Last year was the warmest year on record for the United States and the spring-like temperatures during the final three weeks of the year led to the fourth warmest December since records began in 1895, according to a report Tuesday from the National Oceanic and Atmospheric Administration (NOAA).

Using the Residential Energy Demand Temperature Index, an index developed at NOAA to relate energy usage to climate, NOAA scientists determined that the nation’s residential energy demand was 13.5% lower than what would have occurred under average climate conditions for the first half of the winter heating season.

The average annual temperature for the contiguous U.S. in 2006 was 55 degrees Fahrenheit, 2.2 degrees above the 20th century mean and 0.7 degrees warmer than the previous annual record set in 1998. Seven months in 2006 were much warmer than average, including December.

NOAA originally estimated in mid-December that the 2006 was going to be the third warmest year on record, slightly cooler than 1998 and 1934, according to preliminary data. Further analysis of annual temperatures and an unusually warm December caused the change in records.

After a cold start to December, there was three consecutive weeks of above normal temperatures. For example, the monthly average temperature in Boston was 8 degrees above average, and in Minneapolis-St Paul, the temperature was 17 degrees above average for the last three weeks of December. Even in Denver, which had its third snowiest December on record and endured a major blizzard that brought the city to a standstill during the holiday travel season, the temperature for the month was 1.4 degrees warmer than the 1971-2000 average.

Five states had their warmest December on record (Minnesota, New York, Connecticut, Vermont, New Hampshire) and no state was colder than average in December.

NOAA said a contributing factor to the unusually warm temperatures throughout 2006 is the long-term warming trend, which has been linked to increases in greenhouse gases. “This has made warmer-than-average conditions more common in the U.S. and other parts of the world,” the agency said. “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. It is known that El Nino is playing a major role in this winter’s short-term warm period.”

The agency said U.S. and global annual temperatures are now 1 degree warmer than at the start of the 20th century, and the rate that warming has accelerated over the past 30 years temperatures are increasing globally approximately three times faster than the century-scale trend. The past 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, NOAA said.

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