The observed Arctic sea ice loss of recent decades has raised the question about the existence of a tipping point, a point of no return where sea ice is lost in a large, abrupt and irreversible event.
When forced with steadily increasing CO2 concentrations, complex climate models first show a linear decline in summer sea-ice coverage, in agreement with an observed linear relationship in the real world.
After the complete loss of summer sea ice, the simulated sea ice still forms each winter until the Arctic winter becomes too warm. This transition from a seasonally ice-covered Arctic to a perennially ice-free Arctic Ocean happens substantially faster than the preceding loss of summer sea ice. In a few models, the loss of winter sea ice occurs very abruptly after a long time of relatively little change. In the MPI-M Earth system model (MPI-ESM), an ice area of several million square kilometers disappears within only a few years. To remove a similar area of summer sea ice, a global warming of two degrees would be required, and the melting process would take decades. When shown as an animation, the decline of winter sea ice is therefore not simply a replay of the decline in summer sea ice from some decades before, but a completely different movie (Fig. 1).
Fig. 1. Evolution of sea ice cover (%) in September (left) and March (right) over time in MPI-ESM. While summer sea ice extent decreases gradually from the edge to the centre of the Arctic, winter sea ice coverage remains high until it is abruptly lost between the years 2123 and 2129 in the extended RCP8.5 simulation. To allow a good comparison, the evolution of ice coverage spans a similar range in both animations. Therefore, the time period is not identical on purpose, since winter ice reaches a certain extent much later than summer sea ice.
Interestingly, the abrupt sea-ice loss in MPI-ESM follows the prediction of much simpler column models. They resolve the annual cycle in the Arctic, but their column form allows no horizontal differences. These models show an abrupt loss of winter sea ice at a so-called bifurcation point. The bifurcation scenario suggests that the abrupt transition to an ice-free Arctic is irreversible and a result of large amplifying feedbacks such as the ice-albedo feedback or the convective cloud feedback. The ice-albedo feedback refers to the fact that ice loss and warming amplify each other because more sunlight is absorbed when the bright ice surface is replaced by the dark open water. The convective cloud feedback is associated with the formation of convective clouds above open water which then help to keep the ocean ice-free during the Arctic winter by enhancing the downwelling long-wave radiation.
How important are these feedbacks for the fast winter sea-ice loss in complex models? And is the bifurcation scenario of the simple models a proper analogue for the abrupt sea ice loss in complex climate models? These questions can be addressed by suppressing the ice-albedo feedback and convective cloud feedback in the model MPI-ESM. As a result, the sea-ice loss is still abrupt even without these feedbacks. While cloud feedbacks have very little impact on the result, disabling the positive ice-albedo feedback delays the loss of sea ice. However, the loss of summer sea ice then occurs even more slowly, while winter sea ice still disappears at a very sharp transition point. Hence, neither the ice-albedo feedback nor the radiative cloud feedback can explain the different sensitivity of summer and winter sea ice.
Instead, the large sensitivity of the winter sea-ice area is caused by the asymmetry between melting and freezing. An ice-free summer requires the complete melt of even the thickest sea ice, which is why the ice coverage decreases only gradually as more and more of the thinner ice melts away. In the case of seasonal ice, however, sea-ice areal coverage remains high as long as sea ice still forms in winter, and then drops to zero wherever the ocean warms sufficiently to no longer form ice at any time of the year. As sea-surface temperature in the winterly Arctic Ocean is relatively homogeneous, this can result in a rapid reduction of ice area within a short time span. Hence, the situation resembles the behaviour of a lake that is suddenly covered with a thin film of ice after one cold night. If the next night is slightly warmer, the lake rapidly loses a substantial areal coverage of ice. Hence, the freezing temperature acts as a natural threshold which is independent of radiative feedbacks. As this process is very fundamental, it occurs in every analysed model independent of any specific parameterisation and is therefore likely to be relevant in the real world. The threshold mechanism suggests that rapid winter ice loss is indeed a possibility, but that this loss is still a reversible phenomenon.