We're Feeling the Heat

The warming climate is making itself felt in the daily weather across the United States, tilting the odds in favor of a daily record high temperature to two-to-one over a record low.  In a world without a warming climate, the record daily highs and lows each year would be about even.

In the last decade, as the country experienced unusually mild winters and intense summer heat waves, researchers report, the continental United States set 291,237 record highs and 142,420 record lows.  The trend was felt especially in the west.

A detailed analysis of daily temperatures at 1,800 weather stations since 1950, by a team of researchers from the National Center for Atmospheric Research, Climate Central, The Weather Channel and the National Oceanic and Atmospheric Administration will be published soon in Geophysical Research Letters, a journal of the American Geophysical Union.

"Climate change is making itself felt in terms of day-to-day weather in the United States," said Gerald Meehl, an NCAR researcher and the lead author of the study. "The ways these records are being broken show how our climate is already shifting."  Meehl explains the numbers in this video.

Statistician Claudia Tebaldi of Climate Central put it this way:  "If the climate weren't changing, you would expect the number of temperature records to diminish significantly over time.  As you measure the high and low daily temperatures each year, it normally becomes more difficult to break a record after a number of years.  But as the average temperatures continue to rise this century, we will keep setting more record highs."

The trend has not been a straight line, reflecting changes in average temperatures, which rose in the 1950s, flattened in the 1960s, and began rising with a continuing warming trend that began in the 1970s.  Even in the relatively cool year of 2009, the researchers report, record daily highs out-numbered daily record lows by three to two.

"One of the messages of this study is, you still get cold days," Meehl said. "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."

IMAGE: University Corporation for Atmospheric Research/Mike Shibao