New research from The University of Alabama in Huntsville shows that both extreme heat and extreme cold temperatures across the United States have declined over the past 127 years, offering one of the most comprehensive long-term analyses of climate trends to date.
The study, led by John R. Christy, a professor of atmospheric and earth science and Alabama’s retired state climatologist, examined more than 40 million daily temperature observations using an expanded U.S. Historical Climatology Network dataset dating back to 1899. The findings were published in the journal Theoretical and Applied Climatology.
Researchers analyzed data from 1,211 weather stations across the contiguous United States, reconstructing historical records where gaps existed by using nearby correlated stations. The effort, which included reviewing thousands of original data forms, provides a detailed look at how temperature extremes and record-setting events have evolved over time.
“It was a labor of love and curiosity,” Christy said, noting the extensive work required to build the dataset.
The study found that extreme summer heat has not increased over the long term. Measures including the hottest annual temperatures, frequency of daily heat records and number of heatwave days all show modest declines since 1899. The most intense nationwide heat events remain concentrated between 1925 and 1954, particularly during the 1930s.
Extreme cold temperatures, meanwhile, have declined sharply. Since the 1990s, both the frequency and severity of record-cold winter temperatures have decreased, a trend researchers say may reflect a combination of atmospheric warming and localized development effects near weather stations.
Overall, the gap between the hottest and coldest temperatures recorded each year has narrowed by about 6 degrees Fahrenheit, indicating a long-term reduction in temperature variability.
Regional trends vary, with western parts of the country experiencing increased heatwave activity in recent years, while central and eastern regions show long-term declines. When averaged nationally, those differences largely offset one another.
The study also highlights the role of natural climate variability, noting that early 20th century heat extremes, particularly during the 1930s, still exceed more recent events in magnitude.
Christy said the expanded dataset allows for a more complete evaluation of long-term climate patterns and public claims about extreme weather.
“This dataset allows us to ask and answer questions about extreme heat and cold with more confidence,” he said. “Understanding how extremes have changed over time is essential for interpreting how today’s events actually compare with the past.”
The research underscores Huntsville’s continued role in climate science and atmospheric research, with UAH contributing to national and global understanding of long-term environmental trends.
