Onboard a low flyover of Jakobshavn Isbræ in Greenland, I saw the glacier's ice front extend completely to the horizon. After several years studying images of this gigantic ice body on my computer screen, I was floored by its sheer scale and impressive dynamics -- and how they manifested in a complex network of crevasses. Despite the stunning view, I was looking down on an ice sheet that is currently the largest single contributor to rising sea levels, now estimated to add up to over 3 inches to global sea levels by 2100.

"I was looking down on an ice sheet that is currently the largest single contributor to rising sea levels, now estimated to add up to over 3 inches to global sea levels by 2100."

The Greenland Ice Sheet is changing rapidly. More than three decades of NASA satellite data reveals that large outlet glaciers in Greenland are melting, retreating and draining at accelerated rates. The result is an imbalanced state where sustained mass loss will persist, even if surface melt decelerates.

What does this mean? It means that losing mass is the new normal for the ice sheet. Glaciers are draining greater volumes of ice at the edge, while record amounts of melt are being observed on the ice sheet's surface. This means that even with wintertime snow accumulation, it is very unlikely the Greenland Ice Sheet will gain mass.

"...record amounts of melt are being observed on the ice sheet's surface. This means that even with wintertime snow accumulation, it is very unlikely the Greenland Ice Sheet will gain mass."

In a study I led at the Byrd Polar and Climate Research Center during 2018 through 2020, we referenced three and a half decades of satellite data to measure changes in ice thickness and flow speeds. We found the volume of ice that drained from glaciers increased by about 14% since the 1980's. We also found that this increase was not a steady rise, but rather it largely occurred during the years 2000-2005.

"...even if the air and ocean were to stop warming today, the ice sheet will continue to melt faster than it will be replenished."

The magnitude of the step increase is significant because of its implications for total ice sheet mass balance. As long as the amount of ice loss, called "ice discharge", is matched by an equal mass of snow accumulated on the ice sheet's surface, the ice sheet stays in balance. This was the case prior to 2000. Due to retreat after 2000, however, glaciers are flowing faster on average, which means interior ice is draining more rapidly than surface mass is accumulated. Therefore, a return to mass balance would now require even more snow to accumulate on the ice sheet than what we observed during "healthy" years prior to 2000, which is unlikely given current warming trends and increased surface melt. Outlet glaciers can retreat very quickly, but could take centuries to regrow, which means we would need to sustain cooler climate conditions in order for these glaciers to return to their previous stable conditions. In other words, even if the air and ocean were to stop warming today, the ice sheet will continue to melt faster than it will be replenished.

"Greenland's persistent mass loss over the 21st century ... has been well-documented in scientific literature ... How rapidly the ice sheet loses mass, however, is dependent on rates of atmospheric and oceanic warming and intrinsically linked to greenhouse gases in the earth's atmosphere."

"Although we expect the ice sheet to continue to lose mass in the coming decades, the rate of future mass loss is within our control."

Greenland's persistent mass loss over the 21st century comes as no surprise and has been well-documented in scientific literature. What our study showed is that the impact of widespread glacier retreat played an integral role in nudging the ice sheet to an imbalanced state. Modeling work of large glaciers suggests ice discharge will remain a significant contributor to mass loss in the coming centuries. How rapidly the ice sheet loses mass, however, is dependent on rates of atmospheric and oceanic warming and intrinsically linked to greenhouse gases in the earth's atmosphere.

The quicker we halt increases in air and ocean temperatures, the quicker we can begin restabilizing the ice sheet. Although we expect the ice sheet to continue to lose mass in the coming decades, the rate of future mass loss is within our control. As is true for many important components of our climate system, our actions now are determining how the ice sheet behaves decades and even centuries into the future.