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In understanding the present warming trend, it is absolutely essential to learn more about climate change in the distant past -- or at least during the last 1,000 years. But many scientists, particularly younger ones, prefer to work only with data collected after 1975, when satellite data became available. With only 30 years worth of data, their results are little more than climatological snapshots of what is really a slow, long-term process. The latest accurate satellite images of sea ice distribution in the Arctic Ocean today can be obtained by clicking on a computer screen; but it is impossible to obtain such quality data for periods before 1975.
It is for this reason that only a minority of scientists are studying natural climate change, including multi-decadal oscillations and centurial climate change, which is the true realm of climatology. These areas have not been priorities for the IPCC.
They should be. During winter, England's Thames River would once freeze solid. This occurred on and off between 1400 and 1800 during a period called the "Little Ice Age" when temperatures dropped by as much as 1.5 degrees Celsius, which came after the medieval warm period around 1000. The anomaly of the Little Ice Age corrected itself, of course, through something called rebounding. The rebounding rate is estimated at 0.5 degrees Celsius per century. Since our present warming rate is roughly 0.6 degrees Celsius per century, the greenhouse effect caused by CO2 may represent only a 0.1 degree Celsius increase in temperature over the course of a century.
There is no doubt that global warming is in progress. But much of it can be attributed to the rebounding effect from the Little Ice Age. Recovering from a cool period is, of course, warming -- but it is nothing to panic about. Ice core data from the Greenland ice sheet show many periodic warming and cooling periods during the last 10,000 years. The present warming phase is far from the warmest.
Scientists have no clear knowledge of the cause of the Little Ice Age and of the subsequent rebound; or of the Big Ice Age; or of a warm period when the Arctic Ocean had no ice; or of the medieval warming period. In fact, IPCC scientists do not understand the causes of the rapid increase of temperature from 1910 to 1945; or the decrease from 1945 to 1975, when CO2 levels were rising. Without understanding these recent changes, it is premature for the IPCC to jump to the conclusion that CO2 is the main cause of the last 30 years of global warming.
Mr. Akasofu is the former director of the International Arctic Research Center at the University of Alaska. This essay is adapted from an article appearing in the Sept. 2007 issue of the Far Eastern Economic Review.