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Viewpoints: Climate change is increasing the chances of drought

Whether viewed from a dry-as-dust ground level or from a sky-high planetary perch, the ongoing drought in California is remarkable. As this slow-moving meteorological disaster intensifies, burning forests, fallowed agricultural fields and critically low reservoirs show the increasingly high-stakes interactions among scientists, policymakers and everyday Californians.

This year is California’s third consecutive dry year, but the severity of the drought has dramatically increased over the past 12 to 18 months.

As reported in our new paper, published Monday in the Bulletin of the American Meteorological Society, the drought now encompasses the driest consecutive 12-month period in California’s recorded history.

No single factor is to blame, but our new research shows that human-caused climate change has very likely increased the probability of at least one key cause. For much of the past 18 months, a remarkably persistent ridge of atmospheric high pressure has been centered over the northeastern Pacific Ocean, deflecting storms north of their usual path and bringing two consecutive winter seasons without usually reliable rain and mountain snow.

A system like this may be a chance occurrence. But research shows that global warming is increasing the probability of weather patterns like this one. In particular, our paper demonstrates that human emissions of greenhouse gases have increased the probability of persistently high atmospheric pressure near the western coast of North America. Our ongoing study seeks to determine whether periods of low precipitation in California are becoming more common or more intense.

There are also indirect factors that worsen drought conditions, and these have received less attention. One is high temperature, and we already know that temperatures in California have increased quite substantially since the late 1800s. So far, 2014 has been California’s warmest year in at least 119 years.

Droughts can take place during warm or cool conditions, but when periods of low rainfall coincide with warm conditions, those high temperatures act as an “intensity multiplier” by speeding snowmelt and increasing evaporation rates. When the overall effect of extremely dry conditions and record-high temperatures are taken into account, California’s ongoing drought is the worst in more than a century.

Moreover, data released in conjunction with the U.S. National Climate Assessment this past spring show that years with low precipitation have become much more likely to coincide with warm temperatures over the past few decades, implying that long-term warming has progressively increased the risk of severe drought in California.

What is the prognosis?

California’s drought is likely to worsen further before it improves. Some parts of the state have missed out on more than a year’s worth of precipitation over the past two to three years, meaning that even a very wet winter would be unlikely to erase long-standing water deficits. Water levels in California’s depleted reservoirs and in its overdrawn aquifers will likely continue to fall, at least until the start of the rainy season this November or December and very possibly beyond.

The state’s overall water usage has remained essentially flat since the 1970s drought, despite a large increase in population. While this is an impressive testament to the power of conservation, it also means that future adaptations to water scarcity are likely be more difficult and more expensive. These challenges are likely to intensify in the future, as the climate continues to warm and the probability of drought conditions in California continues to rise.

All Californians hope for an end to the “ridiculously resilient ridge” – as weather watchers now call it – and a return to a wet winter or two ahead. While we don’t know exactly when the current drought will end, we should expect to endure more years like this one in decades to come. Managing drought-related risks will require forward-thinking solutions that take our changing climate into account.

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