February 17, 2022

Is the Southwest U.S. Experiencing a Megadrought Fueled by Global Warming?

During the past week, many major media websites have headlined a study by some UCLA researchers suggesting that the Southwest U.S. is in a megadrought--the worst in 1200 years-- and that global warming is the prime cause.    

To illustrate, below is the front page of the LA Times.  And the Seattle Times highlighted the megadrought claims as well.  

Unfortunately, there are some major problems with this study and many of the hyperbolic claims--as I will explain below.   

The US Southwest has clearly experienced a dry spell recently, but global warming (a.k.a. climate change) is only a minor contributor compared to natural variability.  Decadal dry periods are not unusual or unknown for the U.S. Southwest.  They have happened many times before during periods when human-caused climate change could not be the origin.

Why 22 years?

A key aspect of this paper is its claim that the last 22 years were the driest over the southwest U.S. for the past 1200 years.   Their measure of dryness was soil moisture and they secured this indirectly by relating tree ring chronologies to such moisture.   Here is a plot of soil moisture from their paper:

Figure 1b from Williams et al. 2002.  Nature Climate Change.  The soil moisture 
is expressed as standard deviations from the mean.

Tree rings are, of course, imperfect measurements of soil moisture, but let's put that aside for the moment.   

You will notice a lot of ups and down in southwest U.S. soil moisture, and that in most ways, the recent dry period is unremarkable.  In fact, there were many previous events in which the soil moisture was drier.  There were many periods when the soil moisture was low (say less than -1) for longer periods.

Now, I am not a little surprised that none of the "curious" media stopped for a moment and asked:  why did these researchers pick 22 years?   Why not 25 years,  30 years, or 50 years?

The answer is that their whole narrative, their whole claim of unusual drought, would have weakened greatly if they had used 25 years or 30 years or anything longer.

You can see the issue from the plot above.  The soil moisture was in fact VERY HIGH during the 1990s, including 1998.  If they had used a longer period, they would have found more normal conditions.   

If you want to see this more clearly, let me show you the Palmer Drought Index over California during the past 120 years (see below).  This index combines temperature and precipitation and is a reasonable thing to compare to their soil moisture index.

The 22 year period (red, -1.51)) has an average that is much lower than the past 30 years (blue -.93) or the past 25 years (cyan, -1.24).   This effect is even larger if you look at precipitation.  By selecting 22 years they avoided the wet period in the mid to late 1990s.

Now I am not comfortable with their approach, with claims of megadrought dependent on selecting the exact period over which it recently has been dry.

A NOAA website just came up, and using it I have plotted the Palmer Drought Severity Index for the whole Southwest.   I put a line on the year 2000.  You will see that the Williams et al study selected the driest period for analysis...the optimally dry period, with a much wetter period preceding.  Such a rapid transition is not the expected impact of global warming, which would tend to change temperature/moisture gradually.


What is the origin of the two-decade dry period?

There is little doubt that it has been relatively (but not record) dry over the southwest U.S. during the past two decades.  The authors of this paper claim it is mainly due to global warming, but there is strong evidence that this is not the case.

Something unusual has been going on during the past 20 years:  persistent ridging (high pressure) during the winter over the northeastern Pacific. And such high pressure has kept storms away from the southwest U.S.  

Several highly regarded atmospheric scientists have investigated this issue and have found that the persistent high pressure and associated warm sea surface temperatures off the West Coast are due to natural variability, not global warming.   This (Johnstone and Mantua 2014) work has been published in the peer-reviewed literature  (see one below).

To provide one quote from this excellent paper:

These results suggest that natural internally generated changes in atmospheric circulation were the primary cause of coastal NE Pacific warming from 1900 to 2012 and demonstrate more generally that regional mechanisms of interannual and multidecadal temperature variability can also extend to century time scales. 

Specifically, natural variability, like changes in the Pacific Decadal Oscillation, is what forced the unusually warm, dry conditions of the past 20 years, and there is no evidence that global warming is contributing.

The authors of the megadrought paper, for some reason, ignored the obvious role of natural variability but focused on output from global climate models (GCMs).  Such climate models, forced by increasing greenhouse gases, simulated substantial warming during the past 20 years, drying the soils and thus leading to the author's claims that the 22-year megadrought was mainly the result of greenhouse gas emissions.

