"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.
The report, by federal government researchers at the National Oceanic and Atmospheric Administration, found that warming caused by human activity likely accounted for more than half of the high temperatures recorded in 2006.
Released Tuesday, the analysis is being published in the September issue of Geophysical Research Letters, published by the American Geophysical Union.
The study reflects the effort by researchers to separate natural and manmade factors in any given warming trend.
"Now we have the capability, on the spatial scale of the United States, to better distinguish natural climate variations from climate changes caused by humans," study co-author Martin Hoerling said in a statement.
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.
El Nino question mark
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," said Hoerling, a researcher at NOAA's Earth System Research Laboratory in Boulder, Colo. "We decided to quantify the impact of El Nino and compare it to the human influence on temperatures through greenhouse gases."
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."
The team also found that greenhouse gas increases "enhanced the probability of U.S. temperatures breaking a record in 2006 by approximately 15-fold compared to pre-industrial times," NOAA stated.
The authors estimated a 16 percent chance that 2007 will bring record warmth.
Last year marked the ninth straight year of above-normal temperatures, and all 48 continental states reported above-normal annual temperatures.
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.