Extreme Weather #11 – Trends in Droughts - Rainfall

In #10 we saw an overview which included the idea that a rainfall deficit is one part of a soil moisture deficit. But it’s the part we can measure.

AR6 says, p.1573:

Global studies generally show no significant trends in SPI [drought index for rainfall] time series (Orlowsky and Seneviratne, 2013; Spinoni et al., 2014), and in derived drought frequency and severity data (Spinoni et al., 2019), with very few regional exceptions.

Here’s the graph from Spinoni et al 2019 of 12-month drought frequency based on rainfall (I left out the left set which includes evaporation, that’s for another day). The top is 1951-1980, the middle is 1981-2016 and the bottom is the key graphic - the change. Blue is good, red is bad:

This graphic seems to correspond with the text of the report which said that there had been no overall change in “rainfall droughts” globally. But clearly, some regions are better off and some are worse off.

There is a very lengthy table at the end of the report which goes region by region - I’ve included my summary in the notes below.

Another complication is data, as hinted at in the last article. Spinoni’s team used the GPCC dataset, which is one of four major rainfall datasets.

Here’s a graph from Trenberth et al 2014. The paper was trying to answer the question of why climate scientists had reached opposite conclusions at that time about global droughts. One reason is that rainfall datasets have significant differences:

The total annual rainfall over land is about 800mm so the differences between datasets are up to 5%.

So, overall, on the rainfall metric of droughts there are regional ups and downs, but nothing clear globally.

In the next article we’ll look at the other important aspect for soil moisture - evaporation.

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Notes

Here are a few examples from the detailed table on drought by region. Northern Europe:

Medium confidence: Decrease in intensity and frequency; but dependence on considered index, time frame and region, including negligible trends over shorter periods or some subregions (Orlowsky and Seneviratne, 2013; Stagge et al., 2017; Spinoni et al., 2019; Dunn et al., 2020)

South Western South America:

Medium confidence: Increase in drought duration and severity (Skansi et al., 2013; Garreaud et al., 2017, 2020; Saurral et al., 2017; Boisier et al., 2018; Dereczynski et al., 2020; Dunn et al., 2020)

Central Africa:

Medium confidence - Decrease in SPI (Spinoni et al., 2019) and mean rainfall (Aguilar et al., 2009; Hua et al., 2016; Dai and Zhao, 2017)

note - this last one means an increase in drought.

Many of the references are global reports.

I’ve attempted to summarize the lengthy table below. Red is bad - more drought. Blue is good - less drought. Bold is “high confidence” (HC). The other text, MC, is “medium confidence”. We’ll cover attribution in later articles but I’ve highlighted the two changes which are attributed to human activity.

Note that most are “medium confidence” with only one region of “high confidence” - an increase in drought in north eastern South America.

The report isn’t really clear on why there isn’t high confidence on these changes.

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References

Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

A new global database of meteorological drought events from 1951 to 2016, Jonathan Spinoni et al, Journal of Hydrology: Regional Studies (2019)

Global warming and changes in drought, Kevin Trenberth et al, Nature (2014)