Simulation of recent northern winter climate trends by greenhouse-gas forcing

The temperature of air at the Earth's surface has risen during the past cen tury(1), but the fraction of the warming that can be attributed to anthropo genic greenhouse gases remains controversial. The strongest warming bends h ave been over Northern Hemisphere land masses during winter, and are closel y related to changes in atmospheric circulation. These circulation changes are manifested by a gradual reduction in high-latitude sea-level pressure, and an increase in mid-latitude sea-level pressure associated with one phas e of the Arctic Oscillation (a hemisphere-scale version of the North Atlant ic Oscillation)(2). Here we use several different climate-model versions to demonstrate that the observed sea-level-pressure trends, including their m agnitude, can be simulated by realistic increases in greenhouse-gas concent rations, Thus, although the warming appears through a naturally occurring m ode of atmospheric variability, it may be anthropogenically induced and may continue to rise. The Arctic Oscillation trend is captured only in climate models that include a realistic representation of the stratosphere, while changes in ozone concentrations are not necessary to simulate the observed climate trends. The proper representation of stratospheric dynamics appears to be important to the attribution of climate change, at least on a broad regional scale.