Click the image for larger view of U.S. state temperature rankings for 2006.
The 2006 average annual temperature for the contiguous United States was the warmest on record and nearly identical to the record set in 1998, according to a report released Jan. 9 by scientists at the NOAA National Climatic Data Center in Asheville, N.C.
Seven months in 2006 were much warmer than average, including December, which ended as the fourth warmest December since records began in 1895.
Based on preliminary data, the 2006 annual average temperature was 55 degrees F. - 2.2 degrees F. above the 20th Century mean and 0.07 degrees F. warmer than 1998.
NOAA originally estimated in mid-December that the 2006 annual average temperature for the contiguous United States would likely be 2 degrees F. above the 20th Century mean, which would have made 2006 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.
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 NCDC and scheduled for release in 2007 incorporates recent scientific advances that better address uncertainties in the instrumental record. Small changes in annual average temperatures will affect individual rankings. 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 U.S., but with 2006 slightly cooler than 1998.
The unusually warm temperatures during much of the first half of the cold season (October-December) helped reduce residential energy needs for the nation as a whole. Using the Residential Energy Demand Temperature Index (REDTI - an index developed at NOAA to relate energy usage to climate), NOAA scientists determined that the nation's residential energy demand was about 13.5 percent lower than what would have occurred under average climate conditions for the season.
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.
For example, the monthly average temperature in Boston was 8 degrees F. above average, and in Minneapolis-St Paul, the temperature was 17 degrees F. 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 F. 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.
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. A contributing factor to the unusually warm temperatures throughout 2006 also 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. 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 Niño-related circulation pattern. 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 about 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.