he years 1981 and 1983 were very hot, and some parts of the United States suffered severe drought. But no one except the affected farmers worried very much. It was weather. Even the scientists most committed to the greenhouse theories made no claims; "it must be said," wrote Revelle in 1982, "that so far the warming trend has not risen above the 'noise level.'...Confidence in the carbon dioxide hypothesis will be much firmer if a warming trend exceeding the noise level becomes evident."
In 1988, though, the American drought caught everyone's attention. It hit the heart of the grain belt, where most of the nation's and much of the world's food is grown. It followed a dry fall and winter, so its effects were quickly evident; the Mississippi River, for instance, sank to its lowest level since the Navy began taking measurements in 1872. And just about the time that the pictures on the television began to grab everyone's attention it got very, very hot in the urban East, where those in the government and the media establishment, among others, have their homes. It so happened that in late June, right as the anxiety was rising in a great crescendo -- newscasters telling us that the next two weeks were crucial for corn fertilization, meteorologists issuing forlorn sixty-day forecasts -- the Senate Committee on Energy and Natural Resources held a hearing on the greenhouse effect. It was actually the second part of the hearing. Part one had been held the previous November, when, in the words of Louisiana Senator J. Bennett Johnston, the senators listened with "concern" as scientists said that one expected result of the greenhouse effect would be a drying of the Midwest. But now, said Senator Johnston, "as we experience 101-degree temperatures in Washington, D.C., and the [lack of] soil moisture across the midwest is ruining the soybean crops, the corn crops, the cotton crops," "concern" is giving way to "alarm."
As at most congressional hearings, some of the senators on the panel made opening remarks before the witnesses spoke. Several senators said they had already read the report of Dr. Hansen, the day's chief witness, and they predicted that it would startle listeners. Hansen's report, Dale Bumpers of Arkansas said, should be "cause for headlines in every newspaper in America tomorrow morning."
As it turned out, Senator Bumpers was not exaggerating. Hansen said he was ready to state, after exhaustive review of the records, that the warming signal was now apparent above the noise of normal weather. That there was only a one percent chance that the temperature increases seen in the last few years were accidental. That the theories and predictions had come true -- that we now lived in the greenhouse world.
It was a claim no other established scientist had ever made -- certainly not one on a government payroll. And though Hansen delivered his findings in the flat, dry tones of a good researcher, the reaction was much as the senators had expected: the next day's New York Times, for instance, ran a story at the top of the front page under the headline "Global Warming Has Begun, Expert Tells Senate." The message was finally getting across, nearly a century after Arrhenius and three decades past Revelle and Suess. But the heat of the day may have been a mixed blessing -- though it focused everyone's attention on the issue, it led most people to think that what Hansen had said was that the heat and drought of 1988 were greenhouse-related. Strictly speaking, that is not what he had testified to. "It is not possible to blame a specific drought on the greenhouse effect," he said. (Indeed many experts think that most of the drought and heat of 1988 was the result of tropical ocean currents and related natural phenomena.) However, said Hansen, there is evidence that the greenhouse effect "increases the likelihood of such events."
In other words, what we can blame the carbon dioxide and the methane for is a longer-range pattern. Even if the summer of 1988 had been cool and damp, even if there had been mushrooms growing in the wheat fields of Kansas, Hansen would have said the same thing. What had convinced him was not the anguished farmers of the Midwest or the ecstatic air-conditioner salesmen of the Eastern cities, but the numbers his computer kept spitting at him. And if the next Fourth of July should see blizzards burying the Plains or even just the normal heat of an average summer, it might calm down those who still believe that the world is too big and too old to change, but it wouldn't shake Hansen's confidence in the implications of his hundred years' worth of thermometer readings.
"There are two logical time scales to consider," he explained some months after giving his testimony. "One is the thirty years for which we have some measurements of carbon dioxide and other gases. The natural variability in temperature for the years between i950 and 1980 is about .13 degrees Celsius. And our readings show that the global mean temperature has risen about .4 degrees in that period. The other logical choice would be to look at the larger record, the observations back to the 1800s. Over that period there's been about a .6 degree Celsius rise. Now, over a longer period there's also more natural variability -- sources like sunspots, deep ocean circulation, and so forth." The standard deviation, the randomness of temperature, over the longer period is plus or minus .2 degrees Celsius. In both cases, Hansen's observed rise was almost exactly three times the standard deviation. "There's no magic point where you pick out the signal," he said. "There's no point at which it switches over. But when it gets to three sigma -- when it gets to three standard deviations -- you're getting to a level where it's unlikely to be an accidental warming."
As the hearings closed for the day -- after several other authorities supported his findings, forecast a wide range of effects (none pleasant), and called for strong action to reduce fossil fuel emissions -- reporters gathered around the table asking questions. In response to one query, Hansen said, "It's time to stop waffling so much. It's time to say the earth is getting warmer."
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Excerpted from The End of Nature by Bill McKibben. Copyright © 1989 by Bill McKibben. Excerpted by permission of Random House. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher. Anchor trade paper edition published October 1990.