Climate change is one of the most prominent public health issues currently on the CDC’s radar. The organization's Climate and Health Program attempts to help state and city health departments to prepare for the health impacts of climate change, which can come in the form of things like temperature extremes, air pollution, allergens, and changes in disease patterns; they can also be felt indirectly through issues like food security.

Are we seeing some of these effects already? 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.

This projected increase in dry/warm conditions may have serious impacts on human and natural systems. The authors posit that high temperatures during spring and autumn could deplete stores of snowpack and increase wildfire risk, severely affecting the state’s ecology. They also hypothesize that increased dry periods will increase the risk of water shortages for humans and ecological systems and even raise the threat of species extinction due to drought.

This is one of a number of studies to have looked at California’s drought, and they’re coming to similar conclusions: historically it’s not unusual for the state to have either warm or dry winters, but the recent drought is notable for having both at once. Figuring out precisely how likely that is, and how it relates to human emissions, is still an area of ongoing research. But as this study makes clear, the residents of the Golden State, human and otherwise, should be prepared for this to be a recurring problem.

PNAS, 2014. DOI: 10.1073/pnas.1422385112 (About DOIs).