Climate change can explain some 2011 departures from the norm

By Janet Raloff

Web edition: July 11, 2012
Print edition: August 11, 2012; Vol.182 #3 (p. 14)

Texans sweltered through the hottest, driest spring and summer on record last year. Much of the blame can be attributed to a recurring climate pattern known as La Niña, which emerges every few years as surface waters chill in the eastern equatorial Pacific. But Earth’s steadily warming climate contributed as well, a new analysis concludes.

Since the 1960s, the likelihood of Texas seeing extremely hot, dry weather in a La Niña year has mushroomed 20-fold due to human-induced global warming, David Rupp of Oregon State University in Corvallis and his colleagues calculate.

They were among six international teams probing climate’s link to extreme events in late 2010 through 2011. The collected findings appear in the July Bulletin of the American Meteorological Society, or BAMS.

Severe food shortages, in places causing famine, gripped the Horn of Africa last year after drought left the land parched from winter 2010 through the following spring. La Niña played a role there, too. However, computer analyses of global climate conditions since 1979 find that a recent warming of surface waters in the Indian and Pacific Oceans can destabilize La Niña weather patterns. Chris Funk of the U.S. Geological Survey in Santa Barbara, Calif., concludes that these probably intensified 2011’s drought in East Africa.

Other teams pointed to global warming as a likely contributor to excessive heat in central Europe last summer and to unusually balmy temperatures in central England in November 2011. In the British case, that kind of heat could be expected to recur every 20 years now — a 62-fold increase over the 1960s.

Yet global warming can’t be blamed for all monster weather. Unprecedented flooding that submerged large tracts of northern Thailand, including its capital, for up to two months last year resulted from rainfall intensity the region had encountered before. But water management practices and heavy industrialization of a flood plain slowed drainage last year.

These new analyses are pioneering efforts to get near real-time assessments of climate’s role in extreme weather events, says climatologist Thomas Peterson of the National Climatic Data Center in Asheville, N.C.

For years, he says, climate scientists have argued that although global warming can increase the frequency of extreme weather, they couldn’t pin any particular event on human-caused climate change. That appears to be changing, Peterson and his colleagues argue in their introduction to the new report.

Using the developing field of “attribution science,” researchers are beginning to apply massive computing capacity to explore how global temperatures, reflectivity and moisture patterns can affect the odds of localized extreme weather events.

In 2011, droughts beyond Africa and Texas brought billions of dollars in crop losses, says Jessica Blunden of the National Climatic Data Center. The North Atlantic saw above-average hurricane activity (19 named storms, well over the long-term average of 12), and seven separate U.S. tornado outbreaks each wreaked more than $1 billion in damage.

Polar regions racked up their own extremes, says Martin Jeffries of the University of Alaska Fairbanks, who like Blunden, was an editor of a second new analysis: State of the Climate in 2011, released July 10 as a BAMS supplement. Barrow, Alaska, sustained a record 86 consecutive days when the minimum air temperature failed to dip below freezing.

Understanding global warming’s role in extreme events extends well beyond blaming rights. Peterson notes that water managers may need to change policies if evidence begins pointing to persistent changes in the recurrence rates and lengths of droughts or the frequency of heavy rains. Right now, linking these events is difficult, usually works only for events lasting longer than a month, and takes a year to complete. Peterson’s team hopes to see the science mature to the point that assessments might be turned around more quickly and tackle events lasting mere days.