2012 News & Events

Direct Evidence of Summer Climate Change

13 November 2012
adapted from the story written by CIRES Communications

summer sunset
Summer climates in some places are changing – mostly, but not always, warming – according to a new analysis of 90 years of observational data

In many parts of the world, summers just aren't what they were a generation or two ago. Summer climates in some places are changing – mostly, but not always, warming – according to a new analysis of 90 years of observational data led by a scientist working at CSD with the NOAA Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado at Boulder.

"It is the first time that we show on a local scale that there are significant changes in summer temperatures," said lead author Irina Mahlstein, Ph.D. "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. Because the new method involves detecting temperatures outside the expected norm, it identifies changes potentially important to 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. The study "Emerging local warming signals in observational data" is published online in Geophysical Research Letters.

To identify 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 from the 1920 starting 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's findings are consistent with other approaches to answering that 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. "We can actually say that these changes have happened in the lifetime of a person," Mahlstein said.

The study is a good example of the synergies that arise from NOAA's partnership with its cooperative institutes, noted A.R. Ravishankara, Ph.D., who leads the NOAA laboratory in Boulder where Mahlstein works.

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

Mahlstein, I., G. Hegerl, and S. Solomon, Emerging local warming signals in observational data Geophys. Res. Lett., doi:10.1029/2012GL053952, 2012.


The global average temperature of the Earth has increased, but year-to-year variability in local climates impedes the identification of clear changes in observations and human experience. For a signal to become obvious in data records or in a human lifetime it needs to be greater than the noise of variability and thereby lead to a significant shift in the distribution of temperature. We show that locations with the largest amount of warming may not display a clear shift in temperature distributions if the local variability is also large. Based on observational data only we demonstrate that large parts of the Earth have experienced a significant local shift towards warmer temperatures in the summer season, particularly at lower latitudes. We also show that these regions are similar to those that are found to be significant in standard detection methods, thus providing an approach to link locally significant changes more closely to impacts.