Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder

Direct Evidence of Summer Climate Change

That summers “just aren’t what they used to be” no longer seems to be the wistful chant of the world weary looking back on their salad days: Analysis of 90 years of observational data has revealed that summer climates in regions across the globe are changing—mostly, but not always, warming—according to a new study led by a scientist from the Cooperative Institute for Research in Environmental Sciences (CIRES).

“It is the first time that we show on a local scale that there are significant changes in summer temperatures,” said lead author CIRES scientist Irina Mahlstein, who works in NOAA’s Earth System Research Laboratories. “This result shows us that we are experiencing a new summer climate regime in some regions.”

The technique, which reveals location-by-location temperature changes rather than global averages, could yield valuable insights into changes in ecosystems on a regional scale.  As the methodology relies on detecting temperatures outside the expected norm, it is more relevant to understand changes to biota—the animal and plant life of a particular region—which scientists would expect to show sensitivity to changes that lie outside of normal variability.

“If the summers are actually significantly different from the way that they used to be, it could affect ecosystems,” Mahlstein said.  

To identify potential temperature changes, the team used climate observations recorded from 1920 to 2010 from around the globe. The scientists termed the 30-year interval 1920 to 1949 the “base period,” and they compared each 30-year interval—in steps of 10 years later than the starting 1920 date—to the “base period.” The comparison involved statistical tests to determine whether the test interval differed from the base interval beyond what would be expected due to yearly temperature variability for that geographical area.

Their analysis found that some changes began to appear as early as the 1960s, and the observed changes were more prevalent in tropical areas. In these regions, temperatures vary little throughout the years, so the scientists could more easily detect any changes that did occur, Mahlstein said. They found significant summer temperature changes in 40 percent of tropical areas and 20 percent of higher-latitude areas. In the majority of cases, the researchers observed warming summer temperatures, but in some cases they observed cooling summer temperatures.

"This study has applied a new approach to the question: ‘Has the temperature changed in local areas?’" Mahlstein said. The study is in press in the journal Geophysical Research Letters.

The study’s findings are consistent with other approaches answering the same question, such as modeling and analysis of trends, Mahlstein said. But this technique uses observed data only to come to the same result. “Looking at the graphs of our results, you can visibly see how things are changing,” Mahlstein said.

In particular the scientists were able to look at the earlier time periods, note the temperature extremes, and observe that those values became more frequent in the later time periods. “You see how the extreme events of the past have become a normal event,” Mahlstein said.

The scientists used 90 years of data for their study—a little more than the average lifespan of a human being. So if inhabitants of those areas believe that summers have changed since they were younger, they can be confident it is not a figment of their imagination.

“We can actually say that these changes have happened in the lifetime of a person,” Mahlstein said.

Co-authors on the study were Gabriele Hegerl from the University of Edinburgh and Susan Solomon from Massachusetts Institute of Technology.

Irina Mahlstein, CIRES scientist. 303-497 4746,
Karin Vergoth, CIRES, 303-497-5125,

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