Fluctuations in the masses of the world鈥檚 largest ice sheets carry important consequences for future sea level rise, but understanding the complicated interplay of atmospheric conditions, snowfall input and melting processes has never been easy to measure due to the sheer size and remoteness inherent to glacial landscapes.
Much has changed for the better in the past decade, according to a new review paper co-authored by researchers at CU Boulder, NASA, Utrecht University and Delft University of Technology and in the听Reviews of Geophysics.听
The study outlines improvements in satellite imaging and remote sensing equipment that have allowed scientists to measure ice mass in greater detail than ever before.
鈥淲e鈥檝e come a long way in the last 10 years from an observational perspective,鈥� said Jan Lenaerts, lead author of the research and an assistant professor in CU Boulder鈥檚听Department of Atmospheric and Oceanic Sciences (ATOC). 鈥淜nowing what happens to ice sheets in terms of mass in, mass out allows us to better connect climate variations to ice mass and how much the mass has changed over time.鈥�
Ice sheets primarily gain mass from precipitation and lose it due to solid ice discharge and runoff of melt water. Precipitation and runoff, along with other surface processes, collectively determine the surface mass balance. The Antarctic Ice Sheet, the world鈥檚 largest, is cold year-round with only marginal summer melting. A small increase or decrease in yearly snowfall, then, can make a considerable difference in surface mass because the addition or subtraction is compounded over a massive area.听
鈥淪nowfall is dominant over Antarctica and will stay that way for the next few decades,鈥� Lenaerts said. 鈥淎nd we鈥檝e seen that as the atmosphere warms due to climate change, that leads to more snowfall, which somewhat mitigates the loss of ice sheet mass there. Greenland, by contrast, experiences abundant summer melt, which controls much of its present and future ice loss.鈥�
In years past, climate models would have been unable to render the subtleties of snowfall in such a remote area. Now, thanks to automated weather stations, airborne sensors and Earth-orbit satellites such as NASA鈥檚 Gravity Recovery and Climate Experiment (GRACE) mission, these models have been improved considerably. They produce realistic ice sheet surface mass balance, allow for greater spatial precision and account for regional variation as well as wind-driven snow redistribution鈥攁 degree of detail that would have been unheard of as recently as the early 2000s.
鈥淚f you don鈥檛 have the input variable right, you start off on the wrong foot,鈥� Lenaerts said. 鈥淲e鈥檝e focused on snowfall because it heavily influences the ice sheet鈥檚 fate. Airborne observations and satellites have been instrumental in giving a better view of all these processes.鈥�
Ground-based radar systems and ice core samples provide a useful historical archive, allowing scientists to go back in time and observe changes in the ice sheet over long periods of time. But while current technologies allow for greater spatial monitoring, they lack the ability to measure snow density, which is a crucial variable to translate these measurements into mass changes.
The biggest opportunity may lie in cosmic ray counters, which measure surface mass balance directly by measuring neutrons produced by cosmic ray collisions in Earth鈥檚 atmosphere, which linger in water and can be read by a sensor. Over long periods of time, an array of these devices could theoretically provide even greater detail still.
Overall, Lenaerts said, the field of ice sheet observation has come of age in recent years, but still stands to benefit from additional resources.
鈥淭he community of researchers studying these issues is still relatively small, but it鈥檚 already a global community and interest is growing,鈥� he said. 鈥淲e鈥檇 like to get to a point where ice sheet mass processes are factored into global climate and Earth system models, to really show that bigger picture.鈥�
The newly published paper is part of the Grand Challenges special collection created for American Geophysical Union鈥檚 Centennial, highlighting key areas where major future work and discovery are needed to address fundamental questions in understanding the Earth, its space environment and the history of the planet and its solar system. It was co-authored by Brooke Medley of NASA Cryospheric Sciences Laboratory, Michiel van den Broeke of Utrecht University and Bert Wouters of Utrecht University and Delft University of Technology. NASA provided funding for the study.
