Our Role in Earth's Climate

IS THE TEMPERATURE RISING?

THE UNCERTAIN SCIENCE OF GLOBAL WARMING

By S. George Philander

Princeton University Press

262 pp., $29.95

They sound like questions from a curious child. Why is the sky blue? What makes the winds blow? Why is summer hotter than winter? Where do clouds come from? Yet understanding the answers to such deceptively simple questions is essential to dealing intelligently with the biggest yet-to-be-answered atmospheric question of all: Is human activity causing undesirable climate change?

George S. Philander, geosciences professor at Princeton University, guides us through this learning experience with grace, wit, and clarity. Like it or not, humans have become a critical part of the climate machinery.

Humans have taken the climate-weather machine for granted. It's big nature. We've considered ourselves, in comparison, to be bit players whose actions are insignificant on a planetary scale. But by changing the chemical composition of the atmosphere, we're changing the settings of the geophysical machinery.

Wide use of chlorofluorocarbons (CFCs) threatened to weaken the stratospheric ozone layer that shields us from the sun's harmful ultraviolet radiation. The world woke up to that threat and is phasing out the CFCs by international agreement. We have yet to appreciate how significantly human activity is changing atmospheric chemistry in other ways.

Dr. Philander has tried to help us gain that appreciation in "Is the Temperature Rising? The Uncertain Science of Global Warming" and he succeeds. He lays out the basic facts, explains the science, and outlines the unknowns and uncertainties. He gives enough detail for readers to grasp the subject but avoids nerdish technicalities.

The sky is blue because that's the way air molecules scatter sunlight. Clouds appear as water cycles between air, sea, and land. It's a recycling system that helps distribute solar heat and keeps the planet livable. Northern hemisphere summer is warmer than winter even though Earth is farthest from the sun in summer because of the way our planet is oriented in its orbit. Earth's wind systems blow as they do because the circulation distributing heat operates on a rotating planet. We need to be aware of these and other elements of the climate system to understand what scientists tell us about global warming.

There's general scientific agreement that large-scale deforestation and massive emission of heat-trapping "greenhouse" gases such as carbon dioxide and methane will bring climate change and global warming, if continued. What's uncertain is when and how big such changes will be.

Furthermore, as Philander points out, climate change could produce a sudden nasty surprise that scientists don't foresee. That happened with the ozone layer. Scientists expected a gradual global thinning of the layer. What they got was the Antarctic ozone hole. They had overlooked a key mechanism - chemical reactions on tiny ice crystals - that operates over Antarctica.

It's hard to say if global warming has begun. Natural weather variability can obscure long-term trends. Good instrument data only go back about a century. Proxy data, such as tree rings, go back farther. But their interpretation is uncertain. As if to make the point, a report in Nature magazine last week uses such data to indicate 1997 was the warmest year in several centuries - a provocative but not conclusive finding. Humans have to take responsibility for perturbing the climate machinery.

In the face of uncertainty about the effects of that perturbation, they have to decide what actions to take now to moderate their influence. This book can help readers gain the understanding to debate those issues.

* Robert C. Cowen writes on science for the Monitor.