"We have met the enemy, and he is us," the comic-strip character Pogo said decades ago. A new analysis of last year's near-record temperatures in the United States suggests he was right.
Warming caused by human activity was the biggest factor in the high temperatures recorded in 2006, according to a report by researchers at the National Oceanic and Atmospheric Administration.
In January, NOAA's National Climatic Data Center reported that 2006 was the warmest year on record over the 48 contiguous states with an average temperature 2.1 degrees Fahrenheit warmer than normal and 0.07 degree warmer than 1998, the previous warmest year on record.
In May, however, NOAA revised the 2006 ranking to the second warmest year after updated statistics showed the year was actually .08 F cooler than 1998.
At the time the agency said it was not clear how much of the warming was a result of greenhouse-gas induced climate change and how much resulted from the El Nino warming of the tropical Pacific Ocean that was under way.
"We wanted to find out whether it was pure coincidence that the two warmest years on record both coincided with El Nino events," Martin Hoerling of NOAA's Earth System Research Laboratory in Boulder, Colo., said in a statement.
His study looked at the effects of El Nino in the past as well as the effects of the release of gases such as carbon dioxide into the atmosphere by human industrial activities.
The analysis of past El Nino events in the 20th century found that the result was a slightly colder than normal annual average temperature over the 48 contiguous states.
To double check that, the researchers conducted two sets of 50-year computer simulations of U.S. climate, with and without the influence of El Nino. They again found a slight cooling across the nation when El Nino was present.
Then they looked at the effect of the increased greenhouse gases — which are given that name because they can help trap heat from the sun somewhat like a greenhouse traps heat.
They ran 42 different tests using complex computer models to simulate changes in the atmosphere under various conditions and concluded that the "2006 warmth was primarily due to human influences."
While Hoerling's study focused on the United States, NOAA also tracks world climate. Worldwide, 2005 was the warmest year on record, topping 1998, according to the agency.
The research was supported by NOAA's office of Global Programs.