Friday, October 22, 2004

The phrase "on record" doesn’t mean very much, since most records date from the latter part of the nineteenth century. Without accurate records there are still ways of discovering the temperatures of past centuries, and these methods do not confirm the theory of a steady rise. Reading tree rings helps (the rings are further apart when the temperature is warmer and the trees grow faster). Core samples from drilling in ice fields can yield even older data. Some historical reconstruction can help, too—for example, we know that the Norsemen settled Greenland (and named it "green") a millennium ago and grew crops there, in land which is today quite inhospitable to settlement, let alone to agriculture. Other evidence comes from coral growth, isotope data from sea floor sediment, and insects, all of which point to a very warm climate in medieval times. Abundant testimony tells us that the European climate then cooled dramatically from the thirteenth century until the eighteenth, when it began its slow rewarming.

In sum, what we learn from multiple sources is that the earth (and not just Europe) was warmer in the tenth century than it is now, that it cooled dramatically in the middle of our second millennium (this has been called the "little ice age"), and then began warming again. Temperatures were higher in medieval times (from about 800 to 1300) than they are now, and the twentieth century represented a recovery from the little ice age to something like normal. The false perception that the recent warming trend is out of the ordinary is heightened by its being measured from an extraordinarily cold starting point, without taking into account the earlier balmy medieval period, sometimes called the Medieval Climate Optimum. Data such as fossilized sea shells indicate that similar natural climate swings occurred in prehistoric times, well before the appearance of the human race.

Even the period for which we have records can be misread. While the average global surface temperature increased by about 0.5 degrees Celsius during the twentieth century, the major part of that warming occurred in the early part of the century, before the rapid rise in human population and before the consequent rise in emissions of polluting substances into the atmosphere. There was actually a noticeable cooling period after World War II, and this climate trend produced a rather different sort of alarmism—some predicted the return of an ice age. In 1974 the National Science Board, observing a thirty-year-long decline in world temperature, predicted the end of temperate times and the dawning of the next glacial age. Meteorologists, Newsweek reported, were "almost unanimous in the view that the trend will reduce agricultural productivity for the rest of the century." But they were wrong, as we now know (another caution about supposedly "unanimous" scientific opinion), and after 1975 we began to experience our current warming trend. Notice that these fluctuations, over the centuries and within them, do not correlate with human numbers or activity. They are evidently caused by something else.

posted by Shaun Kenney | 10/22/2004 12:02:00 AM | 0 comments

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