Scientists often are presented in fiction as one of two types: cool, emotionless geeks more interested in their discoveries than in their possible effects; or raving lunatics with a secret agenda for taking over the world.

I've never met one of the latter. As for the first category, I've heard science arguments as passionate, violent, vicious and personal as anything in politics or religion. Certain phrases, dropped into a conversation among scientists, are almost guaranteed to cause an uproar. Try "cold fusion," or "punctuated equilibrium," or "Big Bang alternatives," or "origin of consciousness." Or, the one I want to talk about today, "global warming."

Before we come to the point where the debaters on each side froth at the mouth and start to chew the carpet, let's begin with facts about which no one will argue

First, for the past century we have been burning fossil fuels (oil, coal, and gas) that were laid down over millions and tens of millions of years. We are doing this fast, consuming a thousand years of fuel deposition in a single day. No argument here.

Second, when we burn fossil fuels, we release into the atmosphere gases that were held in combination in the fuel. Carbon dioxide is the leading example, though the combustion process also forms and releases nitrogen compounds and water vapor. Again no argument; everyone agrees on this.

Third, the gases released by burning fossil fuels have what is known as a "greenhouse effect." Sunlight that hits the surface of the earth is radiated away at longer wavelengths, and these wavelengths find it harder to pass through "greenhouse gases" like carbon dioxide, just as the same radiation is trapped by the glass walls and roof of a greenhouse. As a result, the earth tends to be warmer than it would be without greenhouse gases. All parties agree on this. In fact, were it not for greenhouse gases, so much heat would escape to space that the Earth would be too cold for life to survive.

With so much general agreement, what is all the argument about?

The first issue: is the Earth actually warming up? Logic says that it is, because we certainly are burning lots of fossil fuels. However, depending on what data you choose to look at, you can find both warming and cooling trends. The problem is, we know from long-term physical data that if you go back thousands or tens of thousands of years, there have been very significant changes in the Earth's overall climate, ranging from bitterly cold Ice Ages to long, warm, inter-glacial periods lasting many millennia.

Human activities had nothing to do with this. The evidence of recent global warming, with measurements available only for 20 or 30 years, forms a tiny blip on that great long-term rise and fall of temperatures. Are we humans "causing" global warming; or are we, like a fly sitting on the axle of a moving chariot, looking around and remarking on the great cloud of dust we are making?

That's where the arguments start. Some, worried about global warming, argue passionately for an immediate reduction in the use of fossil fuels (putting to one side the question of what we use instead for energy generation; nuclear power is not a popular choice). Others believe that we are on the brink of a new Ice Age, and we are staving this off only by accelerated production of greenhouse gases.

Most people, before they take sides on the issue, want to know more. For instance, when we talk of global warming we should ask how much warming? The answer that comes back, maybe a couple of degrees, does not sound impressive. A couple of degrees difference in temperature is too little for most of us to notice. Can such a small change do anything to affect us?

It can, and here we are on firm scientific grounds. We are able to look back through history, and ask what happened last time around, when it was a few degrees warmer or cooler than it is today.

During the last Ice Age, great ice sheets covered most of the Northern Hemisphere, extending down over Canada and into the United States. If that were to happen again, we would lose most of this continent's productive farmlands, and those of Asia and Europe. The water stored in these vast ice sheets was enough to drop ocean levels by more than 300 feet. It's an interesting exercise to draw a map of the world in which the land areas are increased to include everything above the 300-foot depth offshore contours. The result is recognizable, but oddly unfamiliar.

However, it is global warming, not cooling, that we are talking about, and here the effects go the other way. About three-quarters of the world's fresh water is locked up in the Greenland and Antarctic ice caps. The thawing of floating ice, like the West Antarctic Ice Sheet, won't affect sea levels, because floating ice already displaces its own weight in seawater. But the melting of overland ice, which is generally a slower process, could raise sea levels worldwide by over 20 feet. You can see the effects of that by drawing 20-foot contours on land, and watching low-lying shorelines sink beneath the waves. We will lose a little agricultural land in this way, but increased plant growth in response to higher carbon dioxide will more than make up for that.

The ultimate Doomsday Machine of global warming, however, is neither rising sea levels nor more variable weather. It is something known as a "runaway greenhouse effect," in which carbon dioxide levels grow higher and higher, beyond growing plant life's ability to compensate for it. This gives steadily increasing temperatures, until the surface of Earth becomes like the boiling surface of Venus and our kind of life is impossible.

Personally, I find this scenario hard to believe. If we have learned anything over the past half-century, it is the resilience and tenacity of life. Humans are certainly blind enough to ignore the consequences of our actions, and someday we may render Earth uninhabitable; but I think it will take more than global warming to do it.

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