Meteorologists said the past nine years have all been among the 25 warmest years on record for the contiguous United States, a streak which is unprecedented in the historical record.
Seven months in 2006 were much warmer than average, including December, which ended as the fourth warmest December since records began in 1895, the agency said Tuesday.
Based on preliminary data, the 2006 annual average temperature was 55 degrees Fahrenheit. That is 2.2 degrees F (1.2 degrees C) above the 20th century mean and 0.07 degrees F (0.04 degrees C) warmer than 1998.
These values were calculated using a network of more than 1,200 U.S. Historical Climatology Network stations. These data, primarily from rural stations, have been adjusted to remove artificial effects resulting from factors such as urbanization and station and instrument changes, which occurred during the period of record.
An improved data set being developed at the National Climatic Data Center and scheduled for release in 2007 incorporates recent scientific advances that better address uncertainties in the instrumental record.
Although undergoing final testing and development, this new data set also shows 2006 and 1998 to be the two warmest years on record for the contiguous United States, but with 2006 slightly cooler than 1998.
Five states had their warmest December on record - Minnesota, New York, Connecticut, Vermont, New Hampshire - and no state was colder than average in December.
The unusually warm start to this winter reflected the rarity of Arctic outbreaks across the country as an El Niño episode continued in the equatorial Pacific. It is known that El Niño is playing a major role in this winter's short-term warm period.
U.S. and global annual temperatures are now approximately 1.0 degrees F warmer than at the start of the 20th century, 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.