Yes, it's hotter. But why?

Savante August Arrhenius, the Swedish prophet of global warming - would have loved the personal computer. He had to run through tens of thousands of hand calculations in the late-19th century to make the first scientific forecast of carbon-dioxide-driven climate change.

His final answer, published in 1906, predicted that doubling the air's CO2 content "would raise the temperature of the Earth's surface by 4 degrees C." Noting how industry was guzzling fossil fuels, the Nobel laureate also remarked that industrial development could boost the air's CO2 content "to a noticeable degree in the course of a few centuries." Bingo! That's squarely within the 21st-century forecast of the Intergovernmental Panel on Climate Change (IPCC), based on supercomputer climate simulations and the judgement of hundreds of scientists. It estimates that doubling atmospheric CO2 could raise Earth's average surface temperature anywhere from 1.5 to 4.5 degrees C, depending on how the climate system reacts.

Most experts expect atmospheric CO2 to double over the next hundred years even with the most stringent emission controls now contemplated. "The real emerging greenhouse controversy" will be over how much the world is willing to do to hold down atmospheric CO2 growth beyond that, according to Jerry Mahlman, director of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Arrhenius was more interested in how natural CO2 variations affected the ice ages than in humanity's assault on climate. But given the importance global warming has assumed in human affairs, his work must rank as one of the seminal prophesies of the past millennium.

At the same time, it's too simplistic to focus on CO2. There are other heat-trapping "greenhouse" gases, such as methane and nitrogen oxide. Climate modeler James Hansen, who directs NASA's Goddard Institute for Space Studies in New York City, has explained: "We really have to understand the cycle of these greenhouse gases if we're going to reliably forecast what's going to happen."

So far, that understanding "is not all that good," he said. As if to make the point, W.T. Sturges at the University of East Anglia in Norwich, England, and colleagues recently identified a new greenhouse gas - trifluoromethyl sulfur pentafluoride. It has no natural source.

There are caveats with computer climate simulations, too. It's hard to model a climate system that involves complex interactions of air, sea, and ice fields when these interactions are not well understood. As the IPCC points out, these simulations still can't take realistic account of some key factors, such as cloudiness.

The earth has warmed about 0.6 degrees C on average over the past century. This is consistent with computer global-warming forecasts as are the thinning of Arctic ice and worldwide melting of glaciers. Putting all the data and theoretical predictions together, the draft of the IPCC's report now circulating concludes, "there has been a discernable human influence on global climate."

Yet as Dr. Hanson has noted, such a conclusion "does not tell us how much of observed climate variations are a result of human influence and it does not provide the information needed for policymakers to take the appropriate actions."

Arrhenius started a scientific study that has aroused one of the great public-policy issues of the new millennium: How and to what extent should industry, agriculture, and other human activities change to reduce greenhouse gas emissions? Supercomputer simulations can no more decide that issue than could the pencil and paper calculations made a century ago.

(c) Copyright 2000. The Christian Science Publishing Society