WEATHER WATCHERS have a pretty good idea what was going on in the atmosphere last weekend, and how that succeeded in whipping up more than 70 tornadoes along a front that stretched from Ohio to Alabama. It was an unusual confluence of a lot of cold air sweeping quickly in from one direction and rubbing up against an equal amount of warm air piling up not quite as quickly from another.

Whole communities were wrecked and at least 36 people killed.

But how come? What's with all this extreme weather? Floods in Europe and China, drought in Africa and Afghanistan, now twisters from Canada to Mexico -- this isn't like it used to be, is it?

It's tempting to finger global warming as the villain. The eastern part of the country has lived through a warm decade, a hot summer -- all that energy added to the atmosphere must be doing something.

But evidence is wanting. There have in fact been four such tornado clusters in the United States in the past 30 years. One in 1974 was considerably more devastating than this year's -- but who remembers the weather from 28 years ago? The incidence of reported tornadoes jumped about 25 percent in 1990 and has remained high. But there really isn't enough data going back before that to make a clear comparison.

A more nuanced sort of response would be this: Last week's killer weather may not have been caused by global warming, but Americans had better get used to terrible storms because when global warming comes there'll be a lot more of them.

That's not entirely off-base. Late last month, in New Delhi, an international conference on climate change focused less on how to combat global warming and more on how to live with it. Developing nations favor that approach (because they don't want to hurt growth by cutting emissions); so do American businesses (ditto).

But no one knows whether global warming will cause more tornadoes, or more likely do something completely unexpected. And now comes a study in the journal Nature that found that North America is subject to regular spells of unimaginably vicious storms, every 3,000 years or so, and is about ready for another one. So, move over, global warming -- worse disasters may be afoot.

This, then, is the real answer to what's going on -- no one knows for sure, because it's way too hard to grasp the total picture. Come back in a millennium or five and things may be clearer.

But for now, at least, people and nations can be on guard. It's a nasty world out there; fooling with global climate change is playing with fire. Prepare for trouble, like the manager of that movie theater in Van Wert, Ohio, and duck the next time you see an Oldsmobile flying at you.