But there are substantial problems with such models and their application to this problem.

Climate models are known for their failure to properly simulate (or simulate at all) key modes of natural variability like the Pacific Decadal Oscillation.   The particular models used in this work (CMIP-6) are well known to be overly sensitive to greenhouse gas concentrations.   There are substantial problems with global climate model physics, such as their unphysical cloud coverage (something I am researching) and poor simulation of convection (thunderstorms).    And there are many other known deficiencies.  In addition, these models have been tuned to match the climate of the past century, which may undermine their ability to predict the future reliably.

In short, just because the climate models were producing warming over the southwest U.S. does not mean that increasing greenhouse gases were actually the cause, particularly when there is very good published science that suggests otherwise.  Furthermore, the climate models have well-known major deficiencies.

In Summary

The southwest U.S. has always experienced periodic droughts that have extended over decades and the fact that this has occurred for thousands of years shows that global warming from increasing greenhouse gases is not the cause.  This is part of the meteorology/climatology of the region.

We are in the midst of a dry period that is not particularly unusual in intensity and there is strong evidence that it is the result of natural variability.

The best science we have now suggests that increasing greenhouse gases will have uncertain effects on southwest U.S. precipitation during this century.  Regarding temperatures, at this point in time, global warming effects are probably small compared to natural variability but will increase during this century as greenhouse gas emissions increase and the atmosphere and ocean slowly warm.


I will be having a zoom session with Patreon supporters on Saturday at 10 AM.


  1. To paraphrase Jon Stewart: "Who gets to decide what misinformation is?"

    Seriously, who is the authority of what is true versus not and where is that authority derived from?

    Cliff, you have been on a tear about the MSM and how they distort information for whatever ends. Do we nationalize the media and have them parrot whatever the official government stance is? Or is this just a case of let people believe whatever aligns with their various convictions? Its not like science is respected these days, since it isn't.

    1. Well, the media has to return to trying to communicate what is in peer-reviewed science and being a bit suspicious of big claims...and trying to determine the truth for such claims. It used to be much better. The media has moved into advocacy, which has not gone well.

    2. It has not gone well for journalistic integrity and ethics but it makes lots of profit for the billionaires who own the media outlets. The media is in the advertising business, not the truth business. Freedom of Press doesn't require the Press to be exact on the information it sells. Coupled with the perception on a broad part of the constituency that science is a tool wielded by the elite to squelch what would otherwise be unmitigated freedom for the masses and you have an end result that is an uphill battle, Cliff. Texas and Florida are basically based upon combating that perception...while launching rockets into space.

      If everything wasn't politics and all politics weren't Culture Wars with all Culture Wars translating into profit/power, then there might be a more rational narrative on Climate Change.

    3. I grew up in the 70's, when questioning anything governmental bodies claimed was the prime purview of the MSM, as well as among the US public. It's propaganda all the way down now, which is a very dangerous aspect of the current situation. The Orwellian doublespeak of "Mega Drought" is just one more insidious example of this phenomenon.

  2. so many good points and questions about the paper's selective data analysis ...

  3. People expect certainty more than ever these days. No one likes to hear well gee it turns out Mother Nature is pretty complicated. In regards to advocacy, I will never understand why people who are concerned about the environment (and I am one of them) choose the most complex, hard-to-pin-down issue (climate change) to focus on when in fact we have more obvious and urgent issues like destruction of entire ecosystems, pollution, over-population, hunger and malnutrition right in front of us. It's as if people are more concerned about the faintest, most-distant alarm bell and ignoring the louder ones right in front of them.

  4. What do people like you feel the need to call others nasty names when they have a different viewpoint? I am neither right wing or a denialist. I do believe that society deserves the truth....which you obviously disagree with. Society can only deal with climate change effectively by having the best scientifically supportable information. ...cliff mass

  5. Couldn’t agree with you more Justin GK.

  6. Keep speaking the truth brother. I rarely feel the need to post. I'm not on social media. But I wanted to take a second to praise your fact driven analysis and willingness to do what a good scientist should do and have always done, challenge the status quo. Huge fan. Keep it up!

  7. Keep speaking the truth brother. I rarely feel the need to post. I'm not on social media. But I wanted to take a second to praise your fact driven analysis and willingness to do what a good scientist should do and have always done, challenge the status quo. Huge fan. Keep it up!

