Guest Post by Willis Eschenbach
Drought is measured in many ways. One of them is the Palmer Self-Resolving Drought Severity Index (scPDSI). There is a clear description of it this. As with the original Palmer Drought Severity Index (PDSI), positive values reflect wetter weather and negative values reflect drier weather.
I think I’ll look at drought trends to see if a repeated statement holds true. This confirms that due to the increased CO2, wet areas will be wetter and dry areas will receive dryers. this is a typical version of this mantra, from the University of Maine.
In a warmer world, expect the wetter to be wetter and the dry to be drier
So I got scPDSI data from KNMI . website. Figure 1 shows areas that are frequently drought-prone and frequently wet.
Figure 1. Palmer’s Mean Self-adjusted Drought Severity Index (scPDSI), 1950 – 2020. Positive values are wetter, negative values are drier.
Next, I calculated the 71-year decadence trend of the scPDSI data for each 1° x 1° longitude grid cell in Figure 1. I then plotted the decimal trend, grid cell by grid cell, compared with the mean scPDSI value shown in Figure 1.
But before I give that result, let’s take a look at what we should be looking for. If the wet actually gets wetter and the dry actually gets drier, then when the aridity average is greater then a drying trend will occur, and vice versa. It should look like the imaginary result shown in Figure 2 below.
Figure 2. Expected results if wet get wet and dry get drier.
However, here in the real world we don’t find that kind of slope pattern at all. Instead, the actual results shown in Figure 3 are random – as has been the case since the beginning of time, some areas are wetter and some drier, but there is no pattern for it.
Figure 3. Scatter plot, drought trend vs mean drought, 1° x 1° longitude grid plot, 1950 – 2020.
As you can see, there is no statistically significant trend in the data. So if the wet gets wetter and the dry gets drier… then the Palmer Drought Severity Index didn’t get the memo.
Finally, this result is supported by considering a similar situation regarding precipitation. Here is that data.
Figure 4. Scatter plot, gridcell rainfall trend versus gridcell rainfall, 1950 – 2020.
Again, there is no indication that “wet gets wetter and dry gets drier”. Another climate myth has exploded…
My best regards to everyone,
w.
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