Record High Temperatures Far Outpace Record Lows Across US Over Last Decade

14 November 2009

Daily record high temperatures occurred twice as often as record lows over the last decade across the continental United States, new research shows. 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. The ways these records are being broken show how our climate is already shifting.

—NCAR scientist Gerald Meehl, the lead author

The ratio of record daily highs to lows from 1950-2009 at 1,800 US weather stations. Credit: NCAR. Click to enlarge.

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

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 1 January 2000, to 30 September 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 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 US 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.

However, 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.

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.

—Claudia Tebaldi, a statistician with Climate Central and co-author

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 US average temperatures, which rose in the 1950s, stabilized in the 1960s, and then began a warming trend in the late 1970s.

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.

The study team analyzed several million daily high and low temperature readings taken over the span of six decades at about 1,800 weather stations across the country, thereby ensuring ample data for statistically significant results.

The readings, collected at the National Oceanic and Atmospheric Administration’s National Climatic Data Center, undergo a quality control process at the data center that looks for such potential problems as missing data as well as inconsistent readings caused by changes in thermometers, station locations, or other factors.

Meehl and his colleagues then used temperature simulations from the Community Climate System Model to compute daily record highs and lows under current and future atmospheric concentrations of greenhouse gases.

Resources

• Gerald A. Meehl, Claudia Tebaldi, Guy Walton, David Easterling, and Larry McDaniel. “The relative increase of record high maximum temperatures compared to record low minimum temperatures in the US”. Geophysical Research Letters (in press)