Human activities contribute to California's global warming
Over the past 85 years, humans have helped shape California climate during certain seasons. But that鈥檚 not necessarily good.
Recent research by scientists at the Lawrence Livermore National Laboratory, the University of California, Merced and the National Center for Atmospheric Research shows that California temperatures have jumped statewide by more than 2.1 degrees Fahrenheit between 1915 and 2000. This warming is likely related to human activities.
Using data from up to eight different observational records, the team found the warming has been fastest in late winter and early spring.
鈥淭he trends in daily minimum and maximum temperatures over the last 50 and 85 years are inconsistent with current model-based estimates of natural internal climate variability,鈥� said lead researcher C茅line Bonfils, a former UC Merced postdoc now working at Lawrence Livermore. 鈥淚t鈥檚 pretty clear that natural causes alone just can鈥檛 cut it and external factors such as greenhouse gases and urbanization come into play.鈥�
California is not alone when it comes to warming trends. Late winter and springtime temperatures have increased in nearly all of western North America. They have been associated with a large change in atmospheric circulation in the northern Pacific, likely resulting from greenhouse gas-induced warming.
But all California climate trends during the 20th century aren鈥檛 so clear.
For example, less warming is observed in summer. This warming, which mainly occurs at night but not during daytime, is not well explained by historical climate simulations.
鈥淲e looked at observations and models and they don鈥檛 concur,鈥� said Phillip Duffy, part of the Livermore team and a UC Merced adjunct professor. 鈥淥ne possible reason for this is that most models don鈥檛 include factors such as irrigation, which can influence regional climate.鈥�
The team found the lack of a trend in summertime maximum temperatures may be associated with the rapid expansion of large-scale irrigation during the 20th century, an important factor in California that is not accounted for in the models.
鈥淲e found empirical evidence that irrigation has a large cooling effect on local summer daytime temperatures but minimal effect on nighttime temperatures,鈥� said Bonfils, who investigated that issue in another Livermore study published in Proceedings of the National Academy of Sciences earlier this year.
Until now, cooling from irrigation may have counteracted the daytime warming from mounting greenhouse gases and urbanization.
鈥淚f this hypothesis is verified, the acceleration of CO2 emissions combined with a leveling of irrigation may result in a rapid summertime warming in the Central Valley in the near future,鈥� said David Lobell, co-author in both studies.
What does this mean for the future climate in California?
鈥淭he 21st century may be less climatically complex than today,鈥� Bonfils said. 鈥淕reenhouse warming is likely to be the dominant factor over today鈥檚 many climate influences.鈥�
鈥淥ur study represents a credible first step toward the identification of the effects of human activities on California climate,鈥� said Benjamin Santer, also part of the Livermore team.
An increase in California temperatures could have dire consequences for the state鈥檚 water system. 鈥淚f human-induced climate change is occurring, societal impacts 鈥� such as impacts on our water supply 鈥� cannot be far behind,鈥� Duffy said.
Source: Lawrence Livermore National Laboratory
