Greenhouse Gas Influence on Northern Hemisphere Winter Climate Trends
Global surface temperature has risen during the past century, but it is still uncertain whether warming in particular regions can be directly attributed to human activities. The largest warming has occurred over Northern Hemisphere land masses during winter, where temperatures have risen by several degrees during the 20th century. (In comparison, the global, annual average temperature has increased by only half a degree). These dramatic local changes are related to recent changes in atmospheric wind patterns. Hence, it is more likely that they arise from a redistribution of heat around the globe, rather than purely from an enhanced "greenhouse effect", whereby heat is trapped near Earth's surface by pollutants such as CO2. The question is whether these recent changes in the winds are merely part of a long term cycle or whether they are linked to the increasing greenhouse effect.
Wind speeds in the midlatitudes are strongly correlated with sea-level pressure. Historical variations of sea-level pressures have been studied intensely. The dominant pattern of Northern Hemisphere variability, is referred to as the Arctic Oscillation (AO) (Fig. 1a). The AO is an extension of the more familiar North Atlantic Oscillation to all longitudes, including the North Pacific. Variability of these two oscillation patterns is highly correlated. The AO is the second largest climate cycle on Earth, after the El Niño/La Niña cycle. Strengthened westerly winds at midlatitudes bringing warm, wet air over northern Eurasia and North America occur during the positive phase of the AO, corresponding to a decrease in high-latitude sea-level pressure and an increase in midlatitude sea-level pressure.
We have used several versions of the NASA Goddard Institute for Space Studies General Circulation Model (GCM) to investigate the changes to the AO pattern and surface temperature as greenhouse gases increase. Our model simulations suggest that indeed much of the increase in surface winds (associated with strengthening of the AO; Fig. 1b) and continental surface temperatures are induced by the buildup of greenhouse gases in the atmosphere.
A comparison of our various models indicates that the surface changes are driven to a large extent by the effect of the greenhouse gases on the stratosphere. While Earth's surface has warmed over recent decades, the stratosphere has cooled and stratospheric winds have increased. The same trends appear in our simulations, where increasing greenhouse gas emissions lead to a warmer surface and at the same time, a colder stratosphere. Additionally, the speed of the stratospheric jet stream increases, strengthening the vortex of air that naturally forms over the polar region each winter (Fig. 1d). Strengthening of the west-to-east winds in the polar vortex also causes west-to-east winds near the surface to increase. During the winter, since the ocean retains heat more than does land, increased west-to-east flow at the surface carries warmer air from the North Atlantic over the Eurasian continent, and colder continental air to the North Pacific. The large wintertime continental temperature increases produced in the model correspond quite well with the observed warming pattern. This does not occur, however, when we use a version of the climate model that does not adequately represent the stratosphere. In this case, no systematic change appears in the AO, or onshore winds, as greenhouse gases increase (Fig. 1c). This suggests that stratospheric processes play an important role in changes in climate at the earth's surface.
The AO is a natural climate cycle than can be traced back at least as far as the start of the twentieth century, before human activities produced enough greenhouse gases to noticeably change the climate. However, because recent increases of greenhouse gases cause the observed change in the AO and mid-latitude winds in our computer climate model, the large warming over wintertime Eurasia may therefore be attributable, in large part, to human activities. Though it takes place indirectly via the coupling between the stratosphere and the troposphere, the impact of greenhouse gases on climate through surface wind changes may be as large as, or in some areas, larger than the more direct radiative trapping of heat at the surface.
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