Human influence on the Californian drought

A much more acute situation

Daniel Swain and Noah Diffenbaugh and colleagues have been looking at the current drought in California to see if global warming has played a part. Earlier this year, their work was reported by Stanford:

The current drought is different from many of California's previous droughts. For example, the state's last major dry spell occurred in the early 1990s and was characterized by below-average amounts of rain and snowfall for several years.

"That's what we typically think of when we think of drought – a few years when precipitation is below normal. We don't conceptualize that the precipitation would just shut off," Swain said. "That's what's so remarkable about this drought. It's not a multi-year drought that's getting progressively worse as the years go by. It's that it has barely rained at all this year. That's a much more acute situation in a lot of ways."

Source: US Drought Monitor
(Click image to enlarge it.)

Three times more likely now than before industrialisation

Daniel Swain coined the term "Ridiculously Resilient Ridge" or Triple R, to describe the large region of high atmospheric pressure that's preventing rains from getting to California and is causing the current drought. Results from their work have recently been published in a special supplement to this month's issue of the Bulletin of the American Meteorological Society (BAMS). It turns out that the conditions causing the current drought are three times more likely to occur with global warming than without.


Stanford has issued a press release describing the report.

The team first assessed the rarity of the Triple R in the context of the 20th-century historical record. They found that the combined persistence and intensity of the Triple R in 2013 was unrivaled by any event since 1948, which is when comprehensive information about the circulation of the atmosphere is first available.

To more directly address the question of whether climate change played a role in the probability of the 2013 event, the team collaborated with Bala Rajaratnam, an assistant professor of statistics and of environmental Earth system science and an affiliated faculty member of the Stanford Woods Institute for the Environment. Rajaratnam and his graduate students Michael Tsiang and Matz Haugen applied advanced statistical techniques to a large suite of climate model simulations.

Using the Triple R as a benchmark, the group compared geopotential heights – an atmospheric property related to pressure – between two sets of climate model experiments. One set mirrored the present climate, in which the atmosphere is growing increasingly warm due to human emissions of carbon dioxide and other greenhouse gases. In the other set of experiments, greenhouse gases were kept at a level similar to those that existed just prior to the Industrial Revolution.

The interdisciplinary research team found that the extreme geopotential heights associated with the Triple R in 2013 were at least three times as likely to occur in the present climate as in the preindustrial climate. They also found that such extreme values are consistently tied to unusually low precipitation in California and the formation of atmospheric ridges over the northeastern Pacific.

"We've demonstrated with high statistical confidence that the large-scale atmospheric conditions, similar to those associated with the Triple R, are far more likely to occur now than in the climate before we emitted large amounts of greenhouse gases," Rajaratnam said.

The worst Californian drought in 100 years

This Ridiculously Resilient Ridge has just hung about, moving around a little bit, shrinking from time to time for short spells and generally stopping the rain from falling over California, with particular effect during what should have been the wet season, naturally enough. This is what the scientists say has resulted in the worst drought in living memory and, according to Daniel, "likely the worst in over 100 years".

Daniel Swain has described this very clearly in an article on his website.

Teleconnections and The Blob

The scientists did more than just look at the extremes in geopotential heights. The ocean at some distance away can affect California. Daniel wrote:

While the immediate impact of unusually warm or cool SSTs is usually felt by the atmosphere in the the immediate vicinity, geographically-remote atmospheric effects (known as “teleconnections“) can occur thousands of miles away.

Several recent studies have examined precisely this possibility in assessing cause of the extraordinary persistence of the Ridiculously Resilient Ridge in 2013 and 2014. Wang et al. (2014) find that tropical West Pacific SST warm anomalies (associated with the West Pacific Warm Pool that acts as a precursor of El Niño) played a leading role in causing the strength and longevity of the Triple R by generating a recurring series of atmospheric “Rossby waves” that propagated from west to east across the Pacific Basin. Wang and Schubert (2014) find that the North Pacific SST warm anomalies during early 2013 created a “predilection” for dry conditions during the second half of the 2013-2013 “rainy season” in California, and Funk et al. (2014) also report that the observed Pacific SST anomalies during 2013-2014 contributed to the extremely low precipitation that was observed during 2013-2014.

