Statistical warning signs suggest that Greenland’s ice cap will soon be irreversibly shrinking. The annual fluctuations in the ice melt in West Greenland showed signs of a phenomenon known as “critical slowing down”, report Niklas Boers from the Potsdam Institute for Climate Impact Research (PIK) and Martin Rypdal from the Arctic University of Norway in Tromsø. This statistical peculiarity indicates that a previously stable state is now unstable and could soon change into a new stable state; Experts also expect such behavior at the tipping points of the climate system. The two researchers interpret this as an indication that the Greenland ice sheet is approaching a climatic tipping point, from which it enters a vicious circle and melts irreversibly.
The cause of the collapse is a feedback between the height of the ice cap and the loss of ice through melting. Below a critical temperature, a thin sheet of ice loses less mass because it no longer flows as quickly; conversely, a thick ice sheet is thinned again by its own weight. This means that the ice cap has a stable height over the long term. However, if the temperature rises above a critical value, the melt on the surface of the ice cap begins to determine the outflow. And since it gets warmer and warmer at lower altitudes, the ice sheet melts even faster the thinner it becomes – a vicious circle that only ends when it melts completely.
As the two researchers report in the journal »PNAS«, the development of ice melt over time offers an insight into the state of this system even before this vicious circle begins – if you know how to interpret the signs. According to Boers and Rypdal, Greenland’s ice cap is practically right at the transition between a stable state and a vicious circle. The variance and autocorrelation of the data series have increased significantly since the middle of the 20th century. And these two statistical parameters serve as warning signs.
The statistics of doom
The variance describes how large the bandwidth of the measured values is – i.e. how much the system fluctuates. The autocorrelation, in turn, says how similar the system is from one point in time to the next. If both values rise, then the system deviates more and more from the historical norm and at the same time takes longer and longer to “recover” from such deviations. That is the essence of »critical slowing down«: When a system approaches a critical transition from one stable system to another, it reacts more and more slowly.
In climate research, such critical transitions are called tipping points. When they are reached, the system – for example Greenland’s ice cap – no longer returns to its normal state after a disruption, but “tips” into a new, stable state. In this case, a completely ice-free Greenland and about seven meters higher sea level. The result fits in with independent predictions based on climate models, according to which the Greenland ice cap already reaches a critical point when the temperature is below three degrees above the reference period. It is unclear exactly where this tipping point is; according to models, it could be as far as 1.6 degrees.
The analyzes by Boers and Rypdal now indicate that Greenland is close to irreversible ice loss. Such statistical patterns, originally predicted theoretically, emerge before critical transitions in many different systems. Biological and technical instabilities are on the horizon as well as climatic ones. Experts have already found comparable signals that indicate “critical slowing down” in connection with other abrupt climatic fluctuations, for example the Dansgaard-Oeschger events during the last ice age.
Once the ice has started losing, it could even happen relatively quickly. According to simulations, assuming the pessimistic emission scenario RCP8.5, the ice would have completely disappeared by the year 3000. However, these statements are still fraught with great uncertainty, because the behavior of the ice caps is largely puzzling. That is why the researchers do not dare to predict how close you really are to the tipping point and when the vicious circle will noticeably begin to take hold. It is also unknown whether the ice would actually melt completely – or whether there is another stable state of equilibrium with a thinner ice. For example, warmer, more humid air could bring more snowfall to make up for the loss of ice. At least if the global temperature doesn’t rise too much.
This article was originally published by spektrum.de
Article source: spektrum.de
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