Last year was the warmest on record in the continental United States, the federal government reported Tuesday, attributing the temperatures to the natural El Nino cycle as well as to long-term warming linked to human emissions of greenhouse gases.
The findings are preliminary, and a final review later this year could still put 2006 just below the previous record set in 1998, the National Climatic Data Center reported. But in any case, the two years are pretty much a tie in terms of hot ones since recordkeeping began in 1880.
Both years averaged around 55 degrees Fahrenheit — 2.2 degrees above the 20th century mean.
In a statement, the center said the last nine years have all been "among the 25 warmest years on record for the contiguous U.S., a streak which is unprecedented in the historical record."
The center in December had expected 2006 to be the third warmest on record but that changed given warm temperatures in the latter half of the month.
"After a cold start to December, the persistence of spring-like temperatures in the eastern two-thirds of the country during the final two to three weeks of 2006 made this the fourth warmest December on record in the U.S., and helped bring the annual average to record high levels," the center said.
Worldwide, the agency said, 2006 was the sixth warmest year on record.
Denver snowy but warm
The center cited several examples of above average temperatures in December:
- Boston was 8 degrees above average.
- Minneapolis-St Paul was 17 degrees above average for the last three weeks.
- Denver, even with its third snowiest December on record, saw temperatures average 1.4 degrees warmer than the 1971-2000 average.
Five states — Minnesota, New York, Connecticut, Vermont, New Hampshire — had their warmest December on record and no state was colder than average in December, the center added.
El Nino and greenhouse gases
The center said that a moderate El Nino, a periodic Pacific Ocean pattern that affects weather worldwide, had limited Arctic blasts across the continental United States.
"A contributing factor ... also is the long-term warming trend, which has been linked to increases in greenhouse gases," it added.
A key greenhouse gas is carbon dioxide, which cars and industry emit by burning fossil fuel. The gases add to a natural greenhouse effect around Earth that traps in heat. Many scientists fear that humans are adding too much on top of pre-industrial levels of greenhouse gases.
The center described the El Nino contribution as "major" but added that "it is unclear how much of the recent anomalous warmth was due to greenhouse gas-induced warming and how much was due to the El Nino-related circulation pattern."
Carbon dioxide levels in the atmosphere have increased by 30 percent since the start of the industrial revolution, and that has paralleled a warming trend.
"U.S. and global annual temperatures are now approximately 1 degree F warmer than at the start of the 20th century," the center noted, "and the rate of warming has accelerated over the past 30 years, increasing globally since the mid-1970s at a rate approximately three times faster than the century-scale trend."
The Bush administration's anti-warming strategy is to promote technologies that can reduce emissions. It refuses to require emissions curbs, arguing those would hurt the economy and cost jobs.
Most other industrial nations have agreed to curbs under what's known as the Kyoto climate accord.
Britain's weather service last week predicted that 2007 would likely be the warmest year on record globally, citing El Nino and greenhouse gases.