Climate change is changing the odds of some extreme weather events, according to new research by government scientists in the United States and Britain.

Back-to-back La Niña cycles helped create the scorching heat wave that drove Texas' record-breaking drought last year, but climate change also played a role, the researchers report in the Bulletin of the American Meteorological Society.

That type of severe heat wave is 20 times more likely to occur during a La Niña today than it was during a La Niña in the 1960s, say researchers at the National Oceanic and Atmospheric Administration and the U.K. Meteorological Office.

"What this is saying is it's a combination of La Niña variability, but there's also an additional component from longer-term warming," said study co-author Tom Peterson, a climate scientist at NOAA's National Climatic Data Center.

La Niña's influence was also evident on many of the weather extremes during a year that saw an onslaught of natural disasters, including a record-breaking 14 $1 billion-plus extreme weather events in the United States.

"In addition to East African drought, La Niña also played a role in droughts that occurred thousands of miles away in Mexico and southwestern United States, among the worst on record in both countries," said Jessica Blunden, another NCDC climate scientist.

And the weather pattern also helped cool the planet briefly, though 2011 still ranks among the 15 warmest years on record, NOAA said yesterday in its annual "State of the Climate" report, compiled by 378 scientists in 48 countries.

'Cooler' becomes a relative term
Conditions were unusually warm compared with other years when La Niña was present. While 2011 stands as the coolest year since 2008, "the word 'cooler' is actually relative," Blunden said.

The global average temperature last year still exceeded the average temperature recorded between 1981 and 2010 -- the three warmest decades since record-keeping began 130 years ago.

"Long-term trends are continuing to show what we'd expect in a warmer world," said NCDC Director Tom Karl.

Those trends are especially pronounced in the Arctic, which continues to warm roughly twice as fast as the planet's lower latitudes. Last year, the northwestern Alaska town of Barrow experienced a record 86 consecutive days in which the daily low temperature did not dip below freezing.

"The Arctic is clearly experiencing the impacts of a prolonged and intensified warming trend," said Martin Jeffries, Arctic science adviser to the Office of Naval Research.

This year, the lower 48 states are also sizzling. Yesterday, NOAA said the first six months of the year were the warmest January to June in the contiguous United States since record-keeping began in 1895.

Meanwhile, the level of heat-trapping gases in the atmosphere continues to climb. The global average level of carbon dioxide broke the 390 parts per million barrier for the first time last year, an increase of 2.10 ppm from 2010. Levels of methane and nitrous oxide in the atmosphere also rose.

But determining the role climate change has played in individual extreme weather events is tricky, researchers said.

"A lot of people want to know shortly after an extreme event, what were the causes of it? How are these things changing over time?" said Peterson. "But this is an evolving science."

While it's true that scientists can't say with certainty whether climate change caused a given weather event, that is the wrong question to ask, experts said.

When climate gives events an extra push
Instead, they are examining whether climate change is changing the odds of natural disasters like floods, droughts, heat waves and hurricanes.

In the case of the 2011 heat wave in Texas, authors of the new U.S.-U.K. research find that adding climate change to La Niña makes scorching heat far more likely.

They reached similar conclusions about unusually warm conditions in central England last November, the second-hottest since record-keeping began in 1659. The scientists say that kind of warmth is 60 times more likely today than it was in the 1960s.

The extreme cold that blanketed the same area in December 2010 is now half as likely to occur now as it was 50 years ago.

But for some natural disasters, including catastrophic floods last year in Thailand, there is no clear connection to man-made climate change.

"We do not see evidence of a human role in all weather extremes," said Peter Stott, climate monitoring and attribution team leader at the U.K. Met Office. "Natural variability is still playing an important role."

In the case of the Thailand floods -- the worst in that country since 1942 -- examination of climate records showed the amount of rainfall that fell was not unusual.

The scientists believe other factors -- like changes in reservoir policies and increased construction in floodplains -- are to blame for the devastating deluge.
