It was hot in July - just ask anyone from Texas. Global warming, right?
Some politicians - like Vice President Al Gore - would agree. But such claims make for better politics than science.
A hot summer -or any other weather event - isn't proof that human activity is causing the Earth to warm, scientists say. And it's by no means clear that worldwide weather trends have gotten worse in the way that global- warming models have long predicted.
Of course, such changes could be just around the corner.
Some scientists argue that the decades of the '80s and '90s have been the warmest in six centuries, although before that the climate may have been even warmer than it is today. It's only a matter of time, they contend, before that warming changes the weather, causing more droughts, floods, heat waves and severe storms.
In December, the White House agreed to the Kyoto Protocol, a treaty that requires developed countries to curb the output of greenhouse gases.
Since then, it has turned up the heat on the issue of global warming. It's looking to build support in a Senate reluctant to ratify the treaty, due to the harm it might inflict on the economy.
To hear Gore tell the story, global warming is already causing catastrophic weather. He says he has little doubt that such disasters will become more common unless cuts occur in greenhouse gases, some of which come from burning fossil fuels like coal and oil.
In fact, Gore has hardly missed a chance in recent years to link extreme weather - from ice storms to wildfires - to global warming.
January's ice storms in the Northeast:
"We live in a time in our country and the world where it seems weather catastrophes are more common. We now have 100-year floods every two years, it seems," Gore told Maine lawmakers, according to The Boston Globe.
The ice storm that crippled Maine "could be a sign of things to come," given global warming, he said.
June's wildfires in Florida:
"These fires offer a glimpse of what global warming may mean to families across America. And that is why it is so critical that we get on with the job of cutting greenhouse gas emissions," Gore said.
Warmer-than-normal temperatures for the month of July:
"It would be hard to ignore that something's going on - and that something is global warming," Gore said at a White House briefing to announce July's temperature data. "Unless we act, we can expect ever more extreme weather - more heat waves, more flooding, more powerful storms and more drought."
There's not much that scientists agree on when it comes to global warming. One point of common ground, though, is that there's no way to say any particular weather event is due to global warming.
"There is a problem with making a lot of hay out of one individual event," such as the drought in Texas, said Gerald North, head of Texas A&M's meteorology department. "The climate is the average of all those events happening all over the world."
The first seven months of this year were the warmest worldwide in a century, the National Climatic Data Center recently reported.
But even that's not necessarily hard proof of global warming, scientists say. There are simply too many natural climate variations - which have nothing to do with humans - to tell for sure what's behind the changes.
"There are probably multiple causes, and we don't know for sure what warming may be human induced or what may just be natural fluctuations," said Robert Quayle, who heads the NCDC in Asheville, N.C.
For starters, global-warming theory is meant to predict only long-term changes in climate, not short- term spikes, North said.
And temperatures this year have already started to return to normal, according to Patrick Michaels, a climatologist at the University of Virginia. In fact, for the year as a whole, temperatures could come in below normal, he says.
A more likely explanation is the El Nino weather pattern, which typically causes a spike in global temperatures. This winter's El Nino was stronger than any since '82-'83, when several massive volcano eruptions helped mask El Nino's warming. (Volcanoes spew ash into the atmosphere, and the ash acts as an umbrella to shade and cool the planet).
Still, if global warming did come to pass, climate models predict that weather around the world would change. And average worldwide temperatures have increased - about a half-degree Celsius - over the 20th century.
"The globe has warmed since the 19th century," said John Christy, climatologist at the University of Alabama at Huntsville. "But that was probably the coldest century of the last ten. It's difficult to detect what warming is human induced and what is the result of the natural fluctuations of the global temperature."
If the globe were to warm substantially, more droughts, heat waves and severe storms would occur, climate models predict.
Has this happened?
"Overall there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20th century," a '95 report by the U.N. Intergovernmental Panel on Climate Change found.
While there's a drought in Texas and the Southeast right now, there hasn't been an increase in the frequency of droughts worldwide, says the NCDC's Quayle.
What about more numerous and more severe storms? Here, the evidence is mixed. Research by Tom Karl of the National Oceanic and Atmospheric Administration suggests that the world has seen more rain and snow coming in heavy storms, and less in constant showers.
But a '96 study of hurricanes and tropical storms in the Atlantic basin over the last 50 years shows they have not become more severe - and they have happened less often.
"What bothers me is some of the scientists saying this (that the weather is getting worse)," said Peter Rogers, professor of environmental engineering at Harvard University. "The data just don't support that."
"It may be that a lot more damage is done because we're much more at risk," Rogers said. "More people may now live in places that are vulnerable to extreme weather."
So while disasters may not happen more often, they do more damage when they do occur.
In fact, Texas A&M's North says scientists can't be absolutely sure what would happen to the weather if the globe warmed.
"Some of the climate models say we will have more severe weather, but I'm skeptical of that," North said.
Why? The science of modeling the immensely complex climate is new and uncertain, North says. Each way of producing the most basic measure -global temperature - is hotly disputed.
"This is a science with a lot of bad data problems," the NCDC's Quayle said.
For example, scientists don't yet understand how the oceans that cover about 70% of the earth's surface interact with the atmosphere. So climate models can't yet account for that. Nor do they take stock of the effects of cloud cover.
North figures those models will keep improving with more research. Within a decade, he thinks most of these problems will be solved.
Indeed, models have improved a great deal over the past decade. As they have improved, they tend to predict less global warming, not more.
The models used by the IPCC for its '95 report predict warming of between 1 and 2 degrees Celsius over the next century. Just five years before, the IPCC predicted 3.3 degrees.
"The models overestimated the effect of carbon dioxide in the atmosphere," Michaels said. "They also overestimated the rate of increase of carbon dioxide accumulation in the atmosphere. It's being taken up by greater vegetation."
All the uncertainty built into climate models makes it hard to say for sure what's going to happen.
"If you go to one extreme, you can think we're going to boil, and if you go to the other extreme, you can think there is no warming and there ain't going to be any," North said. "And you'd still be pretty consistent with what the models say."
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