California can look forward to more extreme dry-wet “climate whiplash”

As the last few years have reminded us, California weather means you have to be prepared for anything. From 2012 to 2016, the Golden State saw a historic drought that led to water restrictions—and saw land areas sinking as groundwater use increased to compensate. But the winter of 2016 brought too much rain, producing flooding and evacuations below the Oroville Dam.

Variable rainfall is a natural component of California’s climate, but what will happen as climate change continues to play out? That’s the question a team led by UCLA’s Daniel Swain recently set out to answer.

Sim California

Though climate change projections show a warmer California, total rainfall isn’t expected to change much. But in this case, the researchers used climate model simulations to analyze precipitation variability, specifically, rather than just annual totals. They compared historical weather records, an 1,800-year-long simulation of the climate pre-Industrial-Revolution, and 40 simulations of climate change from 1920 to 2100 (assuming high future greenhouse gas emissions). These long simulations allowed them to accumulate meaningful statistics for different weather patterns.

To make the results easy to relate to, the researchers used noteworthy historical events as points of comparison, like the 2012-2016 drought and the Great Flood of 1862. From historical records, you can calculate how rare such events have been. The climate model simulations can then show whether that type of event is becoming more common in a warming climate.

Take the exceedingly wet winter of 2016-2017, for example. The simulations show that this happens two to three times more often by the end of this century. That Great Flood of 1862 was actually a train of atmospheric river storms over about a month, leading to a 200-year flood. If normal weather patterns held, we should have less than a 50-percent chance of seeing another flood like it by 2100, but most of the simulations had two or more such floods this century.

On the dry side, the 1976-1977 winter drought was a 100-year event, but its frequency in the simulations increases about 80 percent in Northern California and 140 percent in Southern California. The outlook for a three-year drought like 2013-2015, however, is mixed. Only a small part of the state showed a significant increase by the end of the century—perhaps, the researchers speculate, because the odds of a wet winter interrupting a drought go up.

Whiplash

So how about that wild swing from drought to sopping wet winter in 2016? (The researchers describe this as “precipitation whiplash.”) The projections show this behavior becoming twice as common in Southern California and 25 percent more common in Northern California by the end of the century.

There’s also an interesting general pattern to wet season precipitation—it is projected to increasingly fall over a smaller part of the calendar year. While winter precipitation is projected to increase, fall and spring precipitation goes down, making the seasonal cycle a little starker. That is, again, something that’s not apparent from annual rainfall totals alone.

Why more extremes? First, there’s the well-known fact that a warmer atmosphere can hold more water vapor, meaning more moisture can be wrung out of the clouds when it does rain. Beyond that, the atmospheric pressures over the eastern Pacific Ocean that govern California’s weather seem to strengthen in the model simulations. That’s a recipe for drier dry years and wetter wet years.

Beyond the comparisons to historical events, the researchers note that the simulations also include unprecedented extremes that current infrastructure probably won’t be able to handle. Unprecedented or not, the projections imply that the state needs to prepare for more rough weather in the coming decades—though reduced global greenhouse emissions would be a big help in avoiding the worst of it.

In a blog post about his study, Daniel Swain writes, “For quite some time, the biggest uncertainty in climate science has been how we, as a society, choose to respond to this growing global threat. The optimistic view: there is still time for us to take bold, decisive actions that will shape our planet for decades (and even millennia) to come… A California resilient to increasing 21st century climate whiplash won’t emerge overnight, but we now know enough about our plausible climate future to make informed decisions.”

Nature Climate Change, 2018. DOI: 10.1038/s41558-018-0140-y  (About DOIs).

https://arstechnica.com/?p=1299019