Record High Temperatures Far Outpace Record Lows Across U.S.

Spurred by a warming climate, daily record high temperatures occurred twice as often as record lows over the last decade across the continental United States, new research shows.

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

Results of the research, by authors at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., Climate Central, The Weather Channel, and the National Oceanic and Atmospheric Administration (NOAA), has been accepted for publication in the American Geophysical Union journal Geophysical Research Letters.

"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."

The research was funded by the National Science Foundation (NSF), NCAR's sponsor, the U.S. Department of Energy, and Climate Central.

"This intriguing study provides new evidence of climate change," says Steve Nelson, NSF program director for NCAR. "And it's change that's affecting our daily lives."

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

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 one-and-a-half 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.

In addition to surveying actual temperatures in recent decades, Meehl and his co-authors turned to a sophisticated computer model of global climate to determine how record high and low temperatures are likely to change during the course of this century.

The modeling results indicate that, 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.

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.