On the io9 frontpage yesterday there's this article by Mr. Dvorsky about a recent scientific paper that argues that the current California drought is "likely caused" by anthropogenic global warming (AGW).

Because the comments weren't entirely working yesterday, Hotscot, gorzilla, Addison Jacobs, ExLibrisMortis, dalejrfanfreak, and Joseph Giattino all took time to comment with a link to an LA Times article by Ms. Boxall whose headline says that "Studies find no clear link" between "California drought and climate warming"

Studies?

Yeah, studies. That word should have given pause to the six commenters, because it's possible that of the studies that the LA Times is referring to, that one study (the one that was written about on io9's frontpage) came to a conclusion that other studies disagreed with.

So what are the studies?

To quote Ms. Boxall: One of three studies examining the California drought in 2013 found that the kind of high-pressure systems that blocked winter storms last year have increased with global warming.

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That is a reference to Swain et al.'s study, which is the one which Mr. Dvorsky's article was about. Swain et al.'s study is one of 22 studies in an annual report entitled Explaining Extreme Events from a Climate Perspective 2013 Report (I'll refer to that as the EEEfaCP2013R). The EEEfaCP2013R has twenty-five chapters: an intro, 22 studies looking at different climate events of 2013, a summary, and a reference chapter embiggened by lots of references.

Of these 22 studies, three are about the California drought. So let's see what those three studies, and the intro and summary of the EEEfaCP2013R, have to say about the California drought.

Herring et al. 2014 introduction chapter:

From the abstract: The findings indicate that human-caused climate change greatly increased the risk for the extreme heat waves assessed in this report. How human influence affected other types of events such as droughts, heavy rain events, and storms was less clear, indicating that natural variability likely played a much larger role in these extremes.

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From the paper: The evidence from multiple studies, including those mentioned above, indicates that human influence has changed the frequency of high-impact temperature and precipitation extremes on average over land where there are sufficient observational data to make this assessment.

What does this chapter say about a link between the current California drought and AGW: Nothing. It does not directly comment, at all, on the current California drought.

Swain et a. 2014 "The Extraordinary California Drought of 2013/14: Character, Context, and the Role of Climate Change"

From the abstract: California's driest 12-month period on record occurred during 2013/14, and although global warming has very likely increased the probability of certain large-scale atmospheric conditions, implications for extremely low precipitation in California remain uncertain

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From the paper: Thus, we find that anthropogenic forcing—rather than natural external forcing—dominates the simulated response in extreme GPH … The distribution of ratios between the bootstrapped return periods calculated for the 20C and P.I. simulations suggests that it is "likely" ("very likely") that the probability of extremely high GPH is at least a factor of 4.02 (2.86) as great in the current climate as in the preindustrial control climate (Fig. 2.2c) … Thus, our present methodology cannot reject the possibility that the frequency of occurrence of years with anomalous GPH gradients —and the risk of extreme drought associated with a perturbed North Pacific storm track—has not changed between the preindustrial period and the present. … We find that extreme geopotential height values in this region, which are a defining metric of this type of atmospheric configuration, occur much more frequently in the present climate than in the absence of human emissions (Fig. 2.2)

In English, what does this chapter do: By running climate simulation models of the Earth with and without increased greenhouse gas concentrations, this study argues that there is very likely a link between the weather conditions causing the current California drought and AGW.

What does this chapter say about a link between the current California drought and AGW: Although the study cannot reject the possibility that there is no link, it argues that this possibility is much lower than the possibility that there is a link.

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Wang and Schubert 2014 "Causes of the Extreme Dry Conditions Over California During Early 2013"

From the abstract: The 2013 SST anomalies produced a predilection for California drought, whereas the long-term warming trend appears to make no appreciable contribution because of the counteraction between its dynamical and thermodynamic effects.

From the paper: The implication of the above mean circulation changes is that the weaker westerlies may reduce the number of North Pacific storms reaching California and thereby enhance the risk of dry events over California. Figure 3.2c shows that the PDF of precipitation over California shows no notable change, consistent with a similar PDF analysis using the GPCC precipitation observations (not shown). We note that, for both the GEOS-5 AMIP simulations and the GPCC observations, the mean precipitation over California does not show any noticeable changes between the two periods either. This suggests that there was no increased risk of drought in California during 2013 as a result of the long-term warming trend. … The extreme precipitation deficits over California during early 2013 resulted from considerably fewer North Pacific storms reaching California, due to the blocking by persistent high anomalies over the northeast Pacific.

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In English, what does this chapter do: By examining historical records, it argues that the probability/risk of drought events in California has not been increasing, in a complex way: fewer North Pacific storms reach California, but these fewer storms are wetter. The current drought is caused by "bad luck": few North Pacific storms reaching California for a relatively long among of time.

What does this chapter say about a link between the current California drought and AGW: That's tough to summarize. The chapter argues that increased sea surface temperatures (caused by AGW) do make dry events over California more common, although only slightly, because when rain does happen, it happens in larger abundance. This chapter says there is a link between the current California drought and AGW, but a weak one: AGW increases the odds of events like the current California drought, but also increases the odds of much, much wetter events.

Funk et al. 2014 "Examining the Contribution of the Observed Global Warming Trend to the California Droughts of 2012/13 and 2013/14"

From the abstract: Long-term SST warming trends did not contribute substantially to the 2012/13 and 2013/14 California droughts. North Pacific SSTs were exceptionally warm, however, and coupled models indicate more frequent extreme precipitation.

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From the paper: The models indicate more extreme (more wet and more dry) 21st century California precipitation. … The +1.4°C 2013/14 anomaly was extremely unlikely given either prior observed SST values or the 20th century CMIP5 ensemble. Anthropogenic climate change seems likely to have contributed to these extreme SSTs (Fig. 4.1f) and the associated extreme upper- level height anomalies … our results do not indicate that this long-term warming trend contributed substantially to the 2013 and 2014 drought events. … In 2013/14, the North Pacific SSTs (Fig. 4.1f) and the intensity of the upper troposphere geopotential height gradient (Wang et al. 2014) reached historic maxima. These extremes appear very unlikely without anthropogenic climate change. If SST and ridging events like this become more common, California could experience more frequent droughts (Favre and Gershunov 2009). ... In addition, given the strong thermal control on evaporation, snowmelt, and water resources in California, the long-term warming is continuing to exert a growing stress on water availability (Barnett et al. 2008), potentially amplified by both more frequent dry days and more precipitation extremes (Polade et al. 2014).

In English, what does this chapter do: It runs climate models, quite a few climate models, to explore changes in sea surface temperatures and how that affects California precipitation. Similar to Wang and Schubert (2014), it finds that California's long-term annual precipitation is not noticeably changing, but that more extreme weather events (either low or high amounts of rain) are increasing in frequency.

What does this chapter say about a link between the current California drought and AGW: This study argues that the current California drought is caused by conditions that "appear very unlikely" with AGW. This chapter says that AGW and the current California drought are linked.

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Herring et al. 2014 summary chapter:

From the abstract: This report contributes to the growing body of evidence that human influences on climate have changed the risk of some extreme events and that scientists are increasingly able to detect these changes. A failure to find anthropogenic signals for several events examined in this report does not prove anthropogenic climate change had no role to play. Rather, an anthropogenic contribution to these events that is distinguishable from natural climate variability could not be detected by these analyses. Thus, there may have been an anthropogenic role, but these particular analyses did not find one.

From the paper: [two papers] looked directly at the precipitation deficits associated with the California drought and their link to SSTs and found no appreciable effect from long-term SST warming. [the other] took a different approach and focused on particular circulation patterns that contributed to the drought, rather than examining precipitation directly. While they found global warming to increase the probability of certain large- scale atmospheric circulations, the implications for extremely low precipitation over California were found to be uncertain.

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Note on California drought: Some elements of factors that contributed to California drought were found to be affected by long-term climate change.

Note on Swain et al. study: Swain et al. found a strong influence of global warming upon the large-scale atmospheric conditions linked to the drought but do not make an explicit attribution claim regarding extremely low precipitation

What does this chapter say about a link between the current California drought and AGW: This summary chapter admits that Funk et al. and Wang and Schubert both found that AGW has affected factors that contribute to drought, and that Swain et al. found that AGW strongly influences atmospheric conditions linked to the current California drought. But it for some reason refuses to combine those three conclusions together and state, plainly, that the current California drought and AGW are linked. I don't get why this summary chapter refuses to do such a thing.

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What could have improved Ms. Boxall's article?

It is very obvious from her article that she was told to write a story about a summary chapter without looking at the chapters that the summary chapter was summarizing. Such a lack of looking at primary sources is an unfortunate side-effect of corporate news-production: journalists don't always have time to hunt down primary sources. Maybe the LA Times should give their journalists more time to write stories? Preferably with extra pay. Don't make them work more for less money per hour, that's not cool.

TL;DR:

Yes, there is a link between the current California drought and anthropogenic global warming. Warming the atmosphere results in more extreme weather events in the North Pacific. This results in higher probability of both droughts and flooding. Be safe out there in California and do what you can to prepare for both in the 21st century and beyond.