Credit: California Department of Water Resources
Since 2012, California has been in the midst of a record-setting drought, with extremely warm and dry conditions characterizing the last three years in that state. A new paper published in the Proceedings of the National Academy of Sciences concludes that warming caused by humans is responsible for the conditions that have led to this California drought.
This study, published by scientists affiliated with the Department of Environmental Earth System Science and the Woods Institute for Environment at Stanford University, used historical statewide data for observed temperature, precipitation, and drought in California. The investigators used the Palmer Hydrological Drought Index (PHDI) and the Palmer Drought Severity Index (PDSI), collected by the National Climatic Data Center, as measures of the severity of wet/dry anomalies. They also used global climate model simulations from the Coupled Model Intercomparison Project (CMIP) to compare historical predictions for anthropogenic and non-anthropogenic historical climates.
The authors performed their analysis using an approach called bootstrapping. Bootstrapping techniques allow statisticians to utilize the same sample repeatedly to improve their estimates of specific effects. In this analysis, bootstrapping was used to compare the climate data with measures of populations from different time periods to allow for analysis of how changes in population are associated with different climate conditions.
This analysis found that the statewide warming in California occurs in climate models that include both natural and human factors, but not in simulations that only include natural factors. It’s a difference with a very high (0.001) level of statistical significance.
In their discussion and conclusions, the investigators state that their results strongly suggest that anthropogenic (human-caused) warming has increased the probability of co-occurring temperature and precipitation conditions that have historically led to California’s droughts. They also state that continued global warming is likely to lead to situations where every future dry period, whether it’s seasonal, annual, or multiannual, will be accompanied by historically warm conditions.
More: California’s Hot, Dry Winters Tied to Climate Change