Is the Southwest US experiencing a great drought due to global warming? – Is it good?
Another exploration of Southwest horror and attribution to Climate Change.
from the Cliff High Volume Weather Blog
Dr. Clifford Mass
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.
Now, I’m not surprised at all that no “curious” media stops for a moment and asks: why did these researchers pick 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.
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 drought 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.