Extreme weather: is global warming to blame?

Climate experts warn us that the first sign of a shifting climate will be turbulent, unpredictable weather. So when Hurricane Jeanne crashed ashore in Florida in September of 2004--the fourth hurricane to blow through the region in less than six weeks--environmentalists couldn't help but wonder if this was it: climate change in action. It seems as though the past few years have been characterized by all sorts of weather extremes--driest, hottest, coldest, wettest. We know that the planet has warmed 1.8 degrees Fahrenheit in the past 100 years, but is there really a link between our weather and climate change?

The devastating 2004 hurricane season can only partially be attributed to climate change, experts say. "There are a number of factors that go into making hurricanes," says Ruth Curry, research specialist at Woods Hole Oceanographic Institute. Those factors include El Nino cycles, upper stratospheric circulation patterns and the amount of rainfall in the Sahel region of Africa. Sometimes they combine to create conditions ripe for hurricanes and sometimes they work against each other. The 2004 hurricane season is primarily attributed to alignment of these three critical elements.

Does climate change play any role? "You can't attribute one storm to climate change, but you can look at how the phenomenon fits into a pattern," says Ross Gelbspan, author of the book Boiling Point. "You can't attribute one case of cancer to smoking, but you can look at the epidemiology."

The general scientific consensus on climate change and hurricanes is this: Hurricanes won't necessarily become more frequent, but they will become more intense. While ocean and atmospheric circulation is the engine of a hurricane, heat is the fuel. "In order to form, a hurricane must have ocean temperature of at least 80 degrees down to a depth of 164 feet," says Curry. "Sea surface temperatures all over the tropics are running 1.8 to 3.6 degrees above normal. This is due to global warming." Thus, when other factors line up to form a storm, a warmer ocean means it will be all the more powerful and destructive.

The five-year (and counting) drought afflicting the western states has also stirred murmurings of climate change. Again, climate change is just one factor among many. The relationship between heat and moisture is the key to understanding most weather patterns. While most of us tend to think heat waves cause drought, it is actually the reverse: dryness creates heat.

"The first thing that happens after a rain storm is that the sun comes out and the puddles dry up," explains Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research. Only when the majority of surface water is evaporated does heat begin to kick up. Climate change may contribute to more erratic rainfall, creating conditions ripe for heat waves in some places, but it is difficult to distinguish how much of a heat wave is due to a warmer planet versus natural variation in rainfall.

The current droughts in the western states are reinforced by shorter winters, result a possible of climate change. In the mountains, there are several weeks each year when precipitation that once fell as snow now falls as rain. "In the Sierra Nevadas all of the peak run-offs are now occurring one to two weeks earlier than 30 years ago," says Trenberth. "As you go into the summer there is less snow, so there is less soil moisture and less water to evaporate. Then you get a feedback process; since there is no moisture for clouds to form, there is no further rainfall." Trenberth is, however, cautious about blaming it all on climate change. "I don't know if you want to attribute it entirely to climate change, but it is probably a bit worse than it otherwise would be," he says.

Climate change may be making heat waves more deadly. "Here's the connection," says Gelbspan. "As the temperature is rising, the nighttime lows are going up twice as fast as the daytime highs." Whereas the planet would normally radiate excess heat back into space at night, the greenhouse gasses are trapping it in, meaning less relief at night. "When people get heat stress during the day, they don't get the evening release to let their bodies recuperate," says Gelbspan. Those who lack air conditioning, most often the elderly and poor, are the hardest hit. In 2003, extreme heat killed 30,000 people in Europe. A 1996 heat wave in Chicago killed 800 people.

Cold snaps make it easy to dismiss global warming as myth. But the heat-moisture equation means that increased evaporation in one area can lead to increased precipitation in another. "If the conditions are wet, then more heat goes into evaporating moisture, so things don't warm up," says Trenberth. A more erratic rainfall pattern means that each region experiences an extreme of the expected weather. Right now, while the western states are dry, the eastern states are quite soggy. Under these conditions, the resulting temperature differences are to be expected.

"In the winter of 2003 it was eight degrees below normal in the eastern United States and 12 degrees above normal in the west," Trenberth points out. However, despite a spell of harsh winters, spring is arriving earlier in the Northeast, where the frost-free season is 11 days longer today than 60 years ago, a possible result of climate change. This may be good news for those who dislike the cold, but bad news for plants and animals whose behavior is cued by these elemental signals, allowing pests, for example, to get a jumpstart on crops.

Critics suggest that our perception of these weather extremes is influenced mainly by sensationalized news coverage, and that little about the weather has actually changed in the past century. Patrick Michaels of the libertarian Cato Institute writes in his book Meltdown that any major weather trends that have been observed in the past century can't be definitively linked back to climate change. He challenges the certainty of the temperature-precipitation equation, noting that the 1930s and 1990s, the warmest decades in the U.S. in the 20th century, were the driest and wettest respectively.

What is clear is this: Weather is a mix of constants and variables. The constants are those things that dictate general climate conditions, such as the proportion of the Earth's surface covered in water, atmospheric chemistry, and the orbit and tilt of the Earth, which shift slowly over the millennia. These factors tell us when the seasons will shift and which regions are defined as arid, temperate, tropical and arctic. The variables, such as temperature, humidity, and ocean and air currents, are derived from these climactic constants, giving rise to regionally specific daily weather patterns.

Global warming will result in a shifting of the constants--those factors that we expect to be relatively stable--when the oceans expand, the land masses shrink, and the composition of the atmosphere changes. So within the span of a human lifetime, the implications for the weather may indeed be profound. If, as the saying goes, climate is what you expect and weather is what you get, then climate change means never knowing quite what to expect. CONTACT: National Center for Atmospheric Research, [phone], www.ucar.edu.