Preliminary data from the National Climatic Data Center listed the average temperature for the 48 contiguous states last year as 55 degrees Fahrenheit. That's 2.2 degrees warmer than average and 0.07 degree warmer than 1998, the previous warmest year on record.
Worldwide, the agency said, it was the sixth-warmest year on record.
In December the center had predicted that 2006 would be the United States' third-warmest year, but unusual readings later that month pushed the year into first place.
The center said it is not clear how much of the warming is a result of greenhouse-gas induced climate change and how much resulted from the current El Nino warming of the tropical Pacific Ocean.
Meanwhile, British climate scientists have predicted that a resurgent El Nino climate trend combined with higher levels of greenhouse gases could touch off a fresh round of ecological disasters and make 2007 the world's hottest year on record.
El Nino conditions occur every few years in the Pacific and can affect climate around the world, producing warmer conditions in the United States.
The average U.S. and global temperature are both about 1 degree warmer than at the start of the 20th century, a change many scientists attribute to gases released into the atmosphere by industrial processes.
The temperature data was collected from a network of more than 1,200 stations across the country.
The climate center said the unusual warmth in early winter reduced residential energy needs by 13.5 percent compared to average conditions for the season.
While December started cold, spring-like conditions abounded in the eastern states during the last half of the month, making it the nation's fourth-warmest December. Five states had their warmest December on record — Minnesota, New York, Connecticut, Vermont and New Hampshire. No state was colder than average in December.
Even New Hampshire's Mount Washington, which boasts "the worst weather in America," broke high temperature records last week, CBS News correspondent Sharyn Alfonsi reports. Scientists says it's the fourth warmest winter on record there.