When the first space probes visited Venus and Mars in the 1960s, they didn't find the habitable worlds science fiction writers had dreamed of.

Venus was a cloud-shrouded hell, hot enough to melt lead. Mars was as cold as Antarctica. Yet these differences couldn't be explained solely by distance from the sun.

Scientists realized each planet was governed by a "greenhouse effect." Just as the glass of a parked auto will trap heat even on a cloudy day, the thick atmosphere of Venus retained the sun's heat. The thin atmosphere of Mars allowed it to escape.

Earth has its own greenhouse. If our atmosphere didn't trap heat, our planet would average 60 degrees colder, or about zero Fahrenheit.

We humans have taken a stable climate for granted. Since civilization rose, average temperatures have varied only about one degree. But a drop of just nine degrees produced the last Ice Age, and a similar amount of warming millions of years ago produced forests in Antarctica and breadfruit in Greenland. Can such change happen again?

Most of our atmosphere is made of nitrogen and oxygen. It is water vapor and trace gases such as carbon dioxide and methane that are most crucial in regulating greenhouse temperatures, and these gases are rapidly growing. Carbon dioxide, a byproduct of fossil fuel combustion, has increased 30 percent since the start of the Industrial Revolution, to a level higher than anytime in the last 450,000 years. Methane has increased 145 percent. Nitrous oxide is up 15 percent.

The result, say 2,500 scientists from 60 nations who advise the International Panel on Climate Control (IPCC), is global warming. Average temperatures increased about 1 degree F. in the 20th Century. Rainfall climbed 1 percent as heat evaporated more water. Oceans have also warmed a degree and expanded, raising sea level an average of about eight inches.

The decade of the 1990s was the warmest on record. We're moving into the new century on a continuation of that trend; the US National Climatic Data Center recently announced that 2001 was the second-warmest year on record. The Arctic icecap has thinned 40 percent in the last 20 years. The Alps have lost half their glacial ice mass since the 19th Century and the snow of Africa's Mount Kilimanjaro may be entirely gone in 15 years. In 1995, a piece of Antarctic ice shelf the size of Rhode Island broke off and melted.

Shrubs are growing on Arctic tundra. Disease-bearing mosquitoes that previously were not found above 1,000 meters elevation have now been observed at 1,150 meters in Costa Rica and 2,200 meters in Columbia. Recent increases in storms, El Nino climate oscillations, drought, forest fire, flood and species extinction have been blamed on global warming.

Yet this may be only a prelude. The IPCC recently concluded that global warming in the 21st Century will be worse than the panel originally forecast, with global temperatures averaging 2.2 to 10 degrees Fahrenheit warmer by 2100.

A 10-degree shift could be enough to shift climate zones 500 miles toward each Pole, changing rainfall patterns, snowpack, crop zones, and natural ecology.

Even more worrisome is evidence from pollen, ice, and mud cores. Prehistoric local climates have flip-flopped as much as 18 degrees in as little as a decade, researchers recently learned, meaning climate may be dangerously unstable.

A vocal minority of scientists says such warnings are alarmist. The world actually cooled from about 1940 to 1975, they point out, and temperatures so far have not risen as fast as computer models predict. Observed change may be just natural variation, and Earth's climate may be more self-regulating than the IPCC believes. Besides, maybe a warmer world would be more habitable, not less, they contend.

Yet the evidence for rapid global warming is getting stronger, not weaker. Scientists initially thought satellite measurements showed the upper atmosphere was cooling, not warming, but recalculation that figured in decaying satellite orbits confirmed warming has occurred there, too. Evidence is much firmer today than a decade ago that rising temperatures can be blamed on man-made pollution and not on natural variables such as sunspots, variations in the earth's orbit around the earth, or volcanoes.

Certainly our world is undergoing its biggest environmental transformation since the Ice Age. Global population has tripled since 1920. The IPCC projects that today's 6 billion humans will total from 8.7 billion to 11.3 billion by the end of the 21st Century. Just since 1980, world electricity consumption has doubled, and 79 percent of the world's energy comes from fossil fuel.

Is global warming a real threat? Thousands of scientists supported by billions of dollars in research money are working all over the globe to provide an answer. One vital laboratory is the cloud forest of Monteverde in Costa Rica where researchers are working to give answers before it is too late.