U.S. Record High Temperatures Double Record Lows Since 2000

"Climate change is making itself felt in terms of day-to-day weather in the United States," says Gerald Meehl, lead author of the study and a senior scientist at the National Center for Atmospheric Research. "The ways these records are being broken show how our climate is already shifting."

The ratio of record highs to lows is likely to increase in coming decades if emissions of greenhouse gases continue to climb, he says.

If temperatures were not warming, the number of record daily highs and lows being set each year would be about even.

Instead, for the period from January 1, 2000, to September 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.

This graphic shows the ratio of record daily highs to record daily lows observed at about 1,800 weather stations in the 48 contiguous United States from January 1950 through September 2009. (Image ©UCAR, graphic by Mike Shibao)

The study, by authors at NCAR, Climate Central, The Weather Channel, and the National Oceanic and Atmospheric Administration, has been accepted for publication in the journal "Geophysical Research Letters." It was funded by the National Science Foundation, NCAR's sponsor, the Department of Energy, and Climate Central.

"If the climate weren't changing, you would expect the number of temperature records to diminish significantly over time," says Claudia Tebaldi, a statistician with Climate Central who is one of the paper's co-authors.

"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," Tebaldi predicts.

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 authors 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.

This decade's warming was more pronounced in the western United States, where the ratio was more than two to one, than in the eastern United States, where the ratio was about 1.5 to one.

The study also found that the two-to-one ratio across the country as a whole could be attributed more to a comparatively small number of record lows than to a large number of record highs.

This indicates that much of the nation's warming is occurring at night, when temperatures are dipping less often to record lows. This finding is consistent with years of climate model research showing that higher overnight lows should be expected with climate change.

Meehl and his co-authors also used a sophisticated computer model of global climate to see how record high and low temperatures are likely to change by 2100.

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.

The mid-century ratio could be much higher if emissions rose at an even greater pace, or it could be about 8-to-1 if emissions were reduced significantly, the model showed.

The authors caution that such predictions are, by their nature, inexact. They point out that climate models are not designed to capture record daily highs and lows with precision, and it remains impossible to know future human actions that will determine the level of future greenhouse gas emissions.

The model used for the study, the NCAR-based Community Climate System Model, correctly captured the trend toward warmer average temperatures and the greater warming in the West, but overstated the ratio of record highs to record lows in recent years.

Even so, the researchers say the model results are important because they show that, in all likely scenarios of future greenhouse gas emissions, record daily highs should increasingly outpace record lows over time.

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

Despite the increasing number of record highs, there will still be occasional periods of record cold, Meehl notes.

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

Copyright Environment News Service (ENS) 2009. All rights reserved.