Washington — The year 2011 will be remembered for extreme weather events, most notably two occurrences of the Pacific La Niña phenomenon that contributed to droughts in some places and worse-than-average cyclone seasons in others.
The National Oceanic and Atmospheric Administration (NOAA), in partnership with the American Meteorological Society, released the 2011 State of the Climate Report July 10. Compiling the work of more than 375 scientists from almost 50 countries, the report assesses extreme weather patterns, related through a common link of climate change.
"Every weather event that happens now takes place in the context of a changing global environment," said NOAA Deputy Administrator Kathryn Sullivan. "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."
La Niña is a weather pattern characterized by cooler-than-average water temperatures in the eastern equatorial Pacific. It typically can influence rainfall, drought and storm systems across a wide swath of the globe. When the pattern developed twice in 2011, the climate report says, La Niña triggered historic droughts in East Africa, the southern United States and northern Mexico, while it also influenced an above-average tropical cyclone season in the North Atlantic.
Australians saw the end of a two-year drought with record rainfalls due to the 2011 La Niña influence.
Both the planet's polar environs recorded abnormalities in 2011, the climate report finds. Warming more rapidly than the rest of the planet for some time, the Arctic temperatures increased at twice the rate of lower latitudes. Arctic sea ice showed rapid melting, declining to its second smallest summer minimum, as older ice also reached a new record minimum.
Monitoring stations at the South Pole recorded the highest temperature ever seen, a fraction below 10 degrees Fahrenheit (minus 12.3 Celsius), warmer than the previous record by more than 2 degrees (1.1 C).
A NOAA press release says the climate report draws on four independent data sets, all indicating that 2011 was among the 15 warmest years since reliable record keeping began in the late 19th century. The findings are based on 43 indicators tracking climatic conditions, including greenhouse gas concentrations, lower and upper atmosphere temperatures, cloud cover, sea surface temperature, sea level rise, ocean salinity, sea ice extent and snow cover. Each of those indicators includes thousands of measurements from multiple independent data sets.
Despite global awareness about the connection between greenhouse emissions and a warming planet, the level of carbon dioxide in the atmosphere increased in 2011, the report says, exceeding 390 parts per million (ppm), an increase of about 2 ppm from the previous year.
The report cautions about the difficulty of citing climate change as the direct cause of any single weather event. However, the report said, new research helps scientists understand how the probability of extreme events changes in response to global warming.
For example, the southern U.S. state of Texas is 20 times more likely to experience severe heat waves in connection with La Niña now, the report says, as it was during a La Niña occurrence 50 years ago.