction like alpine glaciers, flow outwards in all directions from the centre (which in the case of the Greenland and Antarctic ice sheets are 3 - 4 km high). Near the summit of an ice sheet, where the slope is slight, the flow rate is typically just a few centimetres to a few metres per year. However, closer to the coast, ice speeds can increase dramatically, reaching hundreds of metres or even several kilometres per year as the ice flows into outlet glaciers. 2)

How fast are glaciers melting?

Glaciers are melting due to our warming climate, but the rate at which that is happening is hard to predict. Glaciers gain mass as snow accumulates and is compressed to form new glacial ice. They also lose mass through exposure to the warm air, warm ocean water, and warm meltwater. These interactions cause ice to crack and icebergs to form, resulting in further loss of glacial mass. Another factor affecting glaciers' melt rate is the topography of the sea floor and bedrock under or around the glacier. 2)

Example Products

Randolph Glacier Inventory

The Randolph Glacier Inventory (RGI) is a collection of all of Earth’s glaciers, excluding the Antarctic and Greenland ice sheets. This inventory’s creation was prompted by the Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC). It was created with satellite data collected between 1999 and 2010, from sensors such as Landsat 5’s thematic mapper, and Landsat 7’s enhanced thermal mapper plus, and sensors onboard Terra, IKONOS-2, and SPOT-5. 7)

Figure 3: RGI with glaciers highlighted in red (Image credit: Pfeffer et al., 2017)

Figure 4: Statistics concerning glacial regions in Figure 3 (Image credit: Pfeffer et al., 2017)

Surface Features

By studying the surface features of glaciers, we are able to better predict their stability. The George IV Ice Shelf (GVIIS) located in Antarctica is in a region where ice shelves are rapidly breaking up. Using Landsat’s optical instruments, scientists collected data that measured the ice shelf’s surface features. This was complemented by synthetic aperture radar (SAR) and laser altimetry data from ERS-1/2 and ICESat, respectively. 6)

Glacier Thickness

Glacier thickness can be measured over time to determine the changes in mass balance. It’s important to measure glacial melt rates as they indicate the effects of the past and act as a predictor for the future. Figure 6 features a map of GVIIS thickness data as collected by ICESat’s geoscience laser altimeter system between 2005 and 2008. 6)

Figure 6: GVIIS glacial thickness (Image credit: Holt et al., 2013)

Glacial Flow Rate

Determining the flow rate of different parts of a glacier is important as flow rates both indicate the effects of the past and act as a predictor for the future. These particular measurements (Figure 7) were done by performing Interferometric SAR with ERS-1/2, which involves comparing pairs of images taken 24 hours apart. 6)

Related Missions

Terra (EOS AM-1)

Launched in 1999, Terra (formerly known as EOS AM-1) is the flagship of the National Aeronautical and Space Administration’s (NASA’s) Earth Observing System (EOS) program. Terra’s mission is to monitor Earth’s atmosphere, ocean, land, snow, and ice to gain insight into how the Earth’s processes and dynamics. Terra’s Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is used to monitor glaciers and ice sheets by imaging the Earth’s surface across a wide spectral range.

CryoSat-2

Cryosat-2 is an ESA Earth Explorer mission launched in 2010 that monitors land and sea ice sheet thickness to investigate how the polar ice caps are shrinking. The mission’s primary instrument is the Synthetic Aperture Radar (SAR) Interferometer Radar Altimeter (SIRAL), which measures the extent to which sea ice protrudes from the sea through the emittance and reflectance of radio signals. Measurements of how far this sea ice protrudes from the water can then be used to infer the sea ice thickness.

Gravity Recovery And Climate Experiment - Follow-On (GRACE-FO)

Launched in 2018, two GRACE-FO satellites became the successors to the original two GRACE satellites. GRACE-FO is jointly operated by NASA and the German Aerospace Centre (DLR) to acquire high-resolution monthly global models of Earth’s gravitational field. Earth’s gravity varies very little from month to month as most variation is caused by solid Earth movement which is very slow. However, a small portion of Earth’s gravity is dictated by water flow through ocean currents, snowfalls, and ice melts.

GRACE-FO monitors glaciers and ice sheets by measuring how the distance between the leading and trailing satellites (which averages 220 km) changes as they pass over a mass. As the satellites approach a mass, the leading satellite (being closer to the object) is accelerated away from the trailing satellite increasing the distance between them. After the leading satellite passes over the mass it is decelerated whilst the trailing satellite is accelerated, decreasing the distance between the two satellites. This change in separation is measured by microwave radiation, at a wavelength of about 1 μm.

Figure 8: Diagram explaining the distance variation of the GRACE-FO satellites as they pass over a mass (Image credit: NASA Earth Observatory)

ICESat

Launched in 2003, the Ice, Cloud, and land Elevation Satellite (ICESat) was operated by the National Aeronautics and Space Administration (NASA) until its end of life in 2010. As part of the Earth Science Enterprise (ESE) program, ICESat’s main mission was to monitor glacier topography over time. This was performed using its primary instrument, the Geoscience Laser Altimeter System (GLAS). The three lasers within GLAS transmitted 4 nanosecond pulses at a frequency of 40 Hz. Infrared (1064 nm) pulses were used to measure glacier topography, whilst green (532 nm) pulses were used to make atmospheric measurements.

References

1) Petiteville, I., Lecomte, P., Ward, S., Dyke, G., Steventon, M., & Harry, J. (n.d.). (publication). Satellite Earth Observations in Support of Climate Information Challenges. Retrieved from https://www.eohandbook.com/cop21/index.html.

2) Haeberli, W. (2016). Glaciers. International Encyclopedia of Geography: People, the Earth, Environment and Technology, 1-16, URL: https://doi.org/10.1002/9781118786352.wbieg0335.pub2

3) Glaciers. National Snow and Ice Data Center. (n.d.). https://nsidc.org/learn/parts-cryosphere/glaciers.

4) Ice sheets. National Snow and Ice Data Center. (n.d). https://nsidc.org/learn/parts-cryosphere/ice-sheets.

5) Smith, E. (n.d.). The Anatomy of Glacial Ice Loss. NASA Climate Change. https://climate.nasa.gov/news/3038/the-anatomy-of-glacial-ice-loss/

6) Holt, T. O., Glasser, N. F., Quincey, D. J., & Siegfried, M. R. (2013). Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 7(3), 797-816.

7) Pfeffer, W. T., Arendt, A. A., Bliss, A., Bolch, T., Cogley, J. G., Gardner, A. S., ... & Randolph Consortium. (2014). The Randolph Glacier Inventory: a globally complete inventory of glaciers. Journal of glaciology, 60 (221), 537-552.