The study, by authors at National Center for Atmospheric Research, Climate Central, The Weather Channel and the National Oceanic and Atmospheric Administration (NOAA), looked at millions of daily high and low temperature readings taken over six decades at about 1,800 weather stations across the country.

If temperatures were not warming, the number of record daily highs and lows being set each year should be approximately even. Instead, for the period from January 1, 2000, to September 30, 2009, the U.S. set 291,237 record highs and 142,420 record lows, as the country experienced unusually mild winter weather and intense summer heat waves

Even in the first nine months of this year, when the U.S. cooled slightly after a string of unusually warm years, the ratio of record daily high to record daily low temperatures was more than three to two.

“One of the messages of this study is that you still get cold days,” said Gerald Meehl, NCAR scientist. “Winter still comes. Even in a much warmer climate, we’re setting record low minimum temperatures on a few days each year. But the odds are shifting so there’s a much better chance of daily record highs instead of lows.”

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1. The State of the Union: Growing Warmer, Hotline 1.30.12