AT the Earth Summit in Rio de Janeiro an army of environmental activists and members of the press talked of the need to "Save the Planet" as if we were on the brink of a climatic apocalypse that will turn this rare blue sphere into a cinder of bedrock. Hardly likely.
Meanwhile, a counterforce of conservatives charged that the threat of global warming is overblown, and any actions to fight it will ruin the economy. George Bush, who continues to label environmentalists "extremists," has embraced this view. Columnist George Will has even written, "Some environmentalism is a `green tree with red roots.' "
With all this hype, it might be useful to check the facts and see if we actually know anything about how humans may be meddling with Earth's atmosphere. There are hard facts. And some sensible people actually have well-reasoned proposals for dealing with this matter.
1. There is a greenhouse effect. Water vapor, carbon dioxide, and a few other atmospheric gases act like the glass panes of a greenhouse, allowing sunlight in to warm the planet but preventing heat from escaping.
The proof is that Earth is warm. Without its insulating blanket of "greenhouse gases," the planet would have an average temperature of zero degrees Fahrenheit instead of the balmy 59 degrees we currently experience.
2. Concentrations of carbon dioxide and several other greenhouse gases have been rising at an accelerating pace since the start of the Industrial Revolution.
Locked in bubbles of ancient air trapped in glaciers is a precise record of carbon dioxide stretching back 160,000 years. In that span, the amount of CO2 in the air fluctuated between 190 and 280 parts per million - low during ice ages and high during warm intervals.
But CO2 never rose higher than 280 parts per million until around 1890, when the burning of fossil fuels and forests began to generate enormous amounts of this gas. The concentration now is 356 parts per million. That's a 22 percent rise in just over 100 years - a rate of change far faster than anything nature has come up with. Barring some miraculous post-Rio conversion, levels of carbon dioxide from human activities will double from pre-industrial times in the coming century.
3. The planet is warming. There has been some criticism from skeptics who say that temperature records are inaccurate. But other data strongly support the idea that things are heating up. Temperature readings in holes drilled through permafrost in the Arctic show a sharp recent warming trend. The winter snow pack covering the northern hemisphere has retreated markedly over the past few decades.
So, these are a few of the facts: certain gases warm Earth. Concentrations of these gases are rising rapidly because of human activities. Finally, the global temperature has risen significantly since levels of these gases began to rise.
Obviously, the case is not open-and-shut. It's still possible that the current warming trend is some natural variation caused by factors we don't understand. One thing seems fairly clear: If levels of carbon dioxide continue to rise at the current rate, there is a significant chance that disruptive climate shifts will occur within the lifetimes of children born today. The list of potential consequences is familiar: coastal flooding, searing droughts, wars over shrinking water supplies, accelerated extinc tion of species.
In essence, by fiddling with the global thermostat, we are conducting a potentially hazardous physics experiment. Things might turn out just fine. Or they might turn out terribly. The trouble is, we're all sitting in the test tube.
Some people maintain that even if a dramatic climate change does occur, we'll adapt without too much fuss. Others insist that actions can be taken today - such as greatly increasing the efficiency with which we use energy - that will cut the risks of climate change while offering society other benefits.
That was the conclusion of the National Academy of Sciences, which last year reported, "Despite the great uncertainties, greenhouse warming is a potential threat sufficient to justify action now." One of the authors of that statement was Robert Frosch, the head of research for General Motors Corporation - hardly a radical environmentalist. Unfortunately, quiet, sensible voices such as his have been drowned out in all the noise from left and right.
Global warming presents a critical test of two uniquely human attributes: reason and foresight. It is up to all of us to seek out the facts and decide on a course of action.