Too Much Rain,Or Not Enough Rain, by Willis Eschenbach

Source:

https://wattsupwiththat.com/2022/02/21/10178840/

"Drought is measured in several ways.

One of them is the Self-Correcting Palmer Drought Severity Index (scPDSI). There’s a clear description of it here.

As with the original Palmer Drought Severity Index (PDSI), positive values reflect wetter weather, and negative values reflect dryer weather.

I thought I’d take a look at the trends in droughts to see if an oft-repeated claim is true.

This claim is that due to increasing CO2, wet areas will get wetter and dry areas will get dryer.

Here’s a typical version of this mantra, from the University of Maine:

In a warmer world, expect the wet to get wetter and the dry to get drier

So I got the scPDSI data from the marvelous KNMI website.

Figure 1 shows the areas subject to frequent droughts and frequent wet spells.

Figure 1. Average self-correcting Palmer Drought Severity Index (scPDSI), 1950 – 2020. Positive values are wetter, negative values are dryer.

Next, I calculated the decadal trend of the 71 years of the scPDSI data for each of the 1° latitude x 1° longitude grid cells in Figure 1.

I then graphed the decadal trend, gridcell by gridcell, against the average scPDSI value shown in Figure 1.

But before I show that result, let’s consider what we should find. If the wet is actually getting wetter and the dry is actually getting drier,

when the drought average gets larger so should the drought trend, and vice versa.

It should look something like the imaginary result shown in Figure 2 below.

Figure 2. Expected result if the wet gets wetter and the dry gets drier.

However, here in the real world we don’t find that kind of sloping pattern at all.

Instead, the actual result shown in Figure 3 is random—as has happened since the start of time, some areas are getting wetter and some drier, but there is no pattern to it.

Figure 3. Scatterplot, drought trend versus average drought, 1° latitude x 1° longitude grid cells, 1950 – 2020.

As you can see, there’s no statistically significant trend in the data.

So if the wet is getting wetter and the dry is getting dryer … the Palmer Drought Severity Index didn’t get the memo.

Finally, this result is supported by looking at the same situation with regards to rainfall.

Here’s that data.

Figure 4. Scatterplot, grid cell rainfall trend versus grid cell rainfall, 1950 – 2020.

Once again, there’s no sign of the claimed “wet getting wetter and the dry getting drier”.

Another climate myth busted"