“Bathtub rings” around California’s Lake Oroville show just how far the waters have receded. Credit: Elena Aravina, CC BY-SA 2.0
Source: Geophysical Research Letters

The Golden State earned its name for the precious metal, but for the agricultural giant, the most precious commodity of all might be water. The recent drought dealt a heavy blow to western states, which manifested in groundwater depletion, fields left fallow, tree die-offs, and the latest rise in wildfires. The 2012–2014 drought broke records in California’s key agricultural regions, and conditions in 2014 were record-breaking statewide.

A recent study evaluates the role of anthropogenic climate change on these severe drought conditions. The new insight is fundamental to better protecting communities and ecosystems that depend on California’s most precious resource. Decreasing soil moisture, lowering river levels, and diminishing groundwater reserves could be disastrous for the state, especially in the Central Valley, the primary agricultural region that relies on steady irrigation.

Williams et al. drew data from the Palmer drought severity index, a proxy for drought conditions that calculates near-surface soil moisture from precipitation and temperature records.  They looked at records between 1901 and 2014 to identify the impact of rising global temperatures on drought severity. The team also worked to distinguish natural temperature variability from anthropogenic warming. The authors conclude that “anthropogenic warming is estimated to have accounted for 8–27% of the observed drought anomaly in 2012–2014 and 5–18% in 2014.”

The researchers found that the current droughts naturally occur but got intensified by the rise in global temperatures since 1901. Although lack of precipitation is still the primary driver of drought, anthropogenic warming has resulted in a significant trend toward drought and has increased the probability of severe drought overall.

The trend toward drought emerged because warming disrupts the natural climate variability that would normally offset drying. Warming since the early 1900s causes an additional 9 centimeters of annual evaporation during a given year, effectively making each rain drop or snowflake less valuable to humans and ecosystems because it is more readily evaporated.

With warming projected to continue throughout the 21st century, this study highlights the need for long-term planning to improve drought resilience. A steadily warming planet will have profound local impacts that require conscientious resource management to safeguard lives and livelihoods. (Geophysical Research Letters, doi:10.1002/2015GL064924, 2015)

—Lily Strelich, Freelance Writer

Citation: Strelich, L. (2015), Global warming intensifies drought conditions in California, Eos, 96, doi:10.1029/2015EO034713. Published on 26 August 2015.

Text © 2015. The authors. CC BY-NC 3.0
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