National Climatic Data Center
21 June 2007
This report was updated on May 1, 2007 to reflect revised statistics for the 2006 annual average temperature for the contiguous U.S. based on updates of preliminary data available in January 2007 as well as changes resulting from the switch from Version 1 to Version 2 of the U.S. Historical Climatology Network (USHCN) data set.
A more detailed discussion about how temperature trends are calculated and why differences can occur is available.
In an update to the 2006 average annual temperature for the contiguous U.S., NCDC scientists report that 2006 was the 2nd warmest year on record and nearly identical to the record set in 1998. Seven months in 2006 were much warmer than average, including December, which ended as the eighth warmest December since records began in 1895.
Using final quality controlled data from the recently completed USHCN Version 2 data set, the 2006 average annual temperature for the contiguous U.S. was the 2nd warmest on record. The 2006 annual average temperature was 54.9°F, 2.1°F (1.2°C) above the 20th Century mean and 0.08°F (0.04°C) cooler than 1998.
These values were calculated using the USHCN Version 2 data set, which exploits recent scientific advances that better address uncertainties in the instrumental record. 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.
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 approximately 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 eighth warmest December on record in the U.S., and helped bring the annual average to near record high levels. For example, the monthly average temperature in Boston was 8°F above average, and in Minneapolis-St Paul, the temperature was 17°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°F warmer than the 1971-2000 average.
Six states had their warmest December on record (New York, New Jersey, Connecticut, Massachusetts, Vermont, New Hampshire) and only Washington 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°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-1970's at a rate 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.
For all climate questions other than questions concerning this report, please contact the National Climatic Data Center's Climate Services Division:Jay Lawrimore