Earlier today, I noticed an odd contradiction in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). The Working Group II (WGII) Summary for Policy Makers 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)
While citing Chapter 10. The Final Draft of Chapter 10 says:
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.
The SPM says “additional temperature increases of ~2°C” will cause a loss of 0.2 and 2.0% GDP. Chapter 10 says a “global mean average temperature rise of 2.5C may lead to global aggregated economic losses between 0.2 and 2.0% of income.” Why does the SPM say those damages will happen half a degree sooner?
I don’t know. What I do know is the previous version of the SPM said damages for a:
global mean temperature increase of ~2.5°C above recent levels are between 0.2 and 2.0% of income
Meaning the SPM was changed at the last minute to say those damages will happen sooner, contradicting the section it cites.
Now then, the revision-display for that version of the SPM has the ~2.5°C highlighted, which I believe indicates it will be rewritten. That would mean the decision to edit it was disclosed in the revision process. It just doesn’t seem to have been explained. How could they? You can’t justify citing a chapter which lists one value while randomly listing a different value? The only way they could justify it is to change the value given in the “Final Draft” of Chapter 10 to match the SPM. But isn’t that backwards?
I don’t know what’s going on. To try to figure things out, I decided to look into the origin of this ~2.5°C/~2°C value. The previous quote from Chapter 10 is in its Executive Summary section, and it cites Section 10-9. None of those numbers show up in the text of that section. In fact, they don’t show up anywhere else in the chapter.
They also don’t appear to show up anywhere in the figures for Section 10-9:
Which are the only figures with any numerical values in the chapter. As far as I can see, those values have been pulled out of thin air. It seems impossible to tell whether we should expect to lose 0.2 – 2.0% of our global GDP at 2°C or 2.5°C because neither value has any documentation to support it.
For thoroughness, we should try checking earlier versions of Chapter 10 to see if perhaps a reference got lost. Unfortunately, that won’t work. The previous draft, the Second Order Draft, doesn’t have any text about this value in its Executive Summary. That means the text I quoted at the start of this post was only added after the last round of external reviews.
To sum up, an apparently calculated value was seemingly pulled out of thin air and added to IPCC WGII Chapter 10 after the last round of external reviews. It was then cited in a draft of the Summary for Policy Makers (WGII), the most read document for the IPCC Report (WGII). It was then inexplicably changed to be 20% more alarming for the final version of that document.