Greenhouse gases, and not the ocean current El Nino, accounted for temperatures in the United States that were close to a record high last year, U.S. government climate scientists said Tuesday.
According to data from the National Oceanic and Atmospheric Administration (NOAA), the average annual temperature in the 48 contiguous states was also 1.2 C above the 20th-century average, the ninth consecutive year of above-normal U.S. temperatures.
The UN-led Intergovernmental Panel on Climate Change (IPCC) issued the first of three reports on climate change in February this year, attributing rising temperatures to rising greenhouse gas levels, particularly from man-made activities such as burning fossil fuels.
But because El Nino was active in both 1998 and 2006, two of the hottest years on record, the researchers had to consider the possibility it, too, could have been responsible for 2006's near-record-high temperatures.
El Nino is a system of currents in the Pacific Ocean that lies off the west coast of South America and periodically forms, causing changing weather patterns throughout the world.
"We wanted to find out whether it was pure coincidence that the two warmest years on record both coincided with El Nino events," said Martin Hoerling of the NOAA's Earth System Research Lab.
"We decided to quantify the impact of El Nino and compare it to the human influence on temperatures through greenhouse gases."
The researchers looked at10 past El Nino events observed since 1965 and conducted two sets of simulations, one with the El Nino and one without. The result, they discovered, was a slight cooling across the country in the El Nino years.
When the group conducted simulations based on climate models provided by the IPCC, they found a warming pattern similar to that observed in 2006.
The study will be published next week in the Sept. 5 issue of the Geophysical Research Letters, a publication of the American Geophysical Union.
The NOAA said 2006 was the second-warmest year in the U.S., approximately 0.4 C cooler than 1998.
However NASA scientists were recently forced to concede that 1998 itself was cooler than 1934 in the U.S. after Toronto blogger Stephen McIntyre found inconsistencies in NASA data collection.
Climatologists at NASA's Goddard Institute of Space Sciencein New York attributed the error to a new data collection system established in 2000 that used different methods than the previous one.
The new figures now show that 1998 is the U.S.'s second-hottest year and that five of the 10 warmest years on record in the U.S. date from before 1939. The worldwide numbers remain unchanged, with 1998 and 2005 tied as the hottest year on record.