  8. I have a problem with the phrase "natural variability" when it refers to the long term ups and downs of weather factors such as temperature and precipitation. It is like it is enough just to say they go up and down and always have. I tend to believe there is a reason for these swings, we just do not understand them yet.

    1. We understand many of the sources of natural variability.

  9. Ahem, Cliff, you've made it clear with your ACTIONS (your blogs) that you thing natural variability is the primary factor changing the climate, which makes you a climate denier.

  10. Well it's either global warming, or the air molecules over the Southwest are racist molecules. Either explanation will work - take your pick.

  11. It's also true, I think, that humans have difficulty accepting that their day-to-day perceptions of climate-scaled phenomena are not particularly interesting. Given our life spans, barring catastrophic events that happen to overlap with our lives (e.g. all-out nuclear war, volcano, asteroid), a 70 year view of climate is just not that important.

  12. Anonymous. That is just silly. Both natural variability and global warming are occurring. The question is how much of each contribute to various weather/climate events. This is mainstream science and calling someone a "denier" who notes this science, displays an unfortunate ignorance and a tendency to attack what you don't understand.

  13. Asking that the current drought be compared with longer periods doesn't seem reasonable to me. It's completely possible that the current drought will continue with additional dry years. In hindsight, the 25-year period 2000-2025 might turn out to be significantly dryer than the 25-year period 1996-2001. (Or it might not). Since the years to the future haven't happened yet and we know that the late 90s were quite wet relative to the average, a 25-year or 30-year period seems to me to, in expectation, be likely to artificially downplay what is going on now.

    On the other hand, analysis on, say, a 15-year and a 20-year basis to go along with the 22-year basis would increase my confidence in the Nature Climate Change paper's results.

  14. I had a similar thought to your second comment. Rather than cherry-picking the beginning of the analysis period, shouldn't they have analyzed a series of time periods that are reasonably unbiased in length, such as identifying the driest 5-, 10-, 20-, or 30-year, etc periods within their 1200-yr data set? That would allow the data to speak for itself.

  15. I would like to read the original paper to learn how the authors justify the 22-year period, but unfortunately it is not open access. Interestingly, the second author (Cook) is affiliated with NASA and the research was funded by several federal grants. So, US tax dollars partially or fully funded the research and yet the authors chose not to pay the open access fee? Huh. Not very nice.

    1. "Not very nice" is the kindest take on it - more like "we're not going to allow any independent verification of our erroneous conclusions."

  16. Not that I am meteorologist but I am fascinated with the it and the study of weather patterns. Totally agree with your stance; some individuals are unable to separate themselves from their political parties agenda.

  17. I am wondering if you know of a web site I could access which would have data for the lowest winter temperatures reached for the last 100 years (or so),for our specific region (say between Bellingham and Sequim) (I am not interested in the average low for each year, but the lowest temperature reached in a weather event)
    thank you

  18. Cliff is right. If you let feelings or preferred outcomes rather than facts determne the results, that is not science. If you bend science based on "what can it hurt" you are undercutting science. You call Cliff Trumpian but in fact climate change alarmists and the mainstream media often act like Trumpers and "Stop the Steal". They use anecdotal information such as specfic droughts, heatwaves, fires, tropical cyclones being worse than ever and linked to climate change (compare to stories of boxes of ballots under the table in Atlanta or someone's dead granpaw voting). In fact careful long-term studies like Cliff's show it is not so (and careful recounts show there is no fraud).

  19. Hi Cliff. I was wondering if droughts in the US' southwest are linked to droughts in the south cone of South America. Here in Chile we've been in drought for 13 years or so; it's become quite dire (they're saying we'll have rationing this year in Santiago). I think Peru and parts of Argentina are also suffering from it. Most years high pressure systems block rains that could arrive from the sea, just like in the US. Of course droughts are common in this area of the world, but this has been very bad and of course the official narrative is that is the worst ever.

  20. Like Anonymous, I too get my Sciency stuff from Buzzfeed and Reddit. Enough with your Palmer Drought Index charts Cliff Mass!


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

First Lowland Snow over Western Washington

It is snowing right now in some favored locations in western Washington....yes, snow falling to sea level. Take a look at a video provided b...