How Hot Will It Get?

By James Trefil

Not so long ago, people talked about global warming in apocalyptic terms--imagining the Statue of Liberty up to its chin in water or an onslaught of tropical diseases in Oslo. Recently, however, advances in our understanding of climate have moved global warming from a subject for a summer disaster movie to a serious but manageable scientific and policy issue.

Here's what we know. Since sunlight is always falling on the earth, the laws of physics decree that the planet has to radiate the same amount of energy back into space to keep the books balanced. The earth does this by sending infrared radiation out through the atmosphere, where an array of molecules (the best known is carbon dioxide) form a kind of blanket, holding outgoing radiation for a while and warming the surface. The molecules are similar to the glass in a greenhouse, which is why the warming process is called the greenhouse effect.

The greenhouse effect is nothing new; it has been operating ever since the earth formed. Without it, the surface of the globe would be a frigid -20[degrees]C (-4[degrees]F), the oceans would have frozen, and no life would have developed. So the issue we face in the next millennium is not whether there will be a greenhouse effect, but whether humans, by burning fossil fuels, are adding enough carbon dioxide to the atmosphere to change it (and our climate) in significant ways.

You might think that, knowing what causes greenhouse warming, it would be an easy matter to predict how hot the world will be in the next century. Unfortunately, things aren't that simple. The world is a complex place, and reducing it to the climatologist's tool of choice--the computer model--isn't easy. Around almost every statement in the greenhouse debate is a penumbra of uncertainty that results from our current inability to capture the full complexity of the planet in our models.

There is one fact, though, that everyone agrees on: the amount of carbon dioxide in the atmosphere is increasing steadily. It is near 360 parts per million today, vs. 315 p.p.m. in 1958 (when modern measurements started) and 270 p.p.m. in preindustrial times (as measured by air bubbles trapped in the Greenland ice sheet).

An analysis of admittedly spotty temperature records indicates that the world's average temperature has gone up about 0.5[degree]C (1[degree]F) in the past century, with the '90s being the hottest decade in recent history. This fact is quoted widely in the scientific community, although there are nagging doubts even among researchers. Recent satellite records, using different kinds of instrumentation, fail to show a warming trend.

If we accept that there has been moderate warming, we turn to computer models to see if humans are to blame and what will happen to the earth's climate in the future. These models are complex because climate depends on thousands of things, from Antarctic sea ice to sub-Saharan soil conditions. While the electronic simulations are monuments to the ingenuity and perseverance of their creators, they provide us with, at best, a fuzzy view of the future. They have difficulty handling factors like clouds and ocean currents (two major influences on climate), and if you fed the climate of 1900 into any of them, they couldn't predict the climatic history of the 20th century. Like everything else in this frustrating field, the models' limitations force us to make important decisions in the face of imperfect knowledge.

The most authoritative predictions about future warming come from the Intergovernmental Panel on Climate Change, a worldwide consortium of more than 2,000 climate scientists. The current forecast is that by 2100 the earth's temperature will go up 1[degree] to 3.5[degrees]C (2[degrees] to 7[degrees]F), with the best guess being an increase of 2[degrees]C (4[degrees]F).

At the lower end of this predicted warming range, the temperature rise would take us back to the conditions that existed between A.D. 950 and 1350, when the climate was 1[degree]C (2[degrees]F) warmer than it is now. This time period is regarded as one of the most benign weather regimes in history. To find temperature swings at the upper end, you have to go back 10,000 years, to when the earth was exiting the last Ice Age. Temperatures during the Ice Age were 5[degrees]C (10[degrees]F) cooler than they are now, and there was a series of incidents during which global temperatures changed as much as 10[degrees]F in a matter of decades. If that were to happen now, expanding oceans might flood coastlines and generate fiercer storms. And as weather patterns changed, some places could get wetter and some dryer, and the ranges of diseases could expand. Civilization has seen--and endured--such changes in the past, but they may come much more swiftly this time, making it harder to withstand the jolts.

The main reason for the spread in the IPCC predictions is uncertainty about how much carbon dioxide will be added to the atmosphere by human activity, because how we will respond to the threat of climate warming is the greatest imponderable of all. We can probably develop technologies to deal with excess carbon--some scientists talk about removing it from smokestacks and stashing it underground--but the most direct way to control carbon dioxide in the atmosphere is not to put it there in the first place. This is the point of the 1997 Kyoto Protocol--signed by 84 nations but not ratified by the U.S. Senate--which would limit developed countries' carbon emissions from cars, power plants and other major users of fossil fuels.

It makes no sense to overreact to the prospect of global warming, but it makes no sense to ignore it either. A prudent policy that stresses conservation and alternate energy sources seems to me to be wise insurance in an uncertain age. After all, our grandchildren will thank us for developing high-mileage cars, energy-efficient appliances and cheap solar energy, no matter how the future of global warming plays out.

James Trefil is a George Mason University physics professor and author of 101 Things You Don't Know About Science and No One Else Does Either