The US sweltered under its hottest year on record in 2012, breaking the previous yearly average by 0.6C (1F), the US government has said.
Last year the average US temperature was 13C (55.3F) amid widespread drought and a mild winter.
Scientists said the heat was caused both by global warming and by natural weather variation.
But they said the size of the increase over the previous record year, 1998, was unprecedented.
"These records do not occur like this in an unchanging climate," Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research, said. "And they are costing many billions of dollars."
In 2012, 11 weather-related disasters caused at least $1bn (£623m) in damage, including "superstorm" Sandy and a months-long drought that hit almost two-thirds of the country, according to the National Oceanic and Atmospheric Administration (NOAA).
The drought - the worst in the US since the 1950s - contributed to higher temperatures. Without enough moisture in the soil to evaporate into rainfall, air becomes hotter and dryer.
US temperature records go back to 1895 and are now based on reports from more than 1,200 weather stations across the contiguous 48 states.
Last year was 1.7C (3.2F) warmer than the average for the entire 20th Century, the National Oceanic and Atmospheric Administration said. July broke the record for hottest month.
Nineteen states set yearly heat records in 2012. Alaska, however, was cooler than average.