Earth's thermostat:

1. NATURAL GREENHOUSE

As radiation from the sun (yellow) enters earth's atmosphere, a portion of it is reflected back into space or absorbed directly by the atmosphere. The 50 percent or so that reaches the ground is converted into infrared radiation, or heat (red). Clouds and greenhouse gases such as water vapor and CO2 absorb most of the heat that the earth radiates back toward space. These gases then reradiate the heat. Over time, the outgoing radiation balances the incoming.
Other energy (orange) includes latent heat released by formation of clouds and convective heat carried by updrafts.

2. CARBON CYCLE

The amount of carbon in the earth's atmosphere is regulated by interaction of air, rock, sea, and all life-forms. Plants, for example, release carbon to the atmosphere as they breathe and as they decay. They take in CO2 by photosynthesis and store carbon within energy-rich carbohydrates. Photosynthesis by plankton, as well as other chemical processes, removes carbon dioxide dissolved in the oceans -- but an equivalent amount is released by chemical diffusion and decay of marine organisms.

Carbon is also stored in fossil fuels - the coal, petroleum. and natural gas remaining from plants and marine animals that died millions of years ago.

3. HUMAN INFLUENCES

Humans have disrupted the natural carbon cycle by burning fossil fuels for energy and by clearing forests to feed and house growing populations. Yearly about 50 million acres of forest are lost to logging, farming, and pasturage.

Industrial and agricultural emissions raise atmospheric carbon by about seven billion metric tons a year. Roughly half is absorbed by the oceans and by vegetation and soils. Emission of greenhouse gases can be reduced, but no technology is available to remove them once they reach the atmosphere.

FRENCH mathematician Jean Fourier in 1822 compared the earth's atmosphere to the glass of a greenhouse: Both let the sun's visible rays enter to warm land, water, plants, and air, yet retard the escape of heat. His comparison was apt -- in part. Without its atmosphere, the earth would be as frozen and lifeless as Mars, instead of averaging 15°C (59°F) and sustaining millions of species. Although the physics of earth's atmosphere and greenhouse glass are quite different, the term caught on even before British engineer G. S. Callendar warned in 1938 that human activities were altering this beneficial greenhouse effect.

What causes this warming, when 99 percent of the atmosphere is nitrogen and oxygen, neither of which absorbs much heat! Scientists have discovered that water vapor, carbon dioxide, and other gases -- though mere traces in the air -- act as powerful heat absorbers. Today the greenhouse effect stands as well established as any theory in the atmospheric sciences, anchored by countless measurements from satellites, weather balloons, and ground stations.

About half of the radiant energy reaching earth from the sun, because of its short wavelengths, can pass through the atmosphere to the earth's surface. But the longer waves of heat that radiate back toward space are absorbed and reradiated by water vapor, carbon dioxide, other gases, and clouds, and the atmosphere warms. "That's the greenhouse effect," Elmer had said. "Without it, earth would be frozen -- at least 60 degrees Fahrenheit colder -- and there would be no more life here than on Mars. But if it were to increase.... Well, some climatologists say we face temperatures three to nine degrees higher in the next century."

Back in the last glacial age, some 20,000 years ago, world temperature averaged about nine degrees colder than today. The carbon dioxide level was only 190 to 200 parts per million, ancient ice samples from Greenland and Antarctica show. As the ice melted back, the CO2 level gradually rose to about 280 ppm by the beginning of the industrial age. "By the middle of the coming century -- in our children's lifetime," Elmer had said, "the level will reach 550 or even 600 at its current rate of rise."

The prospect of doubled CO2 -- and even more rapid rise of other gases, such as methane, which together equal the warming effect of CO2 in the atmosphere -- is what has atmospheric scientists urgently refining their computer models of the climate. World population also is predicted to double by the middle of the next century, from five billion people to ten. And as all nations become more developed and use more fuel to support those people, the release of carbon dioxide and other gases to the air is bound to keep increasing -- despite the care taken or which fuels are burned.

With more warmth and more CO2, some ask, would not more crops grow, in wider areas than today? Would we not benefit from a warmer world? Perhaps, in some areas. The more CO2 in the air, the more productive some plants become. But the biggest unknown is what changes would occur in the planet's weather patterns. Most climate models show that in some regions -- northern Scandinavia, Siberia, and Canada, for example -- more rain would fall and more trees and crops grow. But in today's great mid-continent breadbasket regions, warming would lead to the drying of soil in summer. Destructive droughts, such as that of 1988 in North America, would strike more often, until the Great Plains and Ukraine turn semidesert. Storms such as hurricanes and tornadoes might become more violent. Forests would decline and change under the temperature rise, and wild-life would have to migrate -- if it could -- or perish. The perma-frost under Arctic tundra would thaw, deep peatlands would decompose, and vast new amounts of carbon dioxide and methane could be released.

And just as inevitably, as ocean waters warm and expand and the ice on Greenland and Antarctica melts back, the seas would creep higher onto the edges of the continents. Large parts of such low countries as Bangladesh -- already swept by ruinous floods and typhoons -- would be submerged; cities like Miami, Venice, even New York, would cower behind dikes.

"If a rise of one to three feet, as the models have predicted, seems extreme," says environmental scientist Stephen Leatherman of the University of Maryland, "keep in mind that the oceans rose more than 300 feet after the last ice age -- all in only a few thousand years."

If the ice cap on the island of Greenland were to melt completely, glaciologists estimate the oceans would rise another 20 feet. Sea level in the eastern United States has already risen a foot in this century alone, and it is predicted to go up at least another foot in the century ahead. With that one-foot rise, Leatherman says, the high-water line at Ocean City, Maryland, will move inland 100 to 200 feet; in Florida, 200 to 1,000 feet; in Louisiana, several miles.

Yet paradoxically, say other glaciologists, the huge ice domes on both Greenland and Antarctica may not be shrinking but growing. The paradox is that this too may be a sign of global warming. As the atmosphere warms, it holds more water vapor from evaporation of oceans and soil; hence more snow falls in the polar regions, hence more ice and possibly lower sea levels. But the warmer seas eventually will melt back the fringes of the polar ice, and the oceans will creep inexorably higher.

On the road to understanding sources of methane, scientists from the National Centre for Atmospheric Research in Boulder, Colorado, test what tumid out to be an innocent suspect -- fresh asphalt (above). Wide-ranging research suggests that leaks and venting from coal, oil, and natural gas production may be underestimated.

Cattle, a known source of methane, are increasing world-wide -- faster than the human population. The same bacteria that enable a cow to turn grass into meat or milk cause it to expel methane, roughly 14 cubic feet a day.

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