The CO2 Warming
Anyone who hasn't spent the last few years in the Bush without a radio has probably heard of the greenhouse effect by now. Carbon dioxide is an important "greenhouse" gas: it allows sunlight in to heat the surface of the earth, but blocks thermal radiation from leaving. Mathematical models indicate that an increase of carbon dioxide in the atmosphere should increase the average temperature of the earth. Carbon dioxide is indeed increasing, from about 270 parts per million (ppm) before the industrial revolution and 300 ppm in 1900 to 345 ppm in 1985. The buildup is thought to be due largely to the burning of fossil fuels, but with an additional component due to the destruction of forests, especially tropical rain forests. None of the models, however, are complete, especially with regard to possible effects of clouds. And in spite of the measured increase in carbon dioxide, global temperatures don't seem to have risen much, if at all. Will carbon dioxide really cause a warming?
Our best answer may lie in how its levels have changed with climatic changes in the past. Carbon dioxide levels for the last 150,000 years and more can be deduced from two sources: air bubbles trapped in ice caps as they grew, and stable carbon isotopes in sediment cores from the ocean floors. Both sources indicate that carbon dioxide levels were lower than they are today -- possibly as low as 200 ppm -- during the coldest parts of the last ice age, 16,000 to 70,000 years ago. During the last major interglacial 130,000 or so years ago, when temperatures were warmer than now, and sea levels possibly a few meters higher, carbon dioxide levels were above today's. They also seem to have been higher during the warm period shortly after the last great ice sheets melted than they were before they began to rise during the industrial revolution. Furthermore the ocean cores, which contain simultaneous records of carbon dioxide levels and the amount of ice on land, indicate that carbon dioxide began to rise before the ice sheets began to recede, and to fall before the ice sheets expanded, which suggests that carbon dioxide changes may have helped cause the ice ages. Finally, recent study of ocean temperatures suggests that while local temperatures on land may not yet be showing a detectable increase in temperature, global ocean temperatures have in fact increased slightly.
Our recent warm winters in Alaska have been due to an increase in warm winds blowing northward from the mid-Pacific, which have been balanced by cold air moving southward into the central and eastern United States. This change in the atmospheric circulation could possibly be due to changes in heating caused by the increase in carbon dioxide, but that is not necessarily the case. Circulation changes of this kind have occurred at intervals throughout the Alaskan climatic record, and the current run of warm winters, although it is one of the longest in this century, could be just another fluctuation. If the warm years go on for another decade, however, we may well look back and say that the carbon dioxide warming started in the 1970s.
Update added July 1996: The general trend of CO2 levels is still believed to follow the outline above, though there is now more controversy about whether warming leads or lags carbon dioxide increases in the atmosphere. Our warm Alaskan winters have continued, but they now seem due not so much to an increase in warm air from the south as to a decrease in cold air from the north. And the current magnitude of global climate change is still debatable, due largely to the fact that the earlier records are not strictly comparable to later ones. There seems little doubt that Alaska has been warmer since 1976 than it was in the 50's and 60's, however.
