Guest Post by Willis Eschenbach

So I was wandering through the marvelous KNMI website, and I came across data for the Palmer Self-Correcting Drought Severity Index. This is an index that measures the drought conditions in some given area. The source website says:

The scPDSI (self-correcting Palmer Drought Severity Index) is a variant on the original PDSI of Palmer (1965), with the aim to make results from different climate regimes more comparable. As with the PDSI, the scPDSI is calculated from time series of precipitation and temperature, together with fixed parameters related to the soil/surface characteristics at each location.

Now, the KNMI site only offers linear trends of data. But if you look at the bottom of the KNMI page linked above, or other pages at that level of inquiry, you’ll find that there is an option to download the NetCDF version of the data. As in this case, this NetCDF data is often gridded.

And using that NetCDF gridded file lets me make a graphic showing the average scPDSI for the globe.

Figure 1. Yes, indeed, Australia is a dry country

Note that the long-term averages range from minus 2.5 (very dry) to 1.9 (pretty wet). Here, to the same scale, is the monthly global average scPDSI.

Figure 2. Monthly global average self-correcting Palmer Drought Severity Index (scPDSI).

No overall change in the scPDSI over the last 120 years—droughts are not becoming either more or less frequent or intense … go figure.

Having gotten that far, I thought I’d see what the IPCC Sixth Assessment Report (AR6) has to say about droughts. Here are couple of quotes:

It is an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since pre-industrial time, in particular for temperature extremes. Evidence of observed changes in extremes and their attribution to human influence (including greenhouse gas and aerosol emissions and landuse changes) has strengthened since AR5, in particular for extreme precipitation, droughts, tropical cyclones and compound extremes (including dry/hot events and fire weather).

SOURCE

My rule of thumb is that most of the time when the IPCC says something is an “established fact” … it isn’t. Here’s another of their claims:

In summary, there is high confidence that concurrent heatwaves and droughts have increased in frequency over the last century at the global scale due to human influence.

SOURCE

Both of those made me say “Hmmm”, so I thought I’d take a look to see what they are calling “heatwaves”, “droughts”, and “high confidence”. From the glossary of the Working Group where those quotes are found, we have:

Heatwave: A period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months. Heatwaves and warm spells have various and, in some cases, overlapping definitions.

Drought: An exceptional period of water shortage for existing ecosystems and the human population (due to low rainfall, high temperature, and/or wind).

SOURCE

I suppose it might be possible for definitions of measurable phenomena to be more vague, but you’d have to work at it. How on earth can you have “high confidence” in claims involving totally undefined terms? And a two-day “heatwave”? Say what? That’s not a wave, that’s a tiny ripple.

Next, here’s how they assign “high confidence”. Not with mathematics or statistics, as you might imagine, but by squinting at it from across the room and making a value judgment based on “evidence” and “agreement”.

Figure 3. IPCC matrix for making value judgments regarding “confidence”.

So how can they have “high confidence” that ” concurrent heatwaves and droughts have increased in frequency” over the last century when they haven’t even bothered to establish clear, bright-line definitions for either heatwaves or droughts? This is fast approaching simply throwing darts at the above confidence matrix …

Having seen that the IPCC is merely issuing its usual meaningless pabulum, I continued my investigation of the scPDSI. I moved on to looking at the 120-year trends by geographical areas. Here’s that graph:

Figure 4. Century-long trends in the self-correcting Palmer Drought Severity Index.

Some points of note. First, there’s no overall trend. Next, almost nowhere has there been a change of more than ± 0.1 units per century. Next, the southwestern US has gotten wetter and the rest has gotten drier. And Australia, as usual … drier. But again, not much.

Seeing that graphic made me wonder about the oft-repeated claim that wet areas are getting wetter and dry areas are getting drier. For example, a study in Nature magazine says:

The “dry gets drier, wet gets wetter” (DGDWGW) paradigm is widely accepted in global moisture change.

To determine if this is true, we can use a scatterplot of the trend in scPDSI levels (Fig. 4) versus the average scPDSI levels. This gives us the following:

Figure 5. Scatterplot, trend vs average, self-correcting Palmer Drought Severity Index.

Interesting. Most of the world has an average scPDSI between -1 and +1 (bottom scale). There’s little trend in there. But in the dry areas, less than -1, the dryer it is, the wetter it’s getting. And the same is true above +1, the wetter it is, the dryer it’s getting.

Finally, I made a video of the annual changes in scPDSI around the globe. Here’s that graphic:

Figure 6. Video, changes in annual average scPDSI.

My conclusion from that? Ignore the hype about droughts. There’s almost always a hair-raising drought going on somewhere on this lovely planet.

In closing, to show there’s nothing new under the sun, here’s a long-term look at drought conditions in the American west …

Good thing that the now-ubiquitous Climate Karens weren’t around during the 200-year drought, the folks back then would never have heard the end of their whining …

My very best to everyone,

w.

Heard It Before Note: When you comment please quote the exact words you’re discussing. It avoids endless problems and misunderstanding.

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Editor’s note. For general reference on droughts and other topics, visit EveryThingClimate.com