November 27, 1997

Who Cares About a Few Degrees?

By ANDREW C. REVKIN

cientists and other experts who have spent years trying to get people concerned about the prospect of global warming have always faced a central problem: temperatures change all the time.

	Global Warming
		HOME ARCHIVE FORUMS GLOSSARY RESOURCES

From summer to winter, Minneapolis goes from 90-degree heat waves to 10-below deep freezes. At the end of a spell of Indian summer, Manhattan can go from 65 degrees to 25 in a few hours.

So why should anyone get excited about a global rise of a few degrees in a few decades?

But the debate before negotiators in Kyoto, Japan, is about climate, not weather. It is about long-term shifts in patterns of weather, not seasonal or day-to-day shifts from rain to sun, cold to heat.

Understanding the difference between weather and climate is the first step in understanding why many scientists are predicting that changes that seem trivial in terms of any single day -- a change, say, of five degrees Fahrenheit -- could have large impacts on many facets of life when those changes are on a global scale.

Global warming would not change the range of weather experienced day to day, but it would increase the odds of having weather that is considered troublesome by 20th-century standards: summer droughts, winter deluges, hurricanes and the like.

At the heart of the theory that has dozens of nations poised to act to lessen the threat is the long-established idea that earth's atmosphere behaves like the roof of a greenhouse.

The atmosphere was first compared to a "glass vessel" in 1827 by the French mathematician Jean-Baptiste Joseph Fourier. He recognized that the air circulating around the planet lets in sunlight -- as a greenhouse's glass roof does -- but prevents some of the resulting warmth from leaving.

If the air had no heat-trapping effect, the heat from the sun would quickly radiate back into space, leaving the planet with a surface temperature of nearly zero degrees Fahrenheit.

In the 1850's, a British physicist, John Tyndall, took things further and tried to measure the heat-trapping properties of various components of the atmosphere. Surprisingly, the two most abundant gases -- nitrogen and oxygen -- turned out to have no heat-trapping ability. Ninety-nine percent of the atmosphere has no insulating properties at all. It is all up to a few trace gases -- mainly water vapor and carbon dioxide -- to keep the planet cozy. If the air did not contain carbon dioxide, the planet would be some 20 degrees cooler. Without water vapor, it would be a deep-frozen ball of ice.

By the 1890's, scientists had figured out that the great blossoming of combustion in the Industrial Revolution had the potential to change the atmosphere's load of carbon dioxide. The idea was summarized succinctly in the April 1896 issue of The London, Edinburgh and Dublin Philosophical Magazine by a Swedish chemist, Svante Arrhenius, who wrote, "We are evaporating our coal mines into the air."

Precise monitoring of carbon dioxide concentrations since the 1950's has shown a relentless upward trend. Some of the carbon dioxide has gone into the ocean, and some has been absorbed by growing trees, but the amount in the air has continued to rise.

There has been a simultaneous rise in the planet's average temperature, although at a far slower -- and more uneven -- pace. Other factors appear to have acted like a buffer, including a rise in the amount of sooty particles in the air; they act something like a parasol, reflecting some of the sun's energy back into space before it can warm things up.

Nonetheless, many scientists, using computer models, say that they can account for buffering mechanisms and still see problems ahead, particularly if significant cuts are not made in the rates at which petroleum and coal are burned.

Indeed, many of their calculations indicate that the necessary cuts in emissions of carbon dioxide would actually have to be far greater than even the most ambitious targets on the negotiating table in Kyoto.

There has also been a rise in the atmosphere's burden of methane and nitrous oxide from sources related to human activities, and these gases also trap heat.

Even though the greenhouse gases exist in only trace amounts -- they are measured in parts per million and, in some cases, parts per trillion -- they exert a powerful influence on the temperature of the planet. So a tiny change in their concentrations can cause a big change in the way the atmosphere behaves.

Dr. John W. Firor, an atmospheric scientist at the National Center for Atmospheric Research, likes to compare the situation to that of a corporation that is vulnerable to a takeover. A change of a couple of shareholders' votes can mean the difference between survival and getting swallowed up. It is a "highly leveraged situation," he said, using the parlance of Wall Street.

He and many other scientists say the risks of meddling significantly with the insulating atmospheric greenhouse are simply too great to continue on the current course, adding more than seven billion tons of carbon dioxide and other heat-trapping gases to the air each year.

But they acknowledge that many of the feedback loops and connections between the components of Earth's atmosphere, oceans, ice caps and, ultimately, climate are complex and remain poorly understood. The computer models with which scientists are projecting the range of future consequences from this rise are still a relatively crude representation of the real world of clouds, ocean currents, jet streams and other complexities.

Some consistent critics of the projections say the models are rife with weaknesses -- particularly in predicting how warming could affect cloud cover and in how solar energy moves from the surface to the highest levels of the atmosphere and then out into space.

An increase in cloudiness could act like panels on a greenhouse roof, countering the heat-trapping effect of the greenhouse gases by reflecting the sun and leading to cooling.

Dr. Richard S. Lindzen, a professor of meteorology at the Massachusetts Institute of Technology and a consistent skeptic on the perils of climate change, said the negotiations in Kyoto were mostly focused on bolstering the resumes of diplomats.

Even if the models are correct -- which Dr. Lindzen doubts -- the cuts in greenhouse gases on the table are so small that they will hardly matter, he said.

"Climate always changes, whether man does anything about it or not," Dr. Lindzen said. "Now any changes will be attributed to policy, not nature."

But many of his colleagues disagree, saying that the models have held true despite continuing efforts to weed out spurious results.

And many of them say it is essential to act now. Dr. Stephen H. Schneider, a Stanford University biology professor, said that in the 20th century, there had already been a distinct warming trend and a change in precipitation patterns that was hard to ascribe to anything other than the manmade increase in greenhouse gases.

"Is this nature being perverse or is it us?" Dr. Schneider said. "The only way to prove it for sure is hang around 10, 20 or 30 more years, when the evidence would be overwhelming. But in the meantime, we're conducting a global experiment. And we're all in the test tube."