But... It Doesn't Say That!

I had a post planned for today, one which is half written, but I'm going to have to call an audible. I came across something that's too weird not to discuss. The latest Intergovernmental Panel on Climate Change report makes this claim in its Summary for Policymakers (SPM):

Coral reefs, for example, are projected to decline by a further 70–90% at 1.5°C (high confidence) with larger losses (>99%) at 2ºC (very high confidence). The risk of irreversible loss of many marine and coastal ecosystems increases with global warming, especially at 2°C or more (high confidence). {3.4.4, Box 3.4}

This is a dramatic claim which holds (virtually) all coral reefs in the world will die. Given such a serious claim, I'd expect there to be lot of good research supporting it. Instead, it turned out not even cited portions of the IPCC report make the same claim.

For today's purpose, I'm not going to worry about the validity of any scientific papers or their conclusions. I'm not here today to challenge any results. All I want to do is establish what is actually said. The SPM cited Section 3.4.4 and Box 3.4 from Chapter 3 for this claim. The Executive Summary at the start of the chapter seems to support it, saying:

For example, multiple lines of evidence indicate that the majority of warmer water coral reefs that exist today (70-90%) will largely disappear when global warming exceeds 1.5°C (very high confidence) {3.