According to the National Snow and Ice Data Center in the United States, about 69% of freshwater supplies are stored in Earth’s ice sheets and glaciers. This means measuring water content in ice and glaciers is critical to understanding future changes to water supplies as well as sea level rise, where melting glaciers and ice sheets are the biggest contributing factor in this rise.
Modern satellites now allow us to make more accurate mass measurements to large areas covered by ice, which also allows us to better understand sea level change.
Measuring mass for ice sheets and glaciers is critical to understand future water supply changes to freshwater. Traditionally, this would be done by ground-based measurements at select sites that would then act as a sample for glaciers and ice sheets so that extrapolation models could be used to provide overall mass estimates.
Measuring the mass loss of glaciers and ice sheets is accomplished through gravimetry. Currently, and replacing traditional measures, the Gravity Recovery and Climate Experiment Follow On (GRACE-FO) satellites, which are joint satellites launched by NASA and the German Research Centre for Geosciences (GFZ) in 2018, are the best ways to measure ice mass change. This satellite system succeeds the GRACE satellites that operated between 2002-2017.
As the melting of ice occurs, land surface mass changes as the weight of ice is removed, leading to a decrease of mass, while increasing ice presses down on land and adds mass. The current GRACE-FO satellites measure changes in land mass using two satellites, with one trailing the other, whereby the lead satellite, as it orbits over a land mass, is pulled ahead by that land mass. If the land mass is relatively larger, then the lead satellite is pull further with that change used to measure the land mass the satellites pass over. The opposite occurs when the land mass is smaller or when the trailing satellite orbits over a larger land mass.
These variations in gravitational not only allow instruments to measure total land mass over an area but repeated runs allow variation over time to be measured. With several years of data, scientists have now been able to determine to what extent ice affects land mass and this has now led to estimates on ice loss and gain on overall land mass. In fact, the effect of melting ice has enabled scientists to determine how much sea level change is affected by melting ice, given changes to land mass, with an estimate now being that 1000 gigatons of land ice melting leads to about 2.8 millimeters change in sea level.
Results show that rapid melting of Greenland and Antarctica’s ice has greatly contributed to seal level rise in the last two decades. About 1.2 millimeters per year in sea level change have been because of melting ice in Greenland and Antarctica alone. On the other hand, melting ice from mountain glaciers has contributed about 8 millimeters of sea level change between 2002-2016.
While these instruments have greatly aided scientists, the fact is surface changes are complex, particularly as melting ice also causes a redistribution of Earth’s total land mass, leading to unequal changes across the planet. Overall, using GRACE data, scientists have estimated that there has been 2.14 ± 0.12 mm/yr sea level change between 2002-2014, while variation in seal level across the globe has meant −0.5 mm/yr in the Arctic to about 2.4 mm/yr for that time. In effect, this means that even with these measures some regions witnessed sea level decline, although overall the global average increase has been demonstrated to be substantial.
Given the complexity of change in sea level, scientists are also using laser altimetry to adjust their model data and attempt to calibrate better models that can estimate past sea level change as well as future sea level change based on changing land mass and melting ice.
We are now witnessing much better and more accurate ways to measure ice changes across the planet, particularly with its implications on sea level change. This has allowed not only more accurate predictions but it has also highlighted the rapid changes that have occurred over the last few decades, particularly in very warm years. The measurements indicate that the Earth’s surface is complex and changes are not uniform but better empirical data allow more accurate calibrated models to be developed so that future changes to ice and sea levels could be better understood as we plan for climate change in the coming years.
 For more on the earlier GRACE satellites and their use, see: Cazenave, A., Dominh, K., Guinehut, S., Berthier, E., Llovel, W., Ramillien, G., Ablain, M., Larnicol, G., 2009. Sea level budget over 2003–2008: A reevaluation from GRACE space gravimetry, satellite altimetry and Argo. Global and Planetary Change 65, 83–88. https://doi.org/10.1016/j.gloplacha.2008.10.004.
 For more on using gravimetry in measuring ice and sea level change, see: https://earthobservatory.nasa.gov/images/147437/taking-a-measure-of-sea-level-rise-gravimetry.
 For more on using GRACE to estimate sea level change, see: Jeon, T., Seo, K.-W., Youm, K., Chen, J., Wilson, C.R., 2018. Global sea level change signatures observed by GRACE satellite gravimetry. Sci Rep 8, 13519. https://doi.org/10.1038/s41598-018-31972-8.