United States

By Jillian MacMath, AccuWeather.com Staff Writer

July 19, 2012; 9:39 PM

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2011 had more severe weather events than average, though many were explainable by La Nina, a report issued by NOAA and the American Meteorological Society (AMS) confirmed Tuesday.

La Nina, a weather pattern characterized by below normal sea temperatures across the equatorial central and eastern Pacific Ocean, often causes increased precipitation and tropical activity.

La Nina contributed to some, but not all of the weather events of 2011, the report stated, but "determining the causes of extreme events is difficult."

The report stressed that it is not possible to say that a particular event definitively was or was not caused by climate change, though it is possible to explain "how the odds of such events have changed in response to global warming."

The Texas Heat Wave of 2011, which was the hottest and driest growing season on record for Texas, was associated with La Nina conditions, it said.

A heat wave such as the one that occurred in Texas is now about 20 times more likely to occur during La Nina years than it was in the 1960s.

2011 was also marked by a very warm November in the United Kingdom, contrasting with the very cold December of 2010.

The odds of temperatures as low as in December 2010 have halved as a result of human induced climate change, the report confirmed.

Additionally, the warm November 2011 temperatures are about 60 times more likely than in the 1960s.

LONG TERM TRENDS

For the most part in 2011, long-term trends continued.

Carbon dioxide concentrations in the atmosphere continued to increase, as they have since 1980.

Upper Ocean Heat Content also continued to increase.

"Globally averaged heat stored in the top 2,300 feet of the oceans was the highest since records began in 1992," it stated.

Global temperature was on the rise as expected, too, increasing at a rate of about 0.31 degrees Fahrenheit since 1980.

In general, global surface temperatures were cooler in 2011 than the previous year, likely due to La Nina, though it remained above the 30 year average.

"Climate change has altered the odds of some of the events that have occurred," the report explained.

"Some have become more likely. Some have become less likely. But natural variability also plays an important role."