The ice in Greenland is melting rapidly. Now new data suggests that parts of the Greenland ice sheet may already be approaching a tipping point. Early warning signs for this include increased fluctuations in the glacier dynamics and positive feedback that accelerate thawing. In particular, the central-western part of the Greenland Ice could approach a critical temperature threshold, as the researchers report. However, whether and how this will affect the ice sheet as a whole is still unclear because of the complex interactions and influencing factors.
The Greenland ice sheet is the second largest ice reservoir on our planet and at the same time a sensitive actuator in the global climate system. Because the glaciers on this giant island are already reacting to climate change. Because the air has warmed up disproportionately over the last few decades, especially over the Arctic, the Greenland ice is melting faster and faster. Climate models suggest that there is a critical temperature limit above which the thawing of this ice sheet can hardly be stopped. Then the entire ice sheet could melt completely over hundreds or thousands of years, leading to a global sea level rise of more than seven meters and a collapse of the Atlantic meridional overturning circulation (AMOC). According to previous knowledge, this tipping point should be in the range of 0.8 to 3.2 degrees of warming compared to the pre-industrial level.
Fluctuations as an early warning sign
This begs the question of how close the Greenland Ice Sheet is to this tipping point. Because the global mean, the warming has already exceeded the one-degree mark. Changes in large-scale air currents also mean that Greenland is increasingly exposed to strong sunlight and warm air masses in summer. As a result, glacier melt has accelerated in recent years, especially in the south and west of Greenland. “The non-linear increase in the melting rates of the Greenland ice sheet and the meltwater runoff indicate that the critical temperature threshold could be closer than previously thought,” state Niklas Boers from the Potsdam Institute for Climate Impact Research (PIK) and his co-author Martin Rypdal from the Arctic University of Norway in Tromsø.
In order to gain more clarity about how close the Greenland Ice Sheet is to the tipping point, they looked for certain early warning signs in the data from ice cores, temperature measurements and with the help of computer models. These are shown, among other things, in increasing fluctuations in ice loss and ice growth and in an increasing decoupling of these fluctuations from the long-term average. “The warning signs are caused by characteristic changes in the dynamics of the Greenland ice sheet, which reflect how well the ice sheet can defend itself against disturbances and recover from them,” explains Rypdal. Such early warning signs can be seen, for example, in ice core layers from the last interglacial periods: They usually occurred shortly before an abrupt change in ice and climatic conditions. Boers and Rypdal have now investigated whether these statically identifiable early warning signs are showing up again today.
First signs of destabilization
The evaluations showed that there are already such early warning signs for at least some parts of the Greenland ice sheet. “We have found evidence that the central-western part of the Greenland Ice Sheet has destabilized,” reports Boers. According to this, the ice heights in this part of Greenland are already showing increasing fluctuations, the pattern of which corresponds to the typical signs of destabilization. The researchers conclude that the central-western part of the Greenland ice sheet could experience a critical transition relatively soon. “Our results suggest that there will be a significant increase in meltdown in the future – which is very worrying,” says Boers. This is because the melting of the Greenland ice contributes to a large extent to the rise in global sea levels.
As the scientists explain, positive feedback, known as melt elevation feedback, could play a key role in the stability of the Greenland ice sheet. This is due to the fact that it is colder in high areas than in lower areas. When the glaciers and ice fields lose more and more height due to the thawing, their surface sinks into warmer layers of air. This in turn intensifies the defrosting and leads to a further loss of height – a vicious circle begins. “This mechanism has long been known and it is one of the main suspects for the identified destabilization of the central-western parts of the Greenland ice sheet,” explains Boers. But whether and how this will affect the ice sheet as a whole remains unclear, because a number of other positive and negative feedbacks influence the behavior of the ice masses. For example, the increasing warming of the atmosphere can cause the air to absorb more moisture, which then falls over Greenland as snow. That would at least partially compensate for the loss of ice.
“We urgently need to better understand the interplay between the various positive and negative feedback mechanisms that determine the current stability and future development of the ice sheet,” says Boers. “In view of the signs we are discovering in ice cores from the central-western part, we need to collect more observations and improve our understanding of the mechanisms involved so that we can make more reliable estimates of the future development of the Greenland ice sheet,” adds Rypdal. However, both researchers emphasize that in any case it is urgently necessary to reduce greenhouse gas emissions as quickly as possible. Because of the extremely slow reaction time of the ice masses, it will take a long time anyway before the ice melt that has already started slows down again or comes to a standstill.
Source: Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2024192118