The Discovery of Global Warming [Excerpt]

The basic physics of climate change have been known for more than a century, but it is in recent decades that the fundamental science of global warming has solidified

By Spencer Weart

CLIMATE CHANGE: Scientists have spent the last 150 years proving that human emissions of greenhouse gases have created global warming. Image: Courtesy of NASA

This excerpt is from The Discovery of Global Warming, by Spencer R. Weart (Harvard University Press, 2008).

It is an epic story: the struggle of thousands of men and women over the course of a century for very high stakes. For some, the work required actual physical courage, a risk to life and limb in icy wastes or on the high seas. The rest needed more subtle forms of courage. They gambled decades of arduous effort on the chance of a useful discovery, and staked their reputations on what they claimed to have found. Even as they stretched their minds to the limit on intellectual problems that often proved insoluble, their attention was diverted into grueling administrative struggles to win minimal support for the great work. A few took the battle into the public arena, often getting more blame than praise; most labored to the end of their lives in obscurity. In the end they did win their goal, which was simply knowledge.

People had long suspected that human activity could change the local climate. For example, ancient Greeks and 19th-century Americans debated how cutting down forests might bring more rainfall to a region, or perhaps less. But there were larger shifts of climate that happened all by themselves. The discovery of ice ages in the distant past proved that climate could change radically over the entire globe, which seemed vastly beyond anything mere humans could provoke. Then what did cause global climate change ? was it variations in the heat of the Sun? Volcanoes erupting clouds of smoke? The raising and lowering of mountain ranges, which diverted wind patterns and ocean currents? Or could it be changes in the composition of the air itself?

In 1896 the Swedish scientist Svante Arrhenius published a new idea. As humanity burned fossil fuels such as coal, which added carbon dioxide gas to the Earth?s atmosphere, we would raise the planet?s average temperature. This ?greenhouse effect? was only one of many speculations about climate change, however, and not the most plausible. Scientists found technical reasons to argue that our emissions could not change the climate. Indeed most thought it was obvious that puny humanity could never affect the vast climate cycles, which were governed by a benign ?balance of nature.? In any case major change seemed impossible except over tens of thousands of years.

In the 1930s, people realized that the United States and North Atlantic region had warmed significantly during the previous half?century. Scientists supposed this was just a phase of some mild natural cycle, with unknown causes. Only one lone voice, the amateur G. S. Callendar, insisted that greenhouse warming was on the way. Whatever the cause of warming, everyone thought that if it happened to continue for the next few centuries, so much the better.

In the 1950s, Callendar?s claims provoked a few scientists to look into the question with improved techniques and calculations. What made that possible was a sharp increase of government funding, especially from military agencies with Cold War concerns about the weather and the seas. The new studies showed that, contrary to earlier crude estimates, carbon dioxide could indeed build up in the atmosphere and should bring warming. Painstaking measurements by C. D. Keeling drove home the point in 1960, showing that the level of the gas was in fact rising, year by year.

Over the next decade a few scientists devised simple mathematical models of the climate, and turned up feedbacks that could make the system surprisingly variable. Others figured out ingenious ways to retrieve past temperatures by studying ancient pollens and fossil shells. It appeared that grave climate change could happen, and in the past had happened, within as little as a few centuries. This finding was reinforced by computer models of the general circulation of the atmosphere, the fruit of a long effort to learn how to predict (and perhaps even deliberately change) the weather. Calculations made in the late 1960s suggested that average temperatures would rise a few degrees within the next century. But the next century seemed far off, and the models were preliminary. Groups of scientists that reviewed the calculations found them plausible but saw no need for any policy action, aside from putting more effort into research to find out for sure what was happening.

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