101 THINGS YOU DON'T KNOW ABOUT SCIENCE AND NO ONE ELSE DOES EITHER
Jan. 1, 1996, n.p.

Copyright © 1996 Houghton Mifflin Company

Is the Climate Getting Warmer?

By James Trefil

     We often speak of "global warming" as if it were an established fact, yet there is a good deal of skepticism in the scientific community about whether the data we have reveal anything more than the normal variability of climate. I have to warn you that on this subject I am something of a skeptic, perhaps even a contrarian, so take what I say with a grain of salt. (And take what everyone else says the same way!)

     The first thing to realize about the earth's climate and average temperature is that both are changing all the time. Over hundreds or thousands of years, changes of several degrees are common. (In this section I'll follow the general custom and report warming and cooling in degrees Celsius. For a rough conversion to the more familiar and accurate degrees Fahrenheit, multiply all my numbers by two.) Second, human beings have not been keeping temperature records for very long. The first thermometer was invented in 1602 (by Galileo, believe it or not), and the mercury thermometer didn't come into widespread use until about 1670. This leads us to one of the most difficult problems faced by scientists trying to discern trends in the earth's climate--because we don't have a very long record of temperatures, it is very hard to establish a base line to which any warming or cooling can be compared. In North America, for example, one of the best temperature records dates back to the early 1800s, when Thomas Jefferson started keeping records at his home, Monticello, in central Virginia. In Europe, records in several cities date back three hundred years.

     To go beyond that, scientists have used all sorts of proxies for temperature. They have, for example, examined the dates of grape harvests in medieval France to estimate summer temperatures and rainfall, land deeds in Switzerland to track the advance and retreat of glaciers, and iceberg sightings in Iceland to estimate the climate in the Northern Hemisphere. The advent of the thermometer helped, of course, but raised its own problem--which temperature should we measure? Air temperatures (what we normally mean by "temperature") are measured at only a few places on earth (airports, for example). Water temperatures at the ocean surface are easy to measure by satellite. But both of these measurements present problems of interpretation. Airports, for example, are customarily built in rural areas that get covered with concrete. (Did you know that the designation for Chicago's O'Hare airport--ORD--refers to the apple orchards in which the airport was built?) Does a higher temperature twenty years later tell you about climate or about the effect of paving rural landscapes? Similarly, in the old days, sea-surface temperatures were sampled from buckets of water hauled up over the sides of boats. But after 1946, measurements were made of water coming into engine rooms. If the temperatures rose after 1946, do you blame the climate or the warmer environment of the engine rooms?

     Over the past few years, this sort of debate has intensified as questions about global warming have moved from the scientific community into public discussions of the greenhouse effect. There are, in fact, two separate issues that often become confused in the public debate: (1) can we document a global warming, and (2) is that warming attributable to human actions?

     The best long-range records, compiled primarily from sea-surface records over the past 140 years, indicate a warming of a little less than half a degree. Most of this warming occurred in two pulses--one in 1920, the other in 1977. In between these dates the sea-surface temperature was roughly constant, and, for ten years starting in the 1940s, actually dropped. (I remember this period well, because it provoked headlines about a "Coming Ice Age" in the Chicago newspapers.) The first of these pulses occurred well before we would expect any greenhouse warming based on increased carbon dioxide concentrations in the atmosphere. The second, unfortunately, occurred just before we began launching temperature-monitoring satellites.

     Since 1979, when the satellite record began, there has been no discernible global warming, despite predictions from the computer models that the earth should be warming at a rate of about one-quarter degree per decade. In fact, the data actually show a slight cooling.

     Faced with the problem of establishing the human influence on climate, though, the Intergovernmental Panel on Climate Change tried a new tack in 1996. Instead of looking at total warming, they looked at patterns of change, in particular the temperatures of the atmosphere at different altitudes and the pattern of warming and cooling in different parts of the world. They claim that they can just begin to see the imprint of human influence on climate. The big question, of course, is what will happen next. If, between now and the year 2000, global temperatures continue to fluctuate through normal ranges, we will have to question the validity of our computer models. If we see a rapid temperature increase, maybe even I will have to start taking global warming more seriously.