15 February 2013

Seeing the hand of climate change in weather extremes

Posted by kramsayer

A picture taken from the International Space Station in September 2011 shows wildfires across a drought-stricken east-central Texas. (Credit: NASA)

BOSTON – Extreme weather events have been cropping up all around the globe recently, and one place that’s been really hit hard is Texas. The drought that slammed that state in 2011 has already caused $7.6 billion in agricultural losses, sparked the sixth most devastating wildfire in the U.S., and coincided with the windiest spring on record for the state, according to Texas State Climatologist John Nielsen-Gammon. And, still, the dry weather has not let up.

“This was historic,” Nielsen-Gammon said Friday morning at a talk on extreme climate and weather, at the annual AAAS meeting in Boston, Mass. That became glaringly obvious from a plot Gammon projected onto meeting room’s screen, showing record Texas temperatures and precipitation levels since the late 1800s. The dot representing 2011 was all on its own, an extreme outlier for the century of data. That year, the temperature was 5.5 degrees Fahrenheit above normal, almost double the previous record of about 3 degrees above normal.

How much did climate change factor into Texas’s weather woes? The fierceness of this heat and drought “is not the sort of thing we expect to see being normal,”Nielsen-Gammon said. But, with a shifting climate, “it’s the sort of thing that can occur potentially more and more frequently.”

Yet, teasing out the role of climate change in the ongoing Texas drought isn’t simple, Nielsen-Gammon said. Over the last century, the amount of precipitation has actually increased about 10 percent, he noted. So the lack of rain can’t be directly attributed to climate change. And droughts in Texas are strongly influenced by global ocean temperature patterns, like El Niño and La Niña, and scientists remain unsure how climate change will impact those patterns. However, human-caused climate change likely made the drought period hotter than it would have been otherwise, Nielsen-Gammon said.

Another speaker at the Friday AAAS session, atmospheric scientist Donald Wuebbles of the University of Illinois in Urbana argued that, although weather naturally has extremes, climate change is now a regular player throughout the system.

“Every weather event nowadays takes place in the context of a changed background environment,” Wuebbles said. “Nothing is entirely natural anymore.”

Reflecting that new reality, he added, the United States has experienced an increase in several types of extreme weather events in the last five decades, such as heat waves and heavy rainfalls. Also on the rise as well is the number of natural disasters in the U.S. that cause a billion dollars or more damage– with a record 14 in 2011 and 11 in 2012.

Taking yet another perspective, Wuebbles said you can gauge the changing climate of the United States by looking at the number of temperature records broken each year. In the 1950s, it was about equal – as many “hottest ever” days across the country as “coldest ever” reports. By the decade starting in 2000, however, there were twice as many heat-related records as cold-related counterparts. And the ratio soared in 2012.

“This last year,” Wuebbles said, “it was closer to 10 to 1.”

–Kate Ramsayer, AGU science writer