Climate’s effect on extreme weather is no game of chance

Climate change is supposed to be about climate, you’d think — not weather. After all, climate is what you expect in the long term, like how bad the average winter will be; weather is what you get day to day, like whether there will be frost on Halloween night. Predicting even next week’s weather often seems like a crapshoot.

But seasoned gamblers know not to fold. All the cards in play suggest that climate change isn’t only about the long-term future, but can noticeably alter the planet’s day-to-day weather as well — to the extreme.

It doesn’t take dealer smarts to recognize that 2011 has been a year of wild weather. The United States alone has seen 10 billion-dollar disasters so far, starting with the Groundhog Day blizzard that paralyzed all creatures in Chicago, both below ground and above. The Federal Emergency Management Agency declared a record 87 major disasters.

Texas saw its worst drought in recorded history; tree ring records suggest only one other summer since 1550 has been as severe. Meanwhile, too much rain was the problem in the upper Midwest and along the Mississippi, where rivers breached their levees and flooded wide swaths. And in August, downpours from Hurricane Irene drenched towns in Vermont and elsewhere.

For years, scientists have been wagering on such extremes. “The kinds of changes being  recorded are just what we expect and have been predicted for the human influence on climate,” Kevin Trenberth, a climate scientist at the  National Center for Atmospheric Research in Boulder, Colo., said in Denver in August at a meeting of the American Chemical Society.

At its heart, the link between climate change and extreme weather comes down to water. Warmer temperatures cause more moisture to evaporate from the world’s oceans and enter the atmosphere, where it is available to fall out as rain or snow. Each degree Celsius increase in temperature translates to 7 percent more water vapor that the atmosphere can hold.

And there’s no shortage of fuel for warming. Atmospheric levels of heat-trapping carbon dioxide have soared to 40 percent above preindustrial levels, with more than half of that increase coming since 1970. Natural climate fluctuations such as the El Ni±o/La Ni±a cycle also influence year-to-year temperature changes, but the overall background picture is one of inexorably climbing temperatures. Among many similar statistics, June through August of this year clocked in as the second-warmest summer in U.S. history.

Changes in some kinds of freakish weather, such as heavy rains, heat waves and wildfires, clearly trace to globally rising temperatures. A famous 2004 paper in Nature, for instance, concluded that human influences have probably doubled the risk of a heat wave like the one that killed tens of thousands in continental Europe the previous year.

Such “detection and attribution” studies use climate models to estimate what might be  expected in a globally warmed world and how much natural factors might cause variations from that. One hot research topic now, for instance, is how changes in atmospheric circulation patterns last year pushed rains that would normally fall over Russia toward Pakistan.  Russia got drought and devastating wildfires, while Pakistan got record flooding and millions of evacuees along the Indus River.

But not all weather extremes are a sure bet. Take hurricanes. Some studies have linked rising temperatures to more intense storms, but researchers haven’t nailed down the connection. The same holds true for tornadoes. 2011 is in fourth place for deadliest U.S. tornado year since 1950, thanks mainly to the late April outbreak in and around Alabama and the May devastation in Joplin, Mo. But although tornadoes form in unstable atmospheres rich in water, science can’t yet say for sure whether more twisters have been popping up because of climate change.

The stakes are simply too high to argue that “extremes are changing” across the board, says atmospheric scientist Susan Solomon of the University of Colorado Boulder. “It’s not all events.”

Still, in the roulette wheel of climate attribution, the ball is falling more often than not into slots that belong to human activity. The issue is not whether any one weather extreme was caused by climate change. Perhaps the way to think about it, Trenberth argues, is that it’s highly likely that many of these things wouldn’t have happened without climate change.

So understanding weird weather may need to start with the assumption that humans have changed the environment, and the burden is on scientists to disprove that — not the other way around. It’s a radical inversion of traditional scientific thinking, but one that may be necessary in a radically altered world.

Get ready. The dice are loaded.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.