Recent peer-reviewed studies strongly support the view that California’s epic drought was made considerably worse by human-caused global warming. A new report from NOAA seeking to cast doubt on that assertion omits some of the latest science and is deeply flawed, as three leading climatologists told me.

A recent Geophysical Research Letters paper, “How unusual is the 2012-2014 California drought?” by researchers from Woods Hole Oceanographic Institution examined two paleoclimate reconstructions of drought and precipitation for California. They found that the soil moisture deficit in the state is truly unprecedented as measured by the Palmer Drought Severity Index (PDSI):

… the current event is the most severe drought in the last 1200 years, with single year (2014) and accumulated moisture deficits worse than any previous continuous span of dry years.… In terms of cumulative severity, it is the worst drought on record (-14.55 cumulative PDSI), more extreme than longer (4- to 9-year) droughts.

In contrast, the new NOAA assessment, co-led by NOAA’s Martin Hoerling, “Causes and Predictability of the 2011-14 California Drought,” asserts, “The current drought, though extreme, is not outside the range of California hydro-climate variability and similar events have occurred before.” That mistake seems to be driven by their focus primarily on the precipitation, when we have known for at least a quarter-century that it is the combination of reduced precipitation during a time of increasing warming-driven evaporation, which creates the very worst droughts. See, for instance, the 1990 NASA study, “Potential evapotranspiration and the likelihood of future drought.”

In his critique of the NOAA assessment, leading climatologist Michael Mann, explains “most inexplicably, they pay only the slightest lip service to the role of temperature in drought, focusing almost entirely on precipitation alone. This neglects the fact that California experienced record heat over the past year, and this certainly contributed to the unprecedented nature of the current drought.”

The Geophysical Research Letters authors don’t make that mistake. Indeed, they explicitly note:

The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures.

It is the combination of reduced precipitation and record temperatures that make this a 1-in-1200-year drought.

This was the same point made to me by California-based climatologist Dr. Peter Gleick, one of the world’s leading water experts. He pointed out that in fact “the last 36 months are the hottest AND driest 36 months in the instrumental record. for California,” and sent me these NOAA charts:

Note that while California has had 36-month periods in the past 115 years almost as dry as the 36 months ending in November — average temperatures over the same 36-month time-frame have never been close to current levels, and it was far cooler in the deepest droughts of the 20th-century.

Climatologist Kevin Trenberth sent me comments that noted, “this study completely fails to consider what climate change is doing to water in California. It completely misses any discussion of evapotranspiration and the increased drying associated with global warming.”

The new Geophysical Research Letters paper explicitly concluded that “temperature could have exacerbated the 2014 drought by approximately 36% … These observations from the paleoclimate record suggest that high temperatures have combined with the low but not yet exceptional precipitation deficits to create the worst short-term drought of the last millennium for the state of California.”

I was on the press call with Hoerling and Seager, and asked them why they focused on the precipitation deficit and mostly ignored the unprecedented recent warming. The answer they gave was that precipitation is what this drought was about. As evidence, they said farmers “were praying for rain, not cooler temperatures.”

While dramatic reductions in precipitation are the major driver of record droughts, it must be noted that hot weather droughts are considerably worse for humans than cooler weather droughts — and not just because of the greater evaporation. As Gleick noted to me in January, when it’s hotter, you also have “a greater ratio of rain-to-snow” and “faster melting of snow,” both of which dramatically reduce the snowpack that is such a critical reservoir for California and the West during the dry summer months.

The new NOAA-led assessment mentions the word “snow” once, noting “Warming increases evaporative loss, raises water demand and reduces snow pack.” Why does NOAA take such a narrow view of warming impacts on the drought?

Gleick points to the title of the NOAA piece as a reason the authors may have gone astray, “Causes and Predictability of the 2011-14 California Drought.” You may recall that in 2012 climatologist Kevin Trenberth, wrote a peer-reviewed paper on how “to relate climate extremes to climate change,” that concluded, “The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question.”

Gleick similarly wrote me:

The question of whether the current California drought is caused by climate change is scientifically interesting, but a complete straw man — the real issue is the growing, clear evidence that climate changes — including record high temperatures and changes in atmospheric circulation patterns — are now influencing these extreme events.

Indeed, there are many studies directly linking the unique atmospheric circulation pattern behind this drought to climate change. Mann told me that “The methodology used in the current article, in my view, is flawed. Other work using a more appropriate methodology (e.g. by Noah Diffenbaugh of Stanford) has come to the conclusion that the unprecedented current drought was at least partly influenced by human-caused greenhouse gas increases.”

As the National Science Foundation news release explained back in September, the pattern that drove this drought is “a particularly tenacious ‘blocking ridge’ over the northeastern Pacific — popularly known as the Ridiculously Resilient Ridge, or ‘Triple R’ — that prevented winter storms from reaching California during the 2013 and 2014 rainy seasons.” NSF funded researchers including Diffenbaugh to compare two sets of models: “one set that duplicated the current climate, in which carbon pollution is warming the atmosphere,” and the other with carbon pollution levels from the 1700s:

The researchers found that the extreme heights of 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 to the formation of atmospheric ridges over the northeastern Pacific.

Coauthor Prof. Rajaratnam concluded, “We’ve demonstrated with high statistical confidence that large-scale atmospheric conditions similar to those of the Triple R are far more likely to occur now than in the climate before we emitted large amounts of greenhouse gases.”

On the press call, I asked Hoerling about this NSF-funded research, which was published in a special issue of the Bulletin of the American Meteorological Society this fall, “Explaining extreme events of 2013″ — a special issue that Hoerling himself coauthored. He said that analysis was wrong and his was right. In an email, Diffenbaugh writes me, “I stand by what we’ve published.”

For the record, the NSF research matches the finding in an April study that “there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge during winter 2013-14, the associated drought and its intensity.” The lead author of that study, Dr. Simon Wang of the Utah Climate Center, told me in an email earlier this year, “I personally think that the debate over global warming leading to stronger blocking has passed. The ongoing challenge is how we predict WHEN and WHERE those blocking will happen and affect WHICH region.”

Indeed, 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.

In fact, a growing body of 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.”

Bottom Line: Human activity has made droughts longer and stronger and hotter and drier in many places, including California. If we continue on our current path of unrestricted carbon pollution, we will be sharply increasing the chances of civilization-threatening mega-droughts here and abroad.

For more see my January post, “Leading Scientists Explain How Climate Change Is Worsening California’s Epic Drought,” and the literature review at the end of Gleick’s recent post.