We covered this topic before on WUWT, but it showed up again in this week’s AGU highlights. Given the attention to the recent Arctic sea ice low and quick turnaround, I thought it would be appropriate to mention again.
The Arctic has been losing sea ice as Earth’s climate warms, and some studies have suggested that the Arctic could reach a tipping point, beyond which ice would not recover even if global temperatures cool down again. However, a new study by Armour et al. using a state-of-the-art atmosphere-ocean global climate model finds no evidence of such irreversibility. In their simulations, the researchers increase atmospheric carbon dioxide levels until Arctic sea ice disappears year-round and then watch what happens as global temperatures are brought back down. They find that sea ice steadily recovers as global temperatures drop. An implication of this result is that future sea ice loss will occur only as long as global temperatures continue to rise.
Source: Geophysical Research Letters, doi:10.1029/2011GL048739, 2011 http://dx.doi.org/10.1029/2011GL048739
Title: The reversibility of sea ice loss in a state-of-the-art climate model
Authors: K. C. Armour: Department of Physics, University of Washington, Seattle, Washington, USA;
I. Eisenman: Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA, and Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA;
E. Blanchard-Wrigglesworth, K. E. McCusker, and C. M. Bitz: Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA.
- Sea ice loss is reversible within a state-of-the-art global climate model
- We find no evidence of threshold behavior in summer or winter sea ice cover
- Rapid sea ice retreat does not imply irreversibility
Rapid Arctic sea ice retreat has fueled speculation about the possibility of threshold (or ‘tipping point’) behavior and irreversible loss of the sea ice cover. We test sea ice reversibility within a state-of-the-art atmosphere–ocean global climate model by increasing atmospheric carbon dioxide until the Arctic Ocean becomes ice-free throughout the year and subsequently decreasing it until the initial ice cover returns. Evidence for irreversibility in the form of hysteresis outside the envelope of natural variability is explored for the loss of summer and winter ice in both hemispheres. We find no evidence of irreversibility or multiple ice-cover states over the full range of simulated sea ice conditions between the modern climate and that with an annually ice-free Arctic Ocean. Summer sea ice area recovers as hemispheric temperature cools along a trajectory that is indistinguishable from the trajectory of summer sea ice loss, while the recovery of winter ice area appears to be slowed due to the long response times of the ocean near the modern winter ice edge. The results are discussed in the context of previous studies that assess the plausibility of sea ice tipping points by other methods. The findings serve as evidence against the existence of threshold behavior in the summer or winter ice cover in either hemisphere.