Sunday, October 17, 2021

IPCC 6: Drought

I have now looked through all of the latest IPCC report. I say “looked through” because the report, including the Summary for Policymakers, is huge, just under four thousand pages. I looked over all of it, read only bits of particular interest to me.

One complication in writing about it is that the main report has, across every page in large light grey letters, “ACCEPTED VERSION SUBJECT TO FINAL EDITING,” and at the bottom of every page, “Do Not Cite, Quote or Distribute.” I will limit myself to describing what is in it and where but not actually quoting. The restriction is not present in the Summary for Policymakers, however.


Climate change is often blamed for recent droughts in a variety of places. That claim was supported by the fourth IPCC report but retracted in the fifth, so I was curious what the sixth said about it. 

The answer is complicated. The summary contains the figure shown below. It divides the world into 45 regions and reports for each on both confidence that drought had increased (or decreased) and confidence that there was a human contribution to the change. Yellow hexagons are regions where there is at least medium confidence in increased drought, green of decreased. One dot indicates low confidence in human contribution, two dots medium confidence, three dots (of which there are none) high confidence. 

As you can see, twelve of the 45 regions show at least medium confidence in an increase in drought, one of a decrease. In only two of the 45 regions is there medium confidence in a human contribution to the increase. So it is still true that, over most of the world, there is no confidence of increased drought due to humans. On the other hand, one of the two regions is WNA, Western North America, where I live. So the report does support the claim that recent drought in California is at least partly due to human contribution, presumably through climate change.

One complication to all of this is the definition of drought. According to the summary:

Agricultural and ecological droughts are assessed based on observed and simulated changes in total column soil moisture, complemented by evidence on changes in surface soil moisture, water balance (precipitation minus evapotranspiration) and indices driven by precipitation and atmospheric evaporative demand.

The IPCC defines drought by the amount of water in the ground, not the effect on plant growth. The reason this is a problem is that one of the effects of an increased concentration of CO2 in the air is a reduced need by plants for water. This fact gets mentioned in a different context in the main report, along with references to the resulting greening, more plant growth, over time. It follows that the IPCC definition of drought overstates it so far as the effect on plants is concerned. The IPCC ignores this in its discussion of drought.

The report does not provide enough information to redraw the figure, defining drought by plant growth instead of soil moisture. It is at least suggestive that, on the figure, twelve of the forty-five regions show increased drought, only one decreased, and yet according to the body of the report plant growth has, with high confidence, increased globally over the past 2-3 decades. That suggests that revising the figure to show the combined effect of change in soil moisture and CO2 fertilization would show fewer areas of increased drought, more of decreased, very possibly on net a decrease, not increase, in drought.



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