Underground Records of Changing Climate; June 1993; Scientific American Magazine; by Henry N. Pollack and David S. Chapman; 7 Page(s)
Is the earth's climate growing warmer? Persuasive evidence exists to support the proposition. According to meteorologic records, the mean temperature of the atmosphere has increased by slightly more than half a degree in the past century. Preserved air samples and other data show that levels of gases that trap the earth's heat have also risen during this period. The proportion of carbon dioxide in the atmosphere has risen by more than 20 percent and that of methane has roughly doubled. This correlation suggests a possible cause for the apparent effect. The proposition seems reasonable that the greenhouse gases are responsible for the warming trend. Yet the case is not airtight. It is conceivable that the matching increases in temperature and greenhouse gases are a statistical coincidence and that the two variables have nothing to do with each other in the long run.
How can climatologists resolve the ambiguity? Half of the necessary data are clearly available: air bubbles trapped in the polar caps and glacial ice archive change in atmospheric composition across a span of millennia. The temperature record is more problematic: widespread meteorologic data reach back no more than 150 years. Effective coverage of the Southern Hemisphere began only in this century, and until the past few decades there were important gaps in the polar regions [see "Global Warming Trends," by Philip D. Jones and Tom M. L. Wigley; SCIENTIFIC AMERICAN, August 1990]. There is nonetheless an archive to be read if one knows where to look for it. Just as the annual layers of Arctic and Antarctic ice preserve tiny bubbles of primordial air, so the ground retains fossil temperatures whose history can be traced back to the climate of previous centuries.