I was recently reminded of an issue I discovered several months ago. It seems worth revisiting. The Summary for Policymakers of the latest IPCC report (Working Group II) says:
Global economic impacts from climate change are difficult to estimate. Economic impact estimates completed over the past 20 years vary in their coverage of subsets of economic sectors and depend on a large number of assumptions, many of which are disputable, and many estimates do not account for catastrophic changes, tipping points, and many other factors. With these recognized limitations, the incomplete estimates of global annual economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0% of income (±1 standard deviation around the mean) (medium evidence, medium agreement).
The reference given for this text is Chapter 10, Section 9. When we look at that chapter, we find the same text in its Executive Summary. Like in the SPM, it cites 10.9. However, nothing in the text of 10.9 gives those numbers. They seem to have been pulled out of thin air.
Another troubling issue is the text in Chapter 10’s Executive Summary was changed to match that in the SPM. In the “Final Draft” of Chapter 10, the text originally said:
Globally aggregated economic impacts of global warming are a small fraction of income up until 3°C [10.9.2, medium evidence, high agreement]. A global mean average temperature rise of 2.5C may lead to global aggregated economic losses between 0.2 and 2.0% of income (medium evidence, medium agreement) and losses increase with greater warming. Little is known about aggregate economics impacts above 3°C. Impact estimates are incomplete and depend on a large number of assumptions, many of which are disputable.
General differences aside, the thing which stands out is the previous version of the text said a rise of 2.5C may cause 0.2 – 2.0% economic losses. The new version says “economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0%.” They’ve shifted the amount of warming necessary for these damages up by half a degree, and they’ve done so without any explanation.
A clue might be present in a previous version of the SPM. It said:
Global mean temperature increase of 2.5C above preindustrial levels may lead to global aggregate economic losses between 0.2 and 2.0%
Notice it says a rise above preindustrial levels. We could perhaps assume the 2.5C figure is in relation to preindustrial levels while the 2C figure is in relation to modern levels even though there has been a greater amount of warming than .5C. This assumption requires assuming the “Final Draft” of Chapter 10’s text was wrong to refer to a “rise of 2.5C” as ~20% of that rise had already happened.
Even with that assumption though, nothing much is explained. Even if we know exactly what value the IPCC intends, we have no explanation for where the values came from. It seems they were pulled out of thin air. However, if they were somehow based upon Section 10.9, then they would be affected by the changes I highlighted in my recent post. It’s difficult to see how they could get the exact same results for these two sets of data:
So where did the IPCC get its numbers from? Did it just make them up? Did somebody, somewhere, perform some secret calculations we’re not allowed to see?
It’s impossible to read Section 10.9 and think the numbers came from it. Does that mean the IPCC lied about where it got those numbers from?