“Extreme” rain and snow events happening more often in the south, less often up north

A new report suggests that global warming is playing out quite differently in California, depending on whether it’s north or south of San Francisco Bay.

The project, by the Environment California Research & Policy Center, studied precipitation trends between 1948 and 2011, with an eye on “extreme” events — storms that dumped unusual amounts of rain or snow on the state.

They found a dichotomy in California — but not the usual “north has all the water” split. It turns out that north of San Francisco Bay, the extreme precipitation events were happening 26% less often, but south of the Bay, they were happening 35% more often. The authors calculate that a storm that used to come along once a year on average, is happening more like once every nine months to the south, which includes the Central Coast. In fact, Santa Barbara showed the biggest increase in frequency, 72% since 1948.

The report was part of a larger look at precipitation trends across the U.S., entitled “When it Rains, It Pours: Global Warming and the Increase in Extreme Precipitation from 1948 to 2011.” It concludes that nationwide, total annual precipitation is up nine percent over the period, and that on average, extreme rain and snow events are happening 30% more often. The authors define “extreme events” as those which have occurred no more than once a year, according to the historical record. Some might argue that once-a-year is a pretty low threshold for “extreme” but the trend is worth noting. Other scientific studies have taken a different approach, defining “extreme” as events that reside in the upper five percent of rainfall, temperature, or whatever is being studied.

Another finding was that big storms are getting bigger. This showed up most starkly in New England and the Mid-Atlantic states which, “saw the intensity of the largest storm each year increase by 20 percent or more,” according to the report.

The authors aren’t bashful about making the link between these trends and a warming planet, noting that warmer temperatures increase evaporation and the amount of moisture in the air:

“Scientists have found the water content of the atmosphere is now increasing at a rate of about 1.3 percent per decade. The additional moisture loaded into the atmosphere by global warming provides more fuel for intense rainstorms and snowstorms.”

“Heavy rainfall, heavy downpour events have been increasing and that is consistent with the fact that a warming atmosphere holds more moisture,” says David Easterling, who heads the Global Climate Applications Division for NOAA. He helped review the study but he’s at a loss to explain the north-south divergence in California.

The study shows that on average, in southern California, the big storms are getting not only more frequent, but bigger, whereas — again — the reverse seems to be true for most of the state north of San Francisco. Easterling says it appears that something is diverting the wettest winter storms, known as “atmospheric rivers,” to the south. He says the closely studied El Nino Southern Oscillation (ENSO) undoubtedly plays a role but then, he says, you have to ask how climate change is affecting the ENSO.

The study has not gone through a formal scientific peer-review process but Easterling told me that he’s impressed enough with the methodology and results that it should.

The report’s release comes just as state and local officials are gathered in San Diego for a workshop on planning for extreme weather events, and as California’s Natural Resources Agency prepares to release a major update in its series of periodic climate assessments.