The Transient versus the Equilibrium Response of Sea Ice to Global Warming

Abstract

To examine the long-term stability of Arctic and Antarctic sea ice, idealized simulations are carried out with the climate model ECHAM5/Max Planck Institute Ocean Model (MPI-OM). Atmospheric CO2 concentration is increased over 2000 years from preindustrial levels to quadrupling, is then kept constant for 5940 years, is afterward decreased over 2000 years to preindustrial levels, and is finally kept constant for 3940 years. Despite these very slow changes, the sea ice response significantly lags behind the CO2 concentration change. This lag, which is caused by the ocean's thermal inertia, implies that the sea ice equilibrium response to increasing CO2 concentration is substantially underestimated by transient simulations. The sea ice response to CO2 concentration change is not truly hysteretic and is in principle reversible. The authors find no lag in the evolution of Arctic sea ice relative to changes in annual-mean Northern Hemisphere surface temperature. The summer sea ice cover changes linearly with respect to both CO2 concentration and temperature, while the Arctic winter sea ice cover shows a rapid transition to a very low sea ice coverage. This rapid transition of winter sea ice is associated with a sharply enhanced ice–albedo feedback and a sudden onset of convective-cloud feedback in the Arctic. The Antarctic sea ice cover retreats continuously without any rapid transition during the warming. Compared to Arctic sea ice, Antarctic sea ice shows a much more strongly lagged response to changes in CO2 concentration. It even lags behind the surface temperature change, which is caused by a different response of ocean deep convection during the warming and the cooling periods.

Statistical indicators of Arctic sea-ice stability – prospects and limitations

“…As the description of the ice-thickness distribution is similar in E07 and MPI-ESM, the abrupt winter sea-ice loss probably re- sults from the same threshold mechanism . This is corroborated by the fact that the abrupt loss is reversible in MPI-ESM (Li et al, 2013).…”

“…A further similarity with E07 is that an abrupt loss of winter sea-ice area occurs due to the simple representation of the model's icethickness distribution and the homogeneity of the Arctic Ocean . The abrupt ice loss is reversible (Li et al, 2013) and is caused by the same threshold effect as in E07 .…”

“…In particular, we analyse a simulation from MPI-ESM where CO 2 increases linearly until it has quadrupled after 2000 years. This simulation has been performed and reported by Li et al (2013). We perform similar simulations with E07, using different experiment lengths and 1000 realisations per experiment.…”

“…In the extreme case, the tipping point would correspond to a bifurcation point, a point of no return where sea ice is suddenly and irreversibly lost. While this "small ice-cap instability" occurred in simplified models (North, 1984;Thorndike, 1992;Eisenman and Wettlaufer, 2009;Abbot et al, 2011), more comprehensive models show a more gradual and reversible sea-ice loss in scenarios of the future (Armour et al, 2011;Tietsche et al, 2011;Boucher et al, 2012;Ridley et al, 2012;Li et al, 2013). Consequently, Wagner and Eisenman (2015a) recently showed in detail how resolving the seasonal cycle and latitudinal differences can eliminate bifurcations in sea-ice models, explaining why oversimplified models lead to wrong conclusions.…”

Arctic 2.0: What happens after all the ice goes?

“…Notz and his colleagues studied the interplay between greenhouse gases and ocean temperature with a global climate model 8 . They increased CO 2 from pre-industrial concentrations of 280 p.p.m.…”