How Well Does Natural Variability Explain the Difference Between Observed and Modeled Sea Ice Trends in Both Hemispheres?

Abstract:

Observations indicate that the Arctic sea ice cover is rapidly disappearing while the Antarctic sea ice cover is steadily expanding. State-of-the-art climate models, by contrast, predict that global warming should cause relatively moderate rates of sea ice retreat in both hemispheres. A number of recent studies have suggested that, in the Southern Hemisphere, this discrepancy is the result of the global warming signal being overwhelmed by natural variability which causes the observed expansion. Similarly, other studies have suggested that the rapidly diminishing sea ice in the Northern Hemisphere arises from a global warming signal that is being enhanced by natural variability. We investigate these hypotheses using output from climate model simulations carried out for the two most recent IPCC Assessment Reports (CMIP3 and CMIP5). Separating the simulated sea ice change in each hemisphere into a component associated with global warming and a component associated with natural variability, we quantify the likelihood that the model simulations are consistent with observed sea ice changes in both hemispheres. By placing a probability on the scenario in which neither models nor observations contain a significant deficiency, our results may have direct implications for the uncertainty of future sea ice projections generated using current climate models.