Has the CA drought been made worse by human-induced climate change? In his Dot Earth column yesterday Andrew Revkin, A Climate Analyst Clarifies the Science Behind California’s Water Woes, which argued, “No.” More specifically, Revkin interviewed NOAA scientist Martin Hoerling, who doesn’t think so. Hoerling is reality-based. He cites historical data in drawing his conclusions; specifically, the Palmer Drought index and consecutive days without rain. Using those as metrics he concludes that the context for the current drought isn’t significantly different than the context for past droughts. Based on the data he’s looking at, his conclusions are reasonable. That said, while Hoerling’s analysis seems legit as far as it goes I’m not sold that he went far enough. Joe Romm’s post, Climatologist Who Predicted California Drought 10 Years Ago Says It May Soon Be ‘Even More Dire’ addresses the limitations of Hoerling’s analysis. It’s not that Hoerling is wrong about the (lack of) trends he observes. It’s that he’s missing the bigger picture. Romm (emphasis mine):
Climate change can worsen drought in multiple ways. Climate scientists and political scientists often confuse the public and the media by focusing on the narrow question, “Did climate change cause the drought” — that is, did it reduce precipitation?
…scientists a decade ago not only predicted the loss of Arctic ice would dry out California, they also precisely predicted the specific, unprecedented change in the jet stream that has in fact caused the unprecedented nature of the California drought. Study co-author, Prof. Lisa Sloan, told me last week that, “I think the actual situation in the next few decades could be even more dire that our study suggested.”
Back in 2004, Sloan, professor of Earth sciences at UC Santa Cruz, and her graduate student Jacob Sewall published, “Disappearing Arctic sea ice reduces available water in the American west” (subs. req’d). They used powerful computers “to simulate the effects of reduced Arctic sea ice,” and “their most striking finding was a significant reduction in rain and snowfall in the American West.”
“Where the sea ice is reduced, heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air,” Sewall said. “The shift in storm tracks over North America was linked to the formation of these columns of warmer air over areas of reduced sea ice.” In January, Sewall wrote me that “both the pattern and even the magnitude of the anomaly looks very similar to what the models predicted in the 2005 study (see Fig. 3a [below]).”
Here is what Sewall’s model predicted in his 2005 paper:
Figure 3a: Differences in DJF [winter] averaged atmospheric quantities due to an imposed reduction in Arctic sea ice cover. The 500-millibar geopotential height (meters) increases by up to 70 m off the west coast of North America. Increased geopotential height deflects storms away from the dry locus and north into the wet locus
“Geopotential height” is the height above mean sea level for a given pressure level. The “500 mb level is often referred to as the steering level as most weather systems and precipitation follow the winds at this level,” which is around 18,000 feet.
Now here is what the 500 mb geopotential height anomaly looked like over the last year, via NOAA:
That is either a highly accurate prediction or one heck of a coincidence.
The San Jose Mercury News explained that “meteorologists have fixed their attention on the scientific phenomenon they say is to blame for the emerging drought: a vast zone of high pressure in the atmosphere off the West Coast, nearly four miles high and 2,000 miles long, so stubborn that one researcher has dubbed it the Ridiculously Resilient Ridge.” This high pressure ridge has been acting “like a brick wall” and forcing the jet stream along a much more northerly track, “blocking Pacific winter storms from coming ashore in California, deflecting them up into Alaska and British Columbia, even delivering rain and cold weather to the East Coast…
All this isn’t “proof” that human caused climate change helped shift and reduce precipitation in California during its record-setting drought. But a prediction this accurate can’t be ignored, either, especially because of its implications for the future.
“Accurate” is not the word I would have used – I might have said “The level of agreement between prediction and observation can’t be ignored…” – but Romm’s point holds. Sloan and Sewall predicted a change in atmospheric circulation which would induce drought, i.e., ” heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air … [which shifts] in storm tracks over North America.” We’re seeing it now. (FWIW, you can derive very accurate atmospheric temperature profiles from data collected satellite-based sensors. I’m not sure exactly how geopotential heights are determined over water but I’m sure you can do so very accurately via analysis of spectral data from sensors like AIRS and AMSU. Once upon a time I used to dabble in analysis of said data.)
Hoerling’s analysis is fine as far as it goes but it doesn’t address whether the current drought was induced (or the probability that it was induced) by changes in atmospheric circulation brought on by global warming. The current drought may be entirely like past droughts in terms of soil moisture characteristics – and in that sense be an entirely normal drought – but the state of the atmosphere and ocean which led to those conditions may be attributable to human-induced global warming. That’s what I see as the main point of the people Romm interviews.
One last point: The fact there is widespread consensus re basic global warming issues doesn’t mean a damn thing. I’m sure that centuries ago an even greater fraction of the scientific community concurred that the sun orbited the Earth and that the Earth was flat. Consensus is overrated. What matters is evidence. The scientific consensus re global warming should be taken seriously not because there is consensus but because there is strong evidence to support the hypothesis that the mean surface temperature is rising as a result of anthropogenically-generated CO2. Physics applies whether or not people choose to believe in it.