will fluctuate. That is not to say that fears of global warming, lately incited anew by evidence that the Arctic and Antarctic ice caps are thinning, are groundless. Indeed, a growing number of scientists have concluded that human activities are dramatically warming the Earth.

But global climate change is very complicated. Warming of the Earth might be entirely or partly explained by causes other than humankind's relentless pumping of greenhouse gases into the air.

There is obviously the sun, for example, and such other factors as volcanoes and other sources of geothermal heat. For practical reasons, climate models have to assume that sunlight, the overwhelming source of heat on he planet's surface, is constant, though, with sunspots and solar flares, it's obvious that this is not true. And sunlight presumably varies enough to cause widespread climate change.

That might be the most likely explanation for the huge changes of the (geologically) recent past. Varying solar power could explain why some 12,000 years ago, New England was buried under a mile of ice, and yet, a thousand years ago, Norse settlers grew crops on Greenland -- before the invention of gas-guzzling SUVs.

What is most novel about the current situation is not necessarily that there might be elevated levels of carbon dioxide and other greenhouse gases in the atmosphere but rather the much greater amount of information we now have about climate. For instance, we have satellites that can inform us when icebergs the size of Connecticut calve from the Ross Ice Shelf.

In any event, it is good to retain some scepticism about alarming data. The Earth is a big place, and the sun much bigger, and we can never have enough information with which to explain it, or with which to predict dramatic changes.

And more information is coming. One source is the Global Positioning System, satellites that provide surveyor's-transit accuracy for any point on the planet. GPS is best known now as a tool in electronic navigation. But recently, the deliberate scrambling of GPS signals for national security reasons was removed. This means that scientists will finally have a tool that can observe water-vapour distribution in the atmosphere in three dimensions.

This could dramatically improve weather forecasting, as well as our ability to model global climate change.

Ultimately, we will discover all the mechanisms behind climatic change. But until that time, it would be well to remember that humans are not the only things that can affect climate, even if some of them do drive around in cars that are too big. -- Providence Journal-Bulletin