How climate change is making California’s epic drought worse
Persistent high pressure causes a descending motion of air, known as subsidence, which leads to a lack of cloud cover and eventually hotter temperatures on land. And with higher surface temperatures, soils dry as more water evaporates into the air.
Higher temperatures combined with somewhat reduced precipitation — sometimes called “warm droughts” — play a critical role in reducing water availability. In the case of California, precipitation this winter fell in the form of rain instead of snow in the Sierra Mountains due to high temperatures. Snowpack is critical because it stores water and releases it slowly. Thinner snowpack means less water stored on the ground for use in the summer. So even a normal precipitation amount but fallen in the form of rain means less water is available.
Climate fingerprint?
California has experienced sustained droughts in the past. For instance, during the medieval warming period between 900 and 1330 AD, extensive and persistent mega-droughts occurred in western North America. Paleoclimate studies using proxy records such as tree rings indicate that severe drought periods happened during periods of higher temperatures.
The magnitude of the deficit in precipitation from shifting atmospheric conditions in today’s drought is not so unusual compared with past observations. However, there is also a background of unusually high temperature nowadays, clearly exceeding that of the medieval warming period.
So this is a double whammy for the hydrological balance: less precipitation and more evaporation and transpiration, or water evaporation through plants, due to the warm air.
Running the numbers
So the present drought raises at least two questions: first, is this record-breaking twenty-first-century drought related to anthropogenic climate change, and second, has long-term warming altered the probability that precipitation deficits will yield extreme drought in the future?
Detecting and attributing observed or projected impacts to man-caused climate change is not an easy task. But there is some supporting evidence from improving numerical climate models and the record of several diverse meteorological and hydrological events already happening, including heat waves, flooding, or droughts.
Nevertheless, direct causal relationship is nearly impossible to establish in a complex system such as the climate system because interactions and feedbacks all but erase what appear to be causal links.
Still, statistical analysis of past data can help by providing the likelihood of occurrence of particular events. Also, scientists can run climate model experiments that include only natural variability and then include manmade factors, such as greenhouse gases. These tools serve to highlight and distinguish the dominant mechanisms responsible for particular air circulation characteristics.
These climate model simulations show that the extreme and persistent circulation patterns that have caused droughts on the West Coast this century are due to anthropogenic external forces, not natural causes.
For instance, in the case of the persistent region of high pressure — nicknamed the “ridiculously resilient ridge” — the likelihood of an event as observed in 2012-2013 is one in every 420 years!
Studies suggest that climate change might give rise to a new climate regime, one in which the years of low precipitation will be accompanied by warm conditions, creating the aforementioned “warm drought.”
This prediction suggests a growing risk of unprecedented drought in California, and the western United States in general, driven primarily by warmer temperature, reduced snowpack, and late spring and summer soil moisture, even without significantly drier precipitation patterns.
All of this means that sustaining water supplies in parts of the Southwest will be a challenge this year — and in the future.
Catherine Gautier is Professor Emerita of Geography at University of California, Santa Barbara. This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives.
