Ice sheets are continental-scale masses of ice that rest on land. The shape of an ice sheet is a dome that slopes downwards toward its edges, where the interior ice may be thousands of meters thick and the ice around the edges tens of meters thick. Ice sheets are dynamic, flowing from the interior toward the ice sheet’s edge like honey poured onto a mound on a flat surface. The ice sheets serve as large reservoirs of frozen fresh water. This means that over time, when ice sheets gain mass, they contribute to a fall in global mean sea level, and when they lose mass, they contribute to a rise in global mean sea level. An ice sheet gains mass when snow falls on it, especially at high elevations within its interior. An ice sheet loses mass, however, due to various processes.
First, warmer temperatures at lower elevations along the ice sheet’s edges melt ice, and the resulting meltwater flows into the ocean. Secondly, if the ice sheet is large enough, ice can extend to the coasts and flow into the ocean, where it breaks and floats away as icebergs. In some cases, ice will remain intact as it flows into the ocean, staying connected to the ice sheet and creating what is known as an ice shelf. In either of these cases, ice that leaves the land to float in the ocean contributes to the rise of global mean sea level (in the same way that adding an ice cube will raise the water level in a glass). Lastly, the ice sheets are vulnerable at the locations where they connect with ice shelves (or the grounding line). Here, the grounded ice sits on land that is below sea level. Warming oceans can lead to an increase in melting of the ice shelves from underneath, eventually undercutting the grounded ice, causing it to float. This leads to an expansion of the ice-shelf area and to a rise in global mean sea level.
The Greenland Ice Sheet, the largest ice sheet of the Northern Hemisphere, holds enough water to raise global mean sea level by 7.4 meters, while the Antarctic Ice Sheet, the largest ice mass on Earth, sitting on the South Pole, has the potential to increase global mean sea level by 58 meters. To calculate how ice-sheet mass changes over time, scientists use three different types of measurements: gravimetry, altimetry, and the input-output method.
Mass change estimates from gravimetry (such as from twin satellites known as the Gravity Recovery and Climate Experiment, or GRACE) measure changes in Earth’s gravity field, which are caused by redistribution of mass on Earth. After removing what they know about how Earth’s shape changes below the ice sheets, scientists can calculate the change in gravity due just to change in ice mass. GRACE measurements tell us that from 2002 to 2017 the ice sheets have contributed one third of the total mean sea level rise (~1.17 +/- 0.17 mm yr -1).
Repeated satellite and airborne laser and radar altimetry provide detailed surface topography and surface height changes of the ice sheets. Scientists use these measurements of changes in surface topography to estimate changes in ice thickness and, combined with modeled estimates of the density of the snow that sits on top of the ice sheets, scientists can calculate how ice-sheet mass has changed through time at very high spatial resolution. NASA has been collecting laser altimetry measurements over the ice sheets since 1993 and continues to do so using ICESat-2 (launched in 2018). Along with European radar altimetry missions, such as ERS-1, ERS-2, and CryoSat-2 (launched in 2010), these have provided continuous monitoring of ice-sheet height changes.
Ice Velocity and surface climate:
The input-output method combines observations of ice-flow speed with modeled estimates of snow accumulation and melt. Together, these measurements tell us the balance between ice-sheet mass increase due to snow accumulation (input) and how much ice-sheet mass decreases due to melt and flow into the ocean (output). This is the only method that provides long-term estimates of ice-sheet mass change, since observations of ice velocity and surface climate are available over the last four decades.
How the ice sheets will change in the future depends on the atmosphere and ocean. For instance, a warmer atmosphere will result in more ice loss due to increased surface melt and melt runoff into the oceans. Furthermore, at the grounding line, the interior grounded ice is at risk of floating when warmer oceans begin to melt ice shelves at higher rates from below. In many places in West Antarctica, the ice sheet is grounded on land that sits well below sea level and slopes downward towards the ice-sheet interior.
This configuration, combined with warming oceans, promotes a runaway contribution to global mean sea level, as the ice sheet interior continues to unground in an unstable way. This phenomenon is known as the marine ice sheet instability (MISI). Scientists predict ice-sheet mass loss due to the above processes using numerical models to understand present ice-sheet behavior, then to project future contribution to global mean sea level. These models strongly rely on satellite observations to drive simulations of the past and to validate how well the models are able to reproduce past ice-sheet changes. Accurate reproduction of ice-sheet mass change requires precise measurements of the ice-sheet geometry, including the shape of the land that sits underneath the ice, an understanding of how ice may slide along the land, enhancing the speed of ice flow, and realistic estimates of the state of the ocean and atmosphere where they interface with the ice sheets.
With time, ice-sheet models are improving in their ability to reproduce the processes driving mass change, thanks to the increased availability of these key observations. Improvement in the observational coverage and accuracy over the ice sheets will continue to inform ice-sheet models and lead to increased confidence in future projections of sea level.