Why has the weather gone cuckoo?

After historic flooding along the Mississippi River, record tornado sightings, and even a twister in Northeast Philadelphia last week, it is reasonable to ask if the atmosphere has gone out of its mind.

Has worldwide warming pushed it to the brink of riot? Or is its behavior more like that of a beloved relative who has been eccentric all along but we've started noticing?

To the chagrin of some scientists, variants of these questions are being disputed heatedly, polemically - and sometimes even entertainingly - in traditional media and on the Web.

As Richard B. Rood, an atmospheric scientist at the University of Michigan, has noted, the debates are driven by "many agendas," but way down on that list is the "rational pursuit" of answers.

Less visibly on the public radar, however, serious researchers have undertaken ambitious projects to come up with ways to quantify how subtle warming, and human contributions to it, might be affecting day-to-day weather.

"There's been a lot of thought put into that," Rood said last week. And given the events of the last year or so, those projects have taken on fresh urgency. Said Rood: "At this point, there's a lot of demand to quantify it."

Indisputably, this has been a period for wild weather. In the United States, last month's working tornado total, 875, was more than triple the old April record, 267, set in 1974. The Mississippi water levels have exceeded those of the historic 1927 floods.

What might warming have to do with all that?

Various studies have found that water vapor in the atmosphere is on the increase, and that extreme precipitation events have become more common. The World Meteorological Organization reported that the first decade of the 21st century was the wettest in the period of record, dating to 1850.

The Mississippi was engorged by snowmelt and heavy April rains near its junction with the Ohio River. The twisters were fueled by vapor-rich water off an abnormally warm Gulf of Mexico.

Case sealed? Not so fast.

Measurement, it turns out, is a complicated and messy business.

The National Climatic Data Center has been trying its hand at it, publishing a monthly U.S. Climate Extremes Index, which chronicles temperature, precipitation, and drought anomalies. It does show a general increase in extreme weather in the United States in the last 20 years, but the trends are not linear.

Worldwide, while precipitation is on the increase in the Northern Hemisphere's higher latitudes, it has been decreasing in other areas, including tropical Africa, the climate center notes.

Perhaps the most ambitious attempt to track worldwide trends is the 20th Century Reanalysis Project at the Earth System Research Laboratory in Boulder, Colo.

Examining air-pressure patterns, the team led by Gilbert Compo inferred other key variables in the atmosphere and built a database dating all the way to 1871. The analysis has been described as a "climate time machine."

Some of the conclusions were surprising. The team, for example, did not find big changes in key air-circulation patterns over the North Atlantic and Pacific or the tropical Pacific that govern world weather.

Why bother with air pressure?

Barometric pressure, a once-familiar concept that has all but leaked out of our post-agrarian brains, is a critical variable. Areas of high pressure, or heavier air, alternate with masses of lower pressure, or lighter air, around the planet. Pressure contrasts drive winds throughout the atmosphere and spin up storms.

We truly do inhabit the bottom of an ocean of air, all but oblivious to the mighty waves crashing overhead - until they affect us below.

As climate changes, as it has for eons, pressure patterns realign, affecting the weather.

But the question of whether an individual weather event is a result of climate change is not only unanswerable, said Rood, but also misguided.

For one thing, it assumes that the climate system somehow went precipitously from a "natural" to a postindustrial state. It is not at all clear just how much human activity has affected the atmosphere.

He argues further that it holds climate to be utterly walled off from weather, when the two are inextricably related. A warmer world might well mean a wilder one.

"Asserting that an extreme weather event is not representative of climate change is also false," he said in a recent article.

A better question he said, would be: Has human activity made a given extreme event more likely?

"In these cases," he said, "the answer is often yes."