&Bullet; physics 14, 80
A stability analysis of the ice sheet that covers a large area of Greenland suggests that the ice melt is approaching a point of no going back.
The Greenland ice sheet – an important part of the earth’s climate system – is losing mass faster and faster. One major concern is that the ice sheet could be heading for a tipping point beyond which the ice cap would be permanently lost. A new study published in the Proceedings of the National Academy of Science suggests that much of the Greenland ice sheet could be close to such a tipping point. The complete melting of the Greenland ice cap could lead to a sea level rise of up to 7 m and trigger a cascade of other climate instabilities.
Climate tipping points are threshold values beyond which large, sudden changes in the earth’s climate system occur. Paleogeological records have shown that such tipping occurred in the past – examples include the desertification of the Sahara and abrupt temperature changes during the last ice age. However, current climate models, such as those used by the Intergovernmental Panel on Climate Change, have difficulty explaining past climate changes, which calls into question the ability of these models to predict future tipping.
Theories of nonlinear dynamics, which offer powerful tools for describing the development of natural systems over time, offer a promising approach. Previous studies based on nonlinear dynamics suggested that there may be precursor signs of an approaching tipping point associated with a slowdown in the dynamic behavior of the system.
The physicist Niklas Boers from the Potsdam Institute for Climate Impact Research in Germany and the mathematician Martin Rypdal from the University of Tromsø – Arctic University of Norway have now found these “critical slowdown signals” for the Jakobshavn Glacier – the fastest melting of the Greenland ice basins. The duo achieved their result by applying nonlinear dynamics theory to analyze 150 years of data on glacier melting rates derived from ice core samples.
Their study focuses on one of the critical elements that determine the dynamics of the Greenland ice sheet: a feedback mechanism called melt height feedback that links melting with ice elevation. Melting lowers the height of the ice cover, exposing the ice to the warmer air at lower elevations, which in turn accelerates the melting process.
Boers says a key benefit of using sheet heights rather than melt rates as a model parameter is that height is a “state variable” – similar to temperature or pressure. Therefore, the sheet height provides a clearer indication of whether the system is possibly near the phase transition associated with climate tilting, he says. Indeed, the history of the leaf height showed signs of critical slowdown.
A critical slowdown occurs when a system approaches a bifurcation that makes its current state of equilibrium unstable. Under such conditions, the system takes longer to return to equilibrium after a disturbance than if the system were in a stable state. A critical slowdown can be indicated by two parameters that describe the fluctuations of the system. The variance is the size of the fluctuations around equilibrium and increases near the bifurcation as the forces driving the system towards equilibrium become weaker. Autocorrelation quantifies how similar the state of the system at a given point in time is to the state at an earlier point in time. For a system on the way to a tipping point, a slower recovery means that this similarity increases, resulting in a large autocorrelation value.
For the Jakobshavn Glacier, the researchers found that the autocorrelation is approaching unity and the variance is diverging – both point to an approaching turning point. The researchers acknowledge that further research is needed to determine whether a tipping point has already been reached or is near. In particular, other feedback mechanisms must be taken into account, including both destabilizing effects – in particular the decrease in albedo due to the loss of the ice cover – and stabilizing effects – such as the increase in snowfall due to warming.
Alexander Robinson, a climate scientist and glaciologist at Complutense University of Madrid, says these results will help with future modeling. And although the results do not necessarily apply to the entire ice sheet of Greenland, he considers the study of the Jakobshavn glacier region to be particularly important. “You could call it the ‘weak underbelly’ of the Greenland ice sheet,” says Robinson. “It will probably be the warmest here and [ice] Retreat. So it could be the place to see changes as early as possible. “
“The results are sobering, but the way they did it [obtain them] is impressive, ”says climate scientist Tim Lenton at the University of Exeter, UK. While the ice sheet is a sluggish system that changes on very slow timescales, the fact that researchers were able to extract signatures of the slowed fluctuations from observational data is “a real pleasant surprise”. Lenton says findings such as those by Boers and Rypdal support previous calls for the development of tools to anticipate the effects of climate change (see, for example, the 2013 National Academy of Sciences report). “With relatively modest investments, we could develop an early warning signal for a number of climate tipping points,” says Lenton.
Richard Blaustein is a freelance science and environmental journalist based in Washington, DC.