RELEASE NO: 98-181

GREENHOUSE GAS RATE APPEARS SLOWER

The greenhouse gases in the atmosphere that cause global warming are increasing much more slowly than assumed in climate predictions of the Intergovernmental Panel on Climate Change (IPCC), according to a new analysis of the growth rates of the heat-trapping gases.

The study, published in the latest [Oct. 27] issue of the Proceedings of the National Academy of Sciences, also states that science cannot presently make reliable predictions of future warming because several of the key factors influencing global temperatures are so poorly understood.

The paper, "Climate Forcings in the Industrial Era," by Dr. James Hansen and colleagues at NASA’s Goddard Institute of Space Studies in New York, presents a significantly different conclusion than the IPCC report. Instead of a doubling of carbon dioxide in 50 years, which would be expected to cause an eventual warming of 3 to 8 degrees Fahrenheit , the new research estimates it will be 100 years before this level of greenhouse gases occurs, if the current lower growth rates continue.

From 1950 to 1975 the growth rate of greenhouse gases in the atmosphere rose sharply, but since then that rate has slowed. Scientists are not yet able to explain why the slowdown is happening for carbon dioxide during a time when worldwide emissions has rapidly increased. Hypotheses include higher rates of carbon dioxide storage in natural "carbon sinks" such as trees and the ocean. As a result of this uncertainty, researchers are not able to predict how long the lower rates will continue. The growth rate of methane, another important greenhouse gas, has slowed even more dramatically.

But this change in growth rates of greenhouse gases is just one reason why current predictions of future warming are on very shaky ground, Hansen acknowledges. Several key factors influencing global temperature – increases in clouds and small particles (aerosols) that tend to cool the Earth – are so poorly known that without new research and observations in these areas, reliable predictions of future warming are not possible. "These ‘climate forcings’ are complex and we don’t know them as well as we must to predict future climate," says Hansen.

Present evidence suggests that cooling produced by aerosols and clouds partially cancels out the well-established warming capability of greenhouse gases, states the new study. Because of this reduced warming, the relatively small warming caused by a hypothesized increase in the Sun’s radiation output "may play a larger role in long-term climate change than inferred by comparison with greenhouse gases alone," according to the study’s authors.

There are very few measurements of the factors causing global temperatures to rise or fall, such as man-made aerosols, clouds, and land-use patterns, says Hansen. New satellite observations scheduled to begin in 1999 as part of NASA’s Earth Observing System program will provide some of this information. The Earth Observing System program is also sponsoring research into the "missing carbon" question. More research will be needed, Hansen believes, if predictions of future global warming are to improve.