I've put together an animation of the sea surface temperatures (from NOAA), circling the area that I believe Daniel is referring to in the North Pacific. I've only included one image for each month, so it's a bit rough.

The Blob. 
Source: NOAA

Daniel wrote:

...Further evidence suggests that the extremely warm North Pacific ocean temperatures observed during 2013-2014, which are linked to the extremely persistent region of high pressure associated with the Triple R, would have been highly unlikely to occur if not for the human emission of greenhouse gases (Funk et al. 2014). This region of exceptionally high, record-breaking SSTs–which persists as of late September 2014–has started to garner considerable interest in oceanographic circles (where it has come to be known colloquially as “The Blob“).

The full picture

To get the full picture of what has been found so far, I highly recommend reading the paper in BAMS and other papers in that special supplement as well. Also, read Daniel's account on his website. He starts on this page and provides details of his research on the next page here. There is also a press release from Stanford.

Anthony Watts says - "despicable" - tree rings prove something or other

Now you'll be interested to learn that Anthony Watts poo-poos this research (archived here). Why? Well, he reckons that the drought is nothing special. After all, he says, there were longer droughts 800 years ago. And how do we know that? Because scientists looked at tree rings. Suddenly Anthony Watts decides he likes tree rings after all. He wrote:

More despicable climate hype from Noah Diffenbaugh (press release follows) who is ignoring the obvious: California has had far worse droughts before “global warming” was a glimmer in a scientist’s eye, and these were driven by changes in weather patterns that happened long before CO2 became an issue.

(Who'll name the logical fallacy? Is it this one?) Anthony put up an image someone adapted from a 2010 paper by Edward Cook and colleagues, which showed the megadroughts that occurred over western USA from centuries ago to early this century. Here is the actual figure his image was derived from, which also shows the area covered by the chart down the bottom:

Figure 4. Long-term aridity changes in the West as reported by Cook et al. (2004). The top figure shows the overall map covered by the North American Drought Atlas and the area inside the irregular polygon referred to as the West. The bottom figure shows the Drought Area Index (DAI; per cent area covered by PDSI< 1 each year) over time in the West as reconstructed by tree rings, both annual in pale brown and 60-year low-pass filtered in black. The dashed blue curves are two-tailed 95% confidence intervals for the latter. The red and blue lines are mean DAI for the MCA (ca. AD 900–1300) and the 20th century out to 2003, respectively. This record shows that the MCA (ca. AD 900–1300) was much more arid on average than the 20th century. Redrawn from Cook et al. (2004, 2007)
Source: Cook10

Two points:

• The area covered in Anthony's favoured Cook (tree ring) paper is a lot larger than just California.
• As well as that, the above chart only goes to 2003 so it doesn't include the current drought conditions.

If the bottom chart were just for California (which it's not), then this drought would arguably start to rival the megadroughts of 800 years ago and earlier. Not in duration (yet) but in intensity. Also, it's a bit rich for Anthony to suddenly switch from it hasn't warmed in 4, 16, 17, 19 years therefore global warming isn't happening - to - this drought can't be caused by global warming because there were big droughts 800 years ago.

Not only that, but as far as the USA is concerned, the Cook paper states:

Based on intensity and spatial coverage, the most severe 21st century drought year occurred in 2002, with more than 50% of the coterminous USA under moderate to severe drought conditions.

Remember, the bit about 2002 would have been written in 2009. After that there was the 2012 drought.

And it gets better, or worse, depending how you look at it. The Cook paper relies on tree rings for a broad understanding of pre-industrial drought, including the megadroughts of the 13th century in the West, including on the Colorado plateau and in Nebraska. And here I thought that Anthony couldn't abide tree rings.

Then it gets worse again. Although the image Anthony used was from Cook 2010, he linked to a WUWT article which was a copy and paste of a press release about another paper. This one wasn't about California at all, it was about Utah. Specifically, it was about streamflow for northern Utah's Weber River over the past 576 years. There's a press release from Brigham Young University, with a nice big picture of - yep, you guessed it, tree rings!

Why not ENSO or El Niño?

