The massive ice sheet covering much of Greenland has expanded and contracted over hundreds of thousands of years. Today, its thickness reaches 2 miles, rendering it ice-covered, and if it were to melt entirely, global sea levels would rise by 20 feet. However, scientists now understand that in the distant past, the ice sheet contracted so much that it was nearly nonexistent.
For years, scientists have been concerned and have warned that complete or near-complete disintegration of the Greenland Ice Sheet could happen again if global temperatures rise too high. This would lead to a worldwide sea-level rise, further impacting coastal communities. It remains unclear exactly how warm it needs to get to cross this threshold.
The "zone of concern" has fluctuated by about 3.6 degrees Fahrenheit, or 2 degrees Celsius, above pre-industrial levels (Earth has already warmed by approximately 2 degrees Celsius or 1.1 degrees Celsius). Scientists have cautioned that if the ice sheet follows the path of disintegration, it is unlikely to recover.
A new study published in the journal Nature suggests that the Greenland Ice Sheet still has room to maneuver before it irreversibly shrinks. Even if climate change driven by human activity leads to global temperatures surpassing this 2°C threshold (which is quite likely), the Greenland Ice Sheet may avoid complete destruction if temperatures decrease relatively rapidly, according to the study.
"The Greenland Ice Sheet is more resilient than we thought," says Niels Bochow, a researcher at the Arctic University of Norway and the lead author of the article.
However, he emphasizes that resilience has clear boundaries. The likelihood of irreversible ice sheet disintegration within a few thousand years is nearly impossible to avoid if temperatures sharply exceed the 2°C threshold after 2100 or if they remain even slightly above that threshold for more than a few hundred years. But the opportunity to mitigate the damage exists. "If we lower the temperature within a certain time frame, we can prevent this abrupt loss," says Bochow.
There is a limited window to save the Greenland Ice Sheet
Since 2002, the Greenland Ice Sheet has contributed about 20% of the additional water that has led to the rise in global sea levels. "And that fraction is increasing over time," says Helen Seroussi, a glaciologist at Dartmouth College who was not involved in the study. What is even more concerning, in her view, is that "over the last few years, we have seen Greenland reacting faster than models predict."
The ice sheet will continue to discharge more and more water into the ocean in the coming decades as temperatures rise. This cold freshwater elevates coastal oceans and may contribute to other significant climate changes affecting the planet, such as the slowing down of some of the world's most crucial ocean currents.
Ice sheets are less forgiving than the atmosphere. If carbon emissions were to miraculously cease tomorrow, Earth's atmospheric temperature would likely stabilize or even cool relatively quickly. In contrast, the ice sheet would not stop melting right away.
In other words, the melting of the Greenland Ice Sheet observed today began a long time ago. Every year of additional warming locks in future melt.
The changes in the ice sheet caused by global warming, which affect it in a way that accelerates its disintegration even further, are causing significant concern. Scientists refer to this as a positive feedback loop.
The surface of the ice sheet, which is 2 miles high today, is elevated in the atmosphere, where it is colder, similar to the top of a mountain. As the ice melts, the entire surface becomes lower. This leads to contact with warmer air, just like warm air at the base of a mountain. This amplifies the warming effect. After a certain point, this process becomes too powerful to reverse, and it's these positive feedback loops that can lead to the near-total loss of ice.
A scientific group used two different ice sheet models and analyzed various scenarios of time and temperature to see how the ice would react. They raised the temperature by a bit, by quite a bit, and very significantly - by 6.5°C or roughly 12°F - above pre-industrial levels by the year 2100. When the temperature dropped again by approximately 1.5°C above pre-industrial levels after several centuries or even faster, the ice sheet remained on the dangerous thresholds of positive feedback.
"A hundred, two hundred years for the ice sheet is almost instant," says Bochow, because it reacts very slowly. Therefore, swift actions are of paramount importance.
"Overshoot" - when the temperature significantly exceeds global target levels and then decreases again - may become possible if people figure out how to extract carbon from the atmosphere and store it in a safe place. Whether this is achievable on a global scale remains far from certain, emphasized Bochow.
"The more the overshoot, the less time you have to react and start carbon removal, and the more challenging it becomes," says Seroussi.
But asking this question is important, even in the absence of critical technologies to implement it.
"It's a really interesting question," says Ginny Catania, a glaciologist from the University of Texas at Austin who was not involved in the research. "It seems obvious that we won't be able to achieve the [Paris Agreement] goal of keeping temperatures below 1.5°C. We probably won't reach the 2°C goal. So how do we get back from there?"
This study raises as many scientific questions as it provides answers. What is the exact threshold temperature? What processes within the ice sheet can affect its sensitivity? But the question that has the greatest impact is simultaneously the least defined.
"What people are going to do? That's the main player in how the climate will change in the future, and consequently, the ice sheet," says Catania.