Last year was the hottest on record for the contiguous United States, shattering the previous mark set in 1998 by a wide margin, the National Oceanic and Atmospheric Administration announced Tuesday.

The average temperature was 55.3 degrees, 1 degree above the previous record and 3.2 degrees more than the 20th-century average. Temperatures were above normal in every month between June 2011 and September 2012, a 16-month stretch that hasn’t occurred since the government began keeping such records in 1895.

Federal scientists said that the data are compelling evidence that climate change is affecting weather in the United States and suggest that the nation’s weather is likely to be hotter, drier and potentially more extreme than it would have been without the warmer temperatures.

Last year’s record temperature is “clearly symptomatic of a changing climate,” said Thomas R. Karl, who directs NOAA’s National Climatic Data Center.