Scientists say the U.S. heat is part global warming in action and natural weather variations. The drought that struck almost two-thirds of the nation and a La Niña weather event helped push temperatures higher, along with climate change from greenhouse-gas emissions, said Katharine Hayhoe, director of the Climate Science Center at Texas Tech University. She said temperature increases were happening faster than scientists predicted.
"These records do not occur like this in an unchanging climate," said Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colo. "And they are costing many billions of dollars."
Last year was 3.2 degrees warmer than the average for the entire 20th century. July was the also the hottest month on record.
Nineteen states set yearly heat records in 2012 - including New Jersey and Delaware. In Pennsylvania, only two years were hotter than 2012. Alaska was cooler than average. South Carolina reported its hottest temperature since record-keeping began in 1887 - 113 degrees on June 29 in Columbia.
National temperature records go back to 1895 and the yearly average is based on reports from more than 1,200 weather stations across the Lower 48 states.
According to the National Oceanic and Atmospheric Administration, the United States last year also had the second most weather extremes on record, behind 1998. There were 11 disasters that caused more than $1 billion in damage, including Hurricane Sandy and the drought, NOAA said.
"A picture is emerging of a world with more extreme heat," said Andrew Dessler, a Texas A&M University climate scientist. "Not every year will be hot, but when heat waves do occur, the heat will be more extreme. People need to begin to prepare for that future."