Simulation of 21st century Arctic climate change in CCSM4

We summarize the projected response of 21st century Arctic climate to greenhouse forcing in the newest version of NCAR's Community Climate System Model, CCSM4. Under a relatively strong radiative forcing scenario, the model simulates a much warmer, wetter, cloudier, and stormier climate with considerably less sea ice. The high correlation among the variables comprising these changes---temperature, precipitation, cloudiness, sea level pressure, and ice concentration---suggests that their close coupling collectively represents a fingerprint of Arctic climate change.

Although the projected changes in CCSM4's Arctic climate are generally consistent with other GCMs, we identify several noteworthy features in this new model. The Arctic climate response is generally weaker than under comparable greenhouse forcing in the previous model version, CCSM3 (16% smaller Arctic amplification), despite greater global warming in CCSM4. Autumn is expected to become the season of most pronounced Arctic climate change in coming decades among all the primary variables featured in this study. The climate changes are very similar across the five ensemble members we analyzed, although sea level pressure (SLP) displays more variability from one realization to another. The SLP response exhibits a significant trend toward stronger extreme Arctic cyclones on all relevant time scales (daily, monthly, seasonal, and annual), implying greater wave activity in Arctic seas that would promote increasing coastal erosion. Based on a commonly used metric defining the Arctic (the region encompassing the 10^oC July surface air temperature isotherm), the area comprising the Arctic shrinks by about 40% during the 21st century, in conjunction with a nearly 10 K rise in surface temperature poleward of 70^oN. Despite this pronounced long-term warming, CCSM4 simulates a hiatus in the secular Arctic climate trends during a decade-long stretch in the 2040s and to a lesser extent in the 2090s. This pause occurs in spite of averaging over five ensemble members and is remarkable because it happens under the most extreme greenhouse-forcing scenario and in the most climatically sensitive region of the world.