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Updated 11 November 2004

Consequences Vol. 2, No. 1, Winter 1996









One reason why possible global warming is at times discounted may be the size of the numbers in which projected temperature changes are cast. Why should a change of but a few degrees, fifty or a hundred years in the future, be of such concern today? A probable rise in mean global surface temperature of 1 to 3°Centigrade, or 2 to 5°Fahrenheit? The best one can read a window thermometer is about 1°, and the daily forecast of expected highs or lows is accurate to no more than 3°. For those of us in middle latitudes, the variation from day to night in the temperature of the air can be 20°C, and in the course of the seasons the daily mean varies through 50 to 60. What is it about a global average that makes a change of 1° profound?

It is true that a change of 1° in the mean implies larger changes in parts of the globe--particularly polar regions--but also smaller ones in others. And that a small variation in temperature can signal much greater changes in other conditions, such as precipitation and storms and river flow. Still, it's only 1°.

It may help to think of global temperature in the sense of the surface of the sea--that heaves up and down through many meters at any place with passing waves, and with the ebb and swell of the tides. Yet were the mean level of the seas to rise or fall by half a meter, it would alter coasts, and costs, and ecosystems, throughout the world. In the course of these momentous changes you and I would perceive only the tossing of the surface and the splashing on the sand. But it is the lifting or lowering of the global ocean, through relatively small amounts, that has wrought major impacts in the past. So it is with global temperature.

A useful gauge of the impact of a change of a few degrees in global mean temperature can be found in Tom Crowley's article on paleoclimate in this issue of CONSEQUENCES. The full range of global mean temperature variation during the last 1000 years--which has seen the coming and going of a Little Ice Age--is only about one degree C. The difference between the present interglacial period and the last major Ice Age--when a layer of ice a mile or more deep lay year-round over much of North America, including all the Great Lakes and New England--is now thought to be about five degrees C in the global mean value. And the difference between today and the days of the dinosaurs is but six to eight. Tell that to your thermometer.

John A. Eddy

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