Does climate change increase the odds of extreme weather events?

Climate change increased the odds for the kind of extreme weather that prevailed in 2011, a year that saw severe drought in Texas, unusual heat in England and was one of the 15 warmest years on record, scientists reported on Tuesday.

Overall, 2011 was a year of extreme events - from historic droughts in East Africa, northern Mexico and the southern United States to an above-average cyclone season in the North Atlantic and the end of Australia's wettest two-year period ever, scientists from the U.S. National Oceanic and Atmospheric Administration and the United Kingdom's Met Office said.

In the 22nd annual "State of the Climate" report, experts also found the Arctic was warming about twice as fast as the rest of the planet, on average, with Arctic sea ice shrinking to its second-smallest recorded size.

Heat-trapping greenhouse gas concentrations - carbon dioxide, methane and nitrous oxide among others - continued to rise last year, and the global average atmospheric concentration for carbon dioxide went over 390 parts per million for the first time, an increase of 2.1 ppm in 2010.

"Every weather event that happens now takes place in the context of a changing global environment," Deputy NOAA Administrator Kathryn Sullivan said in a statement. "This annual report provides scientists and citizens alike with an analysis of what has happened so we can all prepare for what is to come." 

Beyond measuring what happened in 2011, the international team of scientists aimed to start answering a question weather-watchers have been asking for years: can climate change be shown to be responsible for specific weather events? 

Raising the Odds 

The climate experts acknowledged that event attribution science, as it is called, is in its early stages.

"Currently, attribution of single extreme events to anthropogenic climate change remains challenging," Peterson, Stott and other scientists wrote in a study published in the Bulletin of the American Meteorological Society.

Attribution is possible, they said, as long as it is framed in terms of probability, rather than certainty. So instead of saying climate change caused a heat wave, researchers could gauge how much more or less likely the heat wave was in a world where the climate is changing.

For example, both Texas and England felt the warming effects of the La Nina weather-making pattern but climate change pushed these influences to extremes, Stott said.

La Nina, a recurring patch of cool water in the equatorial Pacific that alternates with the warm-water phenomenon El Nino, would typically bring heat to Texas, the researchers said in an online briefing.

Adding climate change to La Nina makes a Texas heat wave 20 times more likely than it would have been 50 years ago, said Peter Stott of the Met Office. By some measures, 2011 was the warmest, driest growing season in the Texas record, Stott said.

In Britain, November 2011 was the second-warmest in the central England temperature record dating back to 1659, and climate change made that extreme high temperature average 60 times more likely than it would have been in 1960, the researchers found.

By contrast, deadly floods in Thailand last year cannot be blamed on climate change, the scientific team said.

Tuesday's report came one day after NOAA announced statistics for the continental United States, showing that the past 12 months were the hottest such period on record and the first six month of 2012 were the hottest such period on record, with more than 170 all-time heat records matched or broken.

The full report is available online at http://www.ncdc.noaa.gov/bams-state-of-the-climate/2011.php.

Highlights are at http://www.climatewatch.noaa.gov/article/2012/state-of-the-climate-in-2011-highlights/2.

(Reporting By Deborah Zabarenko; Editing by Bill Trott)