Because of global warming, the United Sates has seen more daily record high temperatures than record lows over the past decade, according to the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
For the period from Jan. 1, 2000, to Sept. 30, 2009, the continental United States set 291,237 record highs and 142,420 record lows, as the country experienced unusually mild winter weather and intense summer heat waves, the NCAR said in a study published in the November issue of the Geophysical Research Letters, a journal of the American Geophysical Union.
If temperatures were not warming, the number of record daily highs and lows being set each year would be approximately even, said the study.
The study team focused on weather stations that have been operating since 1950. They found that the ratio of record daily high to record daily low temperatures slightly exceeded one to one in the 1950s, dipped below that level in the 1960s and 1970s, and has risen since the 1980s.
The results reflect changes in U.S. average temperatures, which rose in the 1950s, stabilized in the 1960s, and then began a warming trend in the late 1970s.
The ratio of record highs to lows is likely to increase dramatically in coming decades if emissions of greenhouse gases continue to climb, the study warned.
If nations continue to increase their emissions of greenhouse gases in a "business as usual" scenario, the U.S. ratio of daily record high to record low temperatures would increase to about 20- to-1 by mid-century and 50-to-1 by 2100, said the study.
"Climate change is making itself felt in terms of day-to-day weather in the United States," says NCAR scientist Gerald Meehl, the lead author. "The ways these records are being broken show how our climate is already shifting."
A record daily high means that temperatures were warmer on a given day than on that same date throughout a weather station's history.
The researchers used a quality control process to ensure the reliability of data from thousands of weather stations across the country,
while looking at data over the past six decades to capture longer-term trends.