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Topic Name: A Research Team Shows that Human Activities Changes California Temperatures more than 2.1 Degrees Fahrenheit

Category: Environmental engineering

Research persons: Céline Bonfils

Location: University of California, Merced, United States

Details

Over the past 85 years, humans have helped shape California climate during certain seasons. But that’s 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.

“The 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,” said lead researcher Céline Bonfils, a former UC Merced postdoc now working at Lawrence Livermore.“It’s pretty clear that natural causes alone just can’t cut it and external factors such as greenhouse gases and urbanization come into play.”

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’t 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.

“We looked at observations and models and they don’t concur,” said Phillip Duffy, part of the Livermore team and a UC Merced adjunct professor. “One possible reason for this is that most models don’t include factors such as irrigation, which can influence regional climate.”

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.

“We found empirical evidence that irrigation has a large cooling effect on local summer daytime temperatures but minimal effect on nighttime temperatures,” 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.

“If 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,” said David Lobell, co-author in both studies.

What does this mean for the future climate in California?

“The 21st century may be less climatically complex than today,” Bonfils said. “Greenhouse warming is likely to be the dominant factor over today’s many climate influences.”

“Our study represents a credible first step toward the identification of the effects of human activities on California climate,” said Benjamin Santer, also part of the Livermore team.

An increase in California temperatures could have dire consequences for the state’s water system. “If human-induced climate change is occurring, societal impacts – such as impacts on our water supply – cannot be far behind,” Duffy said.

The research, funded by the California Energy Commission, and including contributions from Livermore scientists Thomas Phillips and Charles Doutriaux, appears in the Dec. 19 online edition of the journal Climatic Change. The research also was included in the “Report to the Governor and Legislature on Climate Change.” California temperature trends also are discussed in a recent article in the American Geophysical journal, Eos, written by Duffy, Bonfils and Lobell.

Note for Urbanization
Urbanization or Urbanisation means the removal of the rural characteristics of a town or area, a process associated with the development of civilization. Demographically, the term denotes redistribution of populations from rural to urban settlements.
Traditional urbanization exhibits a concentration of human activities and settlements around the downtown area. When the residential area shifts outward, this is called suburbanization. A number of researchers and writers suggest that suburbanization has gone so far to form new points of concentration outside the downtown. This networked, poly-centric form of concentration is considered by some an emerging pattern of urbanization. It is called variously exurbia, edge city (Garreau, 1991), network city (Batten, 1995), or postmodern city (Dear, 2000). Los Angeles is the best-known example of this type of urbanization.

Note for Irrigation
Irrigation is the artificial application of water to the soil usually for assisting in growing crops. In crop production it is mainly used in dry areas and in periods of rainfall shortfalls, but also to protect plants against frost. Additionally irrigation helps to suppress weed growing in rice fields. In contrast, agriculture that relies only on direct rainfall is sometimes referred to as dryland farming or as rain fed farming. It is often studied together with drainage, which is the natural or artificial removal of surface and sub-surface water from a given area.
Irrigation is also a term used in the Medical/Dental fields and refers to flushing and washing out anything with water or another liquid.
Various types of irrigation techniques differ in how the water obtained from the source is distributed within the field. In general, the goal is to supply the entire field uniformly with water, so that each plant has the amount of water it needs, neither too much nor too little.