Decreasing Arctic sea ice: Greenhouse gases versus variability and natural forcing (Invited Presentation)

The downward trend in Arctic ice extent, the record minimum ice extent in 2002, and thinner Arctic ice since 1988 are all consistent with a warmer Arctic climate. Is the Arctic sea ice disappearing because of increasing greenhouse gases? Or are these changes attributable to natural variability such as the Arctic Oscillation, as well as natural forcing such as changing solar flux? The spatial patterns of the trends in sea level pressure, ice motion, atmosphere and ocean temperatures are consistent with the Arctic Oscillation. However, global climate model simulations that include a dynamic-thermodynamic ice model show that the increasing greenhouse gases in the late 20th Century produce trends of decreasing ice area and thickness that are beyond the range of the Arctic Oscillation or other natural forcings, such as solar flux variability. Continuing improvements in the Community Climate System Model are reducing known biases and uncertainties in the Arctic response to climate change. A higher resolution (T85) global atmosphere models is being tested that should improve the Arctic circulation pattern. Development of new physics parameterizations based on SHEBA studies are also having an impact on the global climate models.