About of half of the record warmth of 2006 can be attributed to human-caused greenhouse gas warming, according to research by several Colorado researchers.
Based on preliminary data, 2006 was the warmest year in the 112-year record available for the lower 48 states. A paper in press by Boulder-based National Oceanic and Atmospheric Administration scientist Martin Hoerling and colleagues says:
“The annually averaged U.S. temperature was reported to be 55 degrees Fahrenheit (12.8 degrees C) eclipsing the prior record set in 1998. The warmth was widespread, with each of the 48 states being above normal, and warmth in the majority of states ranking among the 10 highest years since 1895.”
U.S. temperatures in degrees C above average, 2006: NOAA
In NOAA’s news release about the record, it was left uncertain how much of the warmth was attributable to human-generated greenhouse gases and how much to other factors. 2006 was also a year in which the natural Pacific fluctuating weather trend called El Nino was active. The previous record year of 1998 was also an El Nino year.
In media reporting on the 2006 record, some outlets (USA Today, for instance) reported that El Nino was largely to blame for the heat, while others (like the Washington Post) tended to blame anthropogenic warming. NOAA itself, as an agency, has been reluctant in the past to attribute the warming to human activity.
“A timely science-based attribution statement is essential,” the scientists wrote. “The question of climate change recently took center stage at both House and Senate hearings in which congressmen discussed their positions on how the Nation should address global warming. Policy and decision makers are requesting expert interpretation. They want to know why the climate is behaving as observed because such knowledge is a key input to informing their debates and responses.”
Hoerling and colleagues conclude, however, that the record warmth was due primarily to human influence on the climate. By looking at observations and model assessments of the factors, they concluded that El Nino could not have been responsible for most of the warming and that human-generated greenhouse gases caused at least 50 percent of it.
They also concluded that 2007 offers a 16 percent chance of setting yet another record as the warmest in the U.S.
The research will be published in the September 5 issue of Geophysical Research Letters.