Greenland may have already committed us to nearly a foot of sea level elevation

Enlarge / View of a survey flight over the Helheim/Kangerlussuaq area in Greenland. NASA/John Sonntag

While it is possible to halt global warming by halting our greenhouse gas emissions, sea level rise is a consequence that keeps happening. Large ice caps like Greenland and Antarctica have enormous inertia - they are slow to melt but continue to melt even after the thermometer stabilizes. There are several reasons for this, including complex processes beneath glaciers that control their downward flow rate. And this complexity makes projecting ice loss over the next century, and centuriess, extremely difficult.

Faced with this formidable complexity, a new study by Jason Box of the Geological Survey of Denmark and Greenland takes a simple approach to projecting the future of Greenland. Rather than trying to simulate as much physics and detail as possible in a model, the team used a simple equation to calculate how much of the ice is vulnerable in the current climate.

find the line

Glacial ice warps under its own weight, flowing out and down like pancake batter in slow motion. The lower elevation and - for an ice sheet like Greenland's - the coastal environment at the edge of the ice is much warmer, and the ice is melting here even as snow accumulates on the colder interior of the ice sheet glacial. The point where the net result changes from gaining mass to losing mass is called the "balance line". Raise the air temperature, and this line will rise to higher elevations, exposing more ice to melt until the glacier shrinks.

The new study maps the average equilibrium line from 2000 to 2019, based on data from NASA's Terra satellite. (You can identify this because the snow cover above the balance line is more reflective than the bare ice below.) Given this position of the balance line, the researchers then apply the geometry of glacial ice to estimate the volume of ice that would disappear to bring the sheet ice into equilibrium with its new climate.

It's scrambling over a ton of critical processes, like meltwater lubricating the base of the ice or ice flow altering the part of the ice sheet that comes in contact with a warming ocean. A consequence of this is that the study cannot give any time frame for its predicted ice loss. Their method assumes that enough time passes for the ice cap to finish shrinking.

The result, they say, is that the Greenland Ice Sheet would lose about 3.3% of its ice, enough to raise global sea levels by about 27 centimeters (nearly 11 inches). This is the long-term warming commitment so far. Since we are no closer to eliminating greenhouse gas emissions, the planet continues to warm, which would drive this number up.

What's in a number?

Is this number shocking? It depends on how much time you give it. The latest IPCC report predicted about 5 to 18 centimeters (2 to 7 inches) of sea level contribution from Greenland by 2100, depending on the greenhouse gas emissions scenario. But he also laid out long-term commitments on sea level. For a world warming only 2°C, for example, total sea level rise reached around 50cm in 2100, but two to six meters when given 2,000 years to react.

Some authors of the new study were quoted saying they suspected their estimated contribution to sea level could show up by 2100 or 2150, but that is their personal judgment and not a conclusion of this study particular. Ice sheet responses are usually discussed in terms of centuries or millennia, depending on the scale, so 2100 would be a very quick timeline.

Another thing to note is that the analysis only had 20 years of data to work with. Greenland experiences great variability in its climate, and ice loss accelerated dramatically in the early 2000s. To illustrate the impact of this variability, the researchers repeat their calculations using the warmest years (2012) and coldest (2018) in the dataset.

Under steady-state 2012 conditions, the committed sea level contribution increases almost by a factor of three to 78 centimeters. But under steady-state conditions in 2018, Greenland's ice would expand, lowering sea levels by 17 centimeters...