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Just a matter of time

Greenland may have already committed us to almost a foot of sea level rise

How long that takes to play out is unclear.

Scott K. Johnson – | 261
View from a survey flight over the Helheim/Kangerlussuaq region of Greenland. Credit: NASA/John Sonntag
View from a survey flight over the Helheim/Kangerlussuaq region of Greenland. Credit: NASA/John Sonntag

While it's possible to halt global warming by halting our greenhouse gas emissions, sea level rise is a consequence that keeps on giving. Great ice sheets like Greenland and Antarctica have tremendous inertia—they're slow to melt but carry on melting even after the thermometer stabilizes. There are many reasons for this, including complex processes beneath glaciers that control their rate of downhill flow. And this complexity makes projecting ice loss over the coming century—and centuries—exceptionally challenging.

In the face of this formidable complexity, a new study led by Jason Box at the Geological Survey of Denmark and Greenland opts for a simple approach to projecting Greenland's future. Rather than attempting to simulate as much physics and detail as possible in a model, the team used a simple equation to calculate what portion of the ice is vulnerable in the current climate.

Finding the line

Glacial ice deforms under its own weight, flowing outward and downhill like slow-motion pancake batter. The low elevation and—for an ice sheet like Greenland's—coastal environment at the ice's edge is much warmer, and ice melts away here even as snow accumulates over the colder interior of the ice sheet. The point where the net result changes from gaining mass to losing mass is called the "equilibrium line." Increase air temperatures, and this line will push up to higher elevations, exposing more ice to melting 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 it because snow cover above the equilibrium line is more reflective than bare ice below it.) Given that equilibrium line position, the researchers then apply the geometry of glacial ice to estimate the volume of ice that would disappear to bring the ice sheet into balance with its new climate.

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That blurs over a ton of critical processes, like meltwater lubricating the base of the ice or ice flow changing the portion of the ice sheet that contacts a warming ocean. One consequence of this is that the study can give no time frame for its predicted ice loss. Their method assumes that enough time passes until the ice sheet finishes shrinking.

The result, they say, is that the Greenland ice sheet would lose about 3.3 percent of its ice—enough to raise the global sea level about 27 centimeters (almost 11 inches). That's the long-term commitment of warming so far. Since we aren't close to eliminating greenhouse gas emissions, the world is continuing to warm, which would push this number higher.

What’s in a number?

Is this number shocking? That depends on the time frame you assign to it. The latest IPCC report projected about 5–18 centimeters (2-7 inches) of sea level contribution from Greenland by 2100, depending on the greenhouse gas emissions scenario. But it also presented long-term sea level commitments. For a world that only warms 2° C, for example, total sea level rise came to about 50 centimeters for 2100—but two to six meters when given 2,000 years to respond.

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

One more thing worth noting is that the analysis only had 20 years of data to work with. Greenland sees a lot of variability in its weather, and ice loss accelerated considerably in the early 2000s. To illustrate the impact of this variability, the researchers repeat their calculations using the dataset's warmest (2012) and coolest (2018) years.

In permanent 2012 conditions, the committed sea level contribution grows almost by a factor of three to 78 centimeters. But in permanent 2018 conditions, Greenland ice would actually grow, lowering sea level by 17 centimeters. Both are unlikely extremes, but they highlight the importance of basing the calculation on a representative average.

In the end, this is another study adding to our understanding of future sea level rise. And applying multiple independent methods helps firm up that understanding, even if no individual answer is the ultimate one. This latest analysis certainly reminds us of a well-worn truth—that warming now commits us to sea level rise long into the future.

Open Access at Nature Climate Change, 2022. DOI: 10.1038/s41558-022-01441-2 (About DOIs).

Listing image: NASA/John Sonntag

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Scott K. Johnson Associate Writer
Scott has written about geoscience and energy at Ars as a freelancer since 2011.
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