Brown-affiliated studies shed light on overestimations in the impact of Greenland Ice Sheet melting
The Greenland Ice Sheet, the second-largest ice sheet on Earth, is experiencing its fastest melting rate in 12,000 years, primarily due to rising surface temperatures caused by human-induced climate change, according to a 2020 study. This rapid melting contributes significantly to global sea level rise and poses a threat to major ocean circulation systems, as reported by various sources.
However, two recent articles co-authored by Brown Professor of Environmental Studies Laurence Smith suggest that current measurements and simulations may not fully account for the processes that retain meltwater on the Ice Sheet, leading to overestimations of meltwater runoff. While current climate models are generally accurate, they often overlook the refreezing or ponding of meltwater, which can result in significant corrections of up to 15%.
Smith and his team conducted fieldwork in Greenland's southwestern region, focusing on the ablation zone, where more snow is lost than accumulated annually. Matthew Cooper, a climate researcher and lead author on one of the papers, explained that the seasonal snowpack melts completely in this zone, exposing dark, bare ice that absorbs more sunlight, driving further melting.
The researchers discovered that meltwater refreezing in the porous ice surface is a significant cause of inaccuracies in climate models. They measured the flow rate of a river draining meltwater on the Ice Sheet's surface using an instrument that beams sound waves into the water to compute its speed and depth. By drilling small holes along the watershed perimeter and inserting bamboo stakes, they quantified the volume of ice lost, revealing that meltwater was being retained.
Through computational modeling of the bare ice surface, the study demonstrated that meltwater produced during the day fills the pore spaces of the top ice layers. At night, temperatures drop, causing the meltwater to refreeze. In subsequent days, heat is directed towards re-melting this ice, rather than generating new runoff. The study projected that the mass of meltwater retained and refrozen within porous bare ice amounts to approximately 11 to 17 gigatons per year, equivalent to 9% to 15% of the modeled annual runoff from that sector.
In flatter areas of the ablation zone, meltwater tends to pond, initiating a positive feedback loop that warms the Ice Sheet and accelerates long-term melting. In a second study, Smith and his team investigated the warming effect caused by these ponds. Ice sheets typically have high albedo (reflectivity) due to their white color. However, when air temperatures rise and ice melts and ponds, the Ice Sheet becomes darker, absorbing more solar radiation and melting faster.
To address the limitation of satellites in capturing smaller surface meltwater reservoirs, the team used drones to take pictures of the Ice Sheet at various GPS points. By overlapping satellite maps of water with albedo maps, they quantified the heating of the newly documented meltwater. According to the study, meltwater ponding accounted for around 1% of the total heating from sunlight across the Greenland Ice Sheet in the summer of 2019, but this proportion can vary significantly depending on the season, elevation, and size of the area.
The fieldwork supporting these studies is demanding, but it offers rewarding experiences. The researchers camp in the melt zone, with water flowing everywhere, requiring them to mark off safe perimeters due to cracks and slippery conditions. This challenging environment fosters a high level of performance and teamwork, making it an exciting and fulfilling experience for those involved.
