For the first time ever, scientists behind one of the world's most comprehensive weather assessments say they can perceive the likely impact of human-influenced climate change on specific extreme weather events.

What the study found was increasing evidence that specific events, and patterns of events, can now safely be attributed to man-made global warming and its growing impact on intense storms, extreme floods, unusual cold spells, prolonged heat waves and drought.

The 'State of the Climate' report, issued jointly each year by the National Oceanic and Atmospheric Administration (NOAA) and the American Meteorological Society (AMS), looks at global weather events, climate patterns, and the implications of flunctuating air temperatures and ocean currents.

This year, the group also released a supplemental paper, titled Explaining Extreme Events of 2011 from a Climate Perspective (pdf), which looks specifically at extreme weather events through the lense of global climate change.

“Every weather event that happens now takes place in the context of a changing global environment."
- NOAA's Kathryn D. Sullivan

“2011 will be remembered as a year of extreme events, both in the United States and around the world,” said Deputy NOAA Administrator Kathryn D. Sullivan, Ph.D. “Every weather event that happens now takes place in the context of a changing global environment. 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.”

Peter Stott, from Britain's National Weather Service which also contributed to the report, said: "We are much more confident about attributing [weather effects] to climate change. This is all adding up to a stronger and stronger picture of human influence on the climate."

"While we didn't find evidence that climate change has affected the odds of all the extreme weather events we looked at, we did see that some events were significantly more likely. Overall we're seeing that human influence is having a marked impact on some types of extreme weather."

The Guardian's Fiona Harvey reports:

Attributing individual weather events, such as floods, droughts and heatwaves, to human-induced climate change – rather than natural variation in the planet's complex weather systems – has long been a goal of climate change scientists. But the difficulty of separating the causation of events from the background "noise" of the variability in the earth's climate systems has until now made such attribution an elusive goal.

To attribute recent extreme weather events – rather than events 10 years ago or more – to human-caused climate change is a big advance, and will help researchers to provide better warnings of the likely effects of climate change in the near future. This is likely to have major repercussions on climate change policy and the ongoing efforts to adapt to the probable effects of global warming.

Researchers found the 2011 crop-destroying drought and heat wave in Texas was "roughly 20 times more likely" the result of man-made climate change -- warming due to greenhouse gasses -- than of natural climate variation, CBS News reported.

Other key findings:

• Warm temperature trends continue: Four independent data sets show 2011 among the 15 warmest since records began in the late 19th century, with annually-averaged temperatures above the 1981–2010 average, but coolest on record since 2008. The Arctic continued to warm at about twice the rate compared with lower latitudes. On the opposite pole, the South Pole station recorded its all-time highest temperature of 9.9°F on December 25, breaking the previous record by more than 2 degrees.
   
• Greenhouse gases climb: Major greenhouse gas concentrations, including carbon dioxide, methane, and nitrous oxide, continued to rise. Carbon dioxide steadily increased in 2011 and the yearly global average exceeded 390 parts per million (ppm) for the first time since instrumental records began. This represents an increase of 2.10 ppm compared with the previous year. There is no evidence that natural emissions of methane in the Arctic have increased significantly during the last decade.
   
• Arctic sea ice extent decreases: Arctic sea ice extent was below average for all of 2011 and has been since June 2001, a span of 127 consecutive months through December 2011. Both the maximum ice extent (5.65 million square miles, March 7) and minimum extent (1.67 million square miles, September 9) were the second smallest of the satellite era.
   
• Ozone levels in Arctic drop: In the upper atmosphere, temperatures in the tropical stratosphere were higher than average while temperatures in the polar stratosphere were lower than average during the early 2011 winter months. This led to the lowest ozone concentrations in the lower Arctic stratosphere since records began in 1979 with more than 80 percent of the ozone between 11 and 12 miles altitude destroyed by late March, increasing UV radiation levels at the surface.
   
• Sea surface temperature & ocean heat content rise: Even with La Niña conditions occurring during most of the year, the 2011 global sea surface temperature was among the 12 highest years on record. Ocean heat content, measured from the surface to 2,300 feet deep, continued to rise since records began in 1993 and was record high.

• Ocean salinity trends continue: Continuing a trend that began in 2004 and similar to 2010, oceans were saltier than average in areas of high evaporation, including the western and central tropical Pacific, and fresher than average in areas of high precipitation, including the eastern tropical South Pacific, suggesting that precipitation is increasing in already rainy areas and evaporation is intensifying in drier locations.

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