The announced objective of the 1992 Global Climate Treaty is to "achieve stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system" [emphasis added]. The problem for policy makers, however, is that no one knows what constitutes a "dangerous" con-centration of greenhouse gases.

Despite forecasts of floods, droughts, and rising sea levels, the climate record actually gives little guidance. Rapid variations of climate occurred during the most recent Ice Age, when carbon dioxide (CO₂) concentrations were as low as 180 parts per million (ppm), about half of what they are today. By contrast, climate over the warm interglacial (the last 11,000 years or so), with higher CO₂ concentrations, has been relatively stable. Scientists don't know how climate variability depends on CO₂, or if it does at all.

The goal of the Climate Treaty then—to prevent "dangerous interference"—remains scientifically undefined. The 1990 report from the U.N. Intergovernmental Panel on Climate Change favored CO₂ stabilization at the 1990 level of about 350 ppm, the 1996 IPCC report aims for the politically more acceptable 550 ppm. Without scientific guidance, the goal is entirely arbitrary, and any stabilization level—or none—will do.

In truth, climate is forever chang-ing, and in many cases for reasons we do not yet understand. On a timescale of years and decades, the most important changes arise from complicated interactions between the atmosphere and the ocean; the El Nino events that cause global changes in temperatures and rain-fall are one example. Volcanic eruptions occur frequently and can have significant effects on climate over the short term.

On a longer time scale, the Earth has experienced some 17 Ices Ages in the last 2 million years. Ice cores, ocean sediment cores, and tree rings all show evidence of large and rapid climate changes, even in recorded history, i.e., the last 3,000 years.

Since about 1880, we've had "global" thermometer records, though large portions of the Southern Hemisphere and the oceans have not been regularly monitored. Also, studies have shown that urban growth around these ground-based temperature stations has created a false increasing temperature trend.

It has only been since 1979, using weather satellites, that scientists have been able to record truly global temperature readings from the lower atmosphere. According to these satellite data, global temperatures over the last two decades have been heading slightly downward.

So what we have is a discrepancy between observations (no warm-ing trend) and theory (which projects a 0.2 to 0.3 degree Celsius warming per decade). What is causing this gap? There are several possible explanations:

The first and most obvious is that global climate computer models still do a less than adequate job of representing the atmosphere. Because of poor spatial resolution, even the most sophisticated models are unable to depict clouds properly. And because the physical processes of clouds are not well enough understood, computer models have difficulty calculating the distribution of water vapor, which is by far the most important atmospheric greenhouse gas, accounting for more than 95 percent of the nat-ural greenhouse warming. Moreover, models cannot as yet reproduce all of the complicated ocean--atmosphere interactions that con-trol natural climate variations.

Another explanation is natural external effects, principally the 11-year cyclical variations of solar ultraviolet radiation or the corpuscular emission from the sun—the so-called "solar wind." These solar variations are believed to produce changes in cloudiness or in atmos-pheric ozone, both of which directly affect climate.

Finally, there are other human influences that have been largely ignored. International efforts focus almost entirely on CO₂, for exam-ple. Yet methane—from landfills, cattle raising, and rice paddies and other wetlands—has increased more than 100 percent in the past century. As a greenhouse molecule, methane is 20 times more potent than a molecule of CO₂.

In addition, an expanding popu-lation is changing the ratio between cropland and forests, and thereby the reflecting power ("albedo") of the surface. Commercial airline traffic, increasing at 5 percent per year, is changing the chemical composition of the lower stratosphere, and may be producing sufficient contrails and cirrus clouds to show regional climate effects.

It has even been suggested recently that the Aswan Darn, by reducing the flow of the Nile, is increasing the already high salinity of the Mediterranean causing its outflow through Gibraltar to disturb the North Atlantic circulation and thus affecting North American and European climate.

All these possible explanations have very different consequences and call for very different policy responses. There is a need, obviously, for more certainty.

But patience is not a virtue of policy-makers. Even with global tem-peratures going down, government delegations are heading to Kyoto in two weeks to attempt to strike an agreement calling for drastic reductions in energy use. They are, in effect, peering into a "black box" called a computer climate model and saying "Trust us. Global warm-ing is in there."

Next: Apocalypse not.