Another thing Anthony complained about was in his headline, which was:

Claim: Cause of California drought linked to climate change – not one mention of ENSO or El Niño

Indeed. I've no idea why Anthony thought ENSO or El Niño should be mentioned. For one thing, doesn't El Nino often bring rains to California? For another thing, the paper was about the 2013-14 Californian drought. There was no ENSO event in that time.

Bob Tisdale probably didn't read the paper

Bob Tisdale has also weighed in, twice. His most recent is today and is archived here. Bob is a one-trick pony. He specialises in sea surface temperature and sees everything through that lens. He wrote at one point:

It’s tough to employ climate models so you can claim that manmade greenhouse gases caused the California drought, when the models used by Swain et al. can’t simulate the lack of warming of one of the key metrics associated with it.

Which is a silly thing to say. Climate models are used for all sorts of things. In this study, they were used to look at geopotential height (GPH) as an indicator of dryness. Bob didn't mention geopotential height once. He spent most of his time writing about sea surface temperatures and CMIP5 models. In the paper, when looking to compare 2013 with historical patterns, the authors wrote:

We define a “2013-magnitude event” as the mean January–December 2013 500-mb GPH over the core area of unprecedented annual GPH (35°–60°N and 210°–240°E; Fig. 2.1e).

Bob wrote a whole heap about stuff that the paper wasn't about and didn't discuss what the paper was about.

Bob wrote about sea surface temperature in the Eastern North Pacific up to 2012. The authors discussed the impact of the extra warm waters in the Western and North Pacific in 2013-2014. Here is the area that Bob spent half his article on:

Blue rectangle covers the area of Bob Tisdales' average SST
Source: NOAA - from 1 Sept 2014

It covers a lot of territory. Averaging it would diffuse the "blob" referred to above, had Bob bothered to focus on the period in question - 2013-14 instead of excluding that period. Most of Bob's charts were from January 1989 to December 2012. I don't know what he was thinking. The authors were writing about the 2013-2014 California drought. Bob did include this chart:

Instead of remarking on the steep rise in sea surface temperature in recent months, all he said was:

And even with the addition of the most recent data, Figure 2, the sea surface temperatures of the eastern extratropical North Pacific show no warming since January 1989, based on the linear trend.

Not that anything Bob wrote had anything to do with the Swain paper. That was about the Californian drought of 2013-14. It was about the Ridiculously Resilient Ridge, which is in the atmosphere not the ocean. The paper was about geopotential height and precipitation. Bob was writing about sea surface temperatures.

Reading Bob's article you'd almost be persuaded that there was no drought in California at all. He barely mentioned the drought itself. He was too busy drawing charts of sea surface temperature writing his own story to pay any attention to the actual research.

Reading Bob's article I have to wonder if he even read the paper. If he did, I wonder did he understand half of it. If he did understand it then he was being deliberately misleading. I expect he didn't understand it at all. He doesn't understand the value of climate models. He just thinks that all the models are wrong and useless, which is a very big mistake on his part. But then again, Bob thinks that global warming is caused by ENSO. We're used to mistakes made by perennially puzzled Bob Tisdale.

Another big mistake that Bob made was to argue that because climate models don't line up exactly in time with observations when it comes to surface temperature, they are of no use at all. He's wrong, but that's another story. In this case, that's not how the scientists used them. What the scientists did was look for similar patterns to what is currently being seen. They looked at models with no added greenhouse gases. Models that were purely subject to natural (not anthropogenic) forcings, and compared them to those with anthropogenic forcings. Time is immaterial as far as I can gather. They weren't looking to see if the models could generate the patterns at a particular time. They were interested in whether the patterns would appear at all. And they did - but mostly only when anthropogenic forcing was included.

Disclaimer and further reading

Here again are links to Daniel's article, the press release, and the BAMS supplement, which includes the paper and others.

I make no assurances that I've interpreted the work properly. I think I've got the gist of it but please point out if you think I've gone astray anywhere.

The BAMS supplement includes a paper by Chris Funk and colleagues to which Bob Tisdale's article would have been more relevant, perhaps. They concluded that the long term warming trend in sea surface temperatures didn't have a substantial impact on the Californian drought. However they did lend support to the conclusions of the Swain paper (and Wang et al 2014), that the extremes in the geopotential gradient would be very unlikely except for human-caused global warming (with the ridge preventing rainfall in California). They wrote: "If SST and ridging events like this become more common, California could experience more frequent droughts."

There are other articles on California and some on the records being set in Australia, too.

From the WUWT comments

johnmarshall is a greenhouse effect denier and wrote:

October 3, 2014 at 4:38 am
Most of us know that climate changes, always has it is the cause that is the problem and that is not CO2.

joelobryan accepts there is a drought but rejects the possibility that increased CO2 could have played a part:

October 3, 2014 at 8:36 am
The CA drought is definitely a very serious problem, and not just for Cal. Anyone in N. America who likes to eat fruits and vegetables are being impacted.
CO2 though at 400ppm in no honest way can be implicated as a casual factor. CO2 is the non-problem.

Gary Pearse is a fake sceptic. He didn't read the press release or the paper. He fell hook, line and sinker for Bob's squirrels about SST and climate models:

October 3, 2014 at 5:24 amWhy didn’t they use the observed temperatures? Do they explain? It is clear from the model they used that it needed a lot of novel statistics and reanalysis though.

earwig42  thinks Bob is honest. Honestly, if he is honest then he doesn't have a clue.

October 3, 2014 at 7:20 am
As usual, clear,concise and honest. Thanks Bob.

Barry is the first person to point out that Bob got it all wrong:

October 3, 2014 at 7:48 am
Bob, if you read Chapters 3 and 4 of the BAMS report, they both indicate that the CA drought cannot be attributed to the long-term warming trend, which seems to align with your argument. The Swain et al. paper, though, does not make a case based on sea surface temperatures, but rather geopotential height and wind anomalies (the high pressure ridge).

I thought in the past there were signs that Pamela Gray had a bit of sense. Not a lot, but a smidgen. I was wrong:

October 3, 2014 at 9:19 am
Current AGW climatologists are just like teenagers. No matter how many generations of teenagers have shown clear evidence of poor decision making skills compared to later in adult life, the current generation of teenagers think they know everything and have the wisdom of Job. History has no place in their brains.

After this California drought goes away the hordes of panicky scientists will too. Until the next drought comes along. At that point the current crop of AGW climatologists will once again be walking the hallowed halls of the Ivory Tower with tall signs saying the World Will End Lest We Repent. History has no place in their brains.

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Daniel L. Swain, Michael Tsiang, Matz Haugen, Deepti Singh, Allison Charland, Bala Rajaratnam, and Noah S. Diffenbaugh, 2014: "The Extraordinary California Drought of 2013/14: Character, Context, and the Role of Climate Change" [in "Explaining Extremes of 2013 from a Climate Perspective"]. Bull. Amer. Meteor. Soc., 95 (9), S15–S18.

Funk, C., A. Hoell, and D. Stone, 2014: "Examining the Contribution of the Observed Global Warming Trend to the California Droughts of 2012/2013 and 2013/2014" [in "Explaining Extremes of 2013 from a Climate Perspective"], Bull. Amer. Meteor. Soc., 95 (9), S11–S15.

Wang, S.-Y., L. Hipps, R. R. Gillies, and J. Yoon, 2014: "Probable causes of the abnormal ridge accompanying the 2013–2014 California drought: ENSO precursor and anthropogenic warming footprint", Geophys. Res. Lett., 41 (9), 3220-3226. DOI: 10.1002/2014GL059748 (open access)

Cook, Edward R., Richard Seager, Richard R. Heim, Russell S. Vose, Celine Herweijer, and Connie Woodhouse. "Megadroughts in North America: Placing IPCC projections of hydroclimatic change in a long‐term palaeoclimate context." Journal of Quaternary Science 25, no. 1 (2010): 48-61. DOI: 10.1002/jqs.1303

Bekker, Matthew F., R. Justin DeRose, Brendan M. Buckley, Roger K. Kjelgren, and Nathan S. Gill. "A 576‐Year Weber River Streamflow Reconstruction from Tree Rings for Water Resource Risk Assessment in the Wasatch Front, Utah." JAWRA Journal of the American Water Resources Association (2014). DOI: 10.1111/jawr.12191