In the past week, many major media sites have reported a study by several UCLA researchers suggests that the southwestern United States is in the midst of a super-drought – the worst in 1200 years – and that global warming is to blame.
To illustrate, here is the front page of the LA Times. And the Seattle Times also highlights claims of a super-drought.
Unfortunately, there are some major problems with this research and many hyperbolic claims – as I will explain below.
The American Southwest has clearly experienced a recent dry spell, but global warming (aka climate change) is only a small contributor to natural variability. Periods of decadent drought are not unusual or unknown for the American Southwest. They have occurred many times before during periods where human-caused climate change could not be the source.
Why 22 years?
A key aspect of this article is its claim that the past 22 years have been the driest period in the Southwest United States in the past 1200 years. Their measure of dryness is soil moisture, and they ensured this indirectly by relating the duration of the tree rings to that moisture. Here’s a chart of soil moisture from their paper:
Figure 1b from Williams et al. 2002. Natural climate change. Soil moisture
expressed as the standard deviation from the mean.
Of course, tree rings are an imperfect measurement of soil moisture, but let’s put that aside for a moment.
Now, I’m not surprised at all that no “curious” media stops for a moment and asks: why did these researchers single out 22 years? Why not 25 years, 30 years or 50 years?
The answer is that their whole story, their whole claim of unusual drought, would have been greatly weakened if they had used 25 years or 30 years or so.
You can see the problem from the plot above. Soil moisture content was actually VERY HIGH throughout the 1990s, including 1998. If they used a longer period, they would have found more normal conditions.
If you want to see this more clearly, let me show you Palmer Drought Index over California for the past 120 years (see below). This index combines temperature and precipitation and is a reasonable comparison to their soil moisture index.
The 22-year period (red, -1.51)) has a much lower average than the past 30 years (in blue -.93) or the last 25 years (cyan, -1.24). This effect is even bigger if you look at precipitation. By choosing 22 years, they avoided the wet period of the mid to late 1990s.
Now I’m uncomfortable with their approach, with the claim that a super-drought depends on choose the exact time period that it has been dry lately.
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Summation
A NOAA website just came out and using it, I charted the Palmer Drought Severity Index for the entire Southwest. I put a line in 2000. You’ll find that Williams et al.’s study chose the driest period for the analysis … the optimal dry period, with a much wetter period before it. Such a rapid transition is not an expected effect of global warming, but will tend towards gradual temperature/humidity changes.
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What is the origin of the two-decade dry period?
There is little doubt that it has been relatively dry (but not on record) in the southwestern United States during the past two decades. The authors of this paper attribute it mainly to global warming, but there is solid evidence that this is not the case.
Something unusual has happened over the past 20 years: a persistent (high pressure) occurrence during winter over the northeastern Pacific. And such high pressure kept the storm away from the American Southwest
A number of highly regarded atmospheric scientists have studied the matter and discovered that the persistently high pressure and warm sea surface temperatures off the West Coast are caused by natural variability, not global warming. This work (Johnstone and Mantua 2014) has been published in peer-reviewed literature (see one below).
To provide a quote from this excellent paper:
These results suggest that naturally occurring internally-generated changes in the atmospheric circulation are the main cause of the Northeast Pacific coastal warming from 1900 to 2012 and demonstrate a totality of these results. More generally, the regional mechanisms of inter-year and multi-period temperature variation can also extend to centuries-old time scales.
Specifically, natural variations, such as changes in Pacific Decay Oscillationhas been responsible for unusually warm, dry conditions over the past 20 years, and there is no evidence that global warming is the cause.
The authors of the super-drought paper, for some reason, ignored the obvious role of natural variability and focused on yields from global climate models (GCMs). Such climate models, forced by an increase in greenhouse gases, simulate significant warming over the past 20 years, drying out the soil and thus leading to the author’s claim that the superdrought drags 22 years is mainly the result of greenhouse gas emissions.
But there are significant problems with such models and their application to the problem.
Climate models are known to fail to properly (or at all) simulate major natural modes of variation such as the Pacific Decay Oscillation. The specific models used in this work (CMIP-6) are known to be overly sensitive to greenhouse gas concentrations. There are significant problems with physics that model global climates, such as their nonphysical cloud coverage (something I’m working on) and poor simulations of convection (thunderstorms). And there are many other known disadvantages. In addition, these models have been adapted to the climate of the last century, which may reduce the ability to reliably predict the future.
In short, just because climate models are generating warming in the southwestern United States doesn’t mean the increase in greenhouse gases is actually the cause, especially when there’s very well published science. good shows the opposite. Furthermore, climate models have well-known major defects.
Summary
The American Southwest has always experienced periodic droughts spanning decades, and the fact that this has been going on for thousands of years suggests that global warming from increased greenhouse gases is not the cause. This is part of the area’s meteorology/climate.
We are in the midst of a dry period of no particularly unusual intensity, and there is solid evidence that it is the result of natural variability.
The best science we have at present suggests that an increase in greenhouse gases will have uncertain effects on precipitation in the southwestern United States this century. In terms of temperature, at this point, global warming effects may be small compared with natural variability but will increase this century as greenhouse gas emissions increase and the atmosphere and the ocean slowly warms up.
Notification
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