If you want to know what climate change means for California’s water supply, consider the last two Februaries.

In 126 years of statewide record-keeping, you can’t find a drier February than the one we just experienced. But February 2019 was the third-wettest on record.

The extremes underscore how global warming is exaggerating the year-to-year swings in California precipitation, which is naturally the most variable in the country.

But surprisingly, the dramatic ups and downs even out. Average precipitation is not declining.

Michael Anderson, the state climatologist, points out that despite periods of withering drought in the last two decades, precipitation in the northern Sierra Nevada — a key water source for the state — hasn’t changed.

Moreover, the region was actually drier during the Dust Bowl era of the 1920s and ‘30s.

“Oddly enough, in the 21st century — because we’ve had those wet extremes and dry years that haven’t been extremely dry — our average comes out really close to” the 20th century, Anderson said.

Statewide, the four wettest Oct. 1-Sept. 30 water years have been recorded since 1980, according to the California Climate Tracker.

That mirrors projections that global warming will hit California with more extreme wet years and boost precipitation a bit in the north.

But how does that fit with a recent scientific paper that concluded the Southwest, including all of California, has been in the grip of a 21st century megadrought intensified by human-caused global warming?

The study, which ranked 2000-2018 as the second-driest 19-year period in 1,200 years, factored in far more than precipitation. Researchers also modeled soil moisture, humidity and temperature and analyzed tree ring records.

Look solely at precipitation and the Southwest has not been stuck in a megadrought — and was drier in the Dust Bowl era — said co-author John Abatzoglou, an associate geography professor at the University of Idaho.

Moreover, using the broader criteria of soil moisture, etc., the paper pointed to 1980-1998 as the wettest 19-year period in at least 1,200 years.

“Basically, we had the wettest multi-decadal period followed by the second-driest multi-decadal period back to back,” Abatzoglou said. “That’s an eye-opening roller coaster.”

All of that is rewriting a water management playbook that is appearing increasingly outdated.


Source: Bettina Boxall, Los Angeles Times