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?
In general, most climate scientists say that is the wrong question — severe drought is much more than just a reduction in precipitation. After a political scientist unjustifiably labeled his mainstream views “zombie science,” the President’s Science Advisor, Dr. John P. Holdren, explained in an extended debunking how climate change worsens Western droughts even if it doesn’t reduce precipitation (see here and below).
First, though, as I’ve reported, 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.
Last year, I contacted Sloan to ask her if she thought there was a connection between the staggering loss of Arctic sea ice in recent years and the brutal drought gripping the West, as her research predicted. She wrote, “Yes, sadly, I think we were correct in our findings, and it will only be worse with Arctic sea ice diminishing quickly.” Last week, Sloan wrote me:
Yes, in this case I hate that we (Sewall & Sloan) might be correct. And in fact, I think the actual situation in the next few decades could be even more dire that our study suggested. Why do I say that? (1) we did not include changes in greenhouse gases other than CO2; (2) maybe we should have melted more sea ice and see what happens; (3) these atmospheric and precipitation estimates do not include changes in land use, in the US and elsewhere. Changing crops, or urban sprawl increases, or melting Greenland and Northern Hemisphere glaciers will surely have an impact on precipitation patterns.
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. That’s doubly true when there is also emerging evidence — documented by Senior Weather Channel meteorologist Stu Ostro and others — that “global warming is increasing the atmosphere’s thickness, leading to stronger and more persistent ridges of high pressure, which in turn are a key to temperature, rainfall, and snowfall extremes and topsy-turvy weather patterns like we’ve had in recent years.”
That’s why it was so puzzling that NOAA’s Martin Hoerling was quoted in the NY Times Thursday saying “to state the obvious, this drought has occurred principally due to a lack of rains, not principally due to warmer temperatures.” He ended by saying, “It is quite clear that the scientific evidence does not support an argument that this current California drought is appreciably, if at all, linked to human-induced climate change.”
Except that it is not quite clear there is no connection to climate change — as we’ve seen. Michael Mann, one of the country’s leading climatologists, told me:
There is credible peer-reviewed scientific work by leading climate scientists, published more than a decade ago, that hypothesized that precisely this sort of blocking pattern would become more frequent with disappearing Arctic sea ice. Moreover, Arctic sea ice has declined precipitously in the intervening decade. So it seems quite clear that there is a potential connection, in a statistical sense, between human-caused global warming, declining Arctic sea ice, and the anomalous blocking pattern this winter that has added to other factors we know are tied to human-caused climate change (warmer temperatures and increased soil evaporation, and decreased winter snowpack and freshwater runoff) to produce the unprecedented drought this year in California.
To claim that it is “quite clear” there is no connection at all turns the burden of scientific evidence completely on its head. Such a statement defies logic.
I asked climatologist and California water expert Peter Gleick for his thoughts. He wrote me:
Dr. Hoerling is answering the wrong question and his wording is conflating different research findings. In addition, his wording is confusing.
“Occurred principally”?? This is NOT the same thing as saying there is “no link between warmer temperatures and the current drought.” But that’s seems to be what he is implying.
Yes, the drought is principally due to lack of rain, not “principally” due to warmer temperatures. But note this is NOT saying that higher temperatures aren’t playing a role. To adopt his wording but saying the opposite: “To state the obvious, higher temperatures already occurring are worsening the impacts of the ongoing drought no matter its cause.”
His second sentence is also correct but perverse and incomplete. In particular, the word “linked” is misleading (does he mean causality or influence; if the former, he is correct; if the latter, he is incorrect).
Gleick has posted a good analysis of the confusion and conflation going on. He notes “the most definitive and well-understood effect (higher temperatures) have decreased current water availability” and shares this chart:
The increasing trend in annual temperature in California over the past 118 years. (Source: NOAA). This trend mirrors the global increase.
“The extra heat from the increase in heat trapping gases in the atmosphere over six months is equivalent to running a small microwave oven at full power for about half an hour over every square foot of the land under the drought,” climatologist Kevin Trenberth explained to me via email. “No wonder wild fires have increased! So climate change undoubtedly affects the intensity and duration of drought, and it has consequences.”
So, yes, climate change has undoubtedly worsened the drought, which was Holdren’s point in the first place. He wrote: “In my recent comments about observed and projected increases in drought in the American West, I mentioned four relatively well understood mechanisms by which climate change can play a role in drought. (I have always been careful to note that, scientifically, we cannot say that climate change caused a particular drought, but only that it is expected to increase the frequency, intensity, and duration of drought in some regions ― and that such changes are being observed.)”
These four mechanisms are:
- In a warming world, a larger fraction of total precipitation falls in downpours, which means a larger fraction is lost to storm runoff (as opposed to being absorbed in soil).
- In mountain regions that are warming, as most are, a larger fraction of precipitation falls as rain rather than as snow, which means lower stream flows in spring and summer.
- What snowpack there is melts earlier in a warming world, further reducing flows later in the year.
- Where temperatures are higher, losses of water from soil and reservoirs due to evaporation are likewise higher than they would otherwise be.
Holdren reviews the scientific literature on those statements in his reply, noting “the second, third, and fourth mechanisms reflect elementary physics and are hardly subject to dispute (but see also additional references provided at the end of this comment).”
This is very mainstream stuff, as I discussed in my 2011 literature review in the journal Nature on “The Next Dust Bowl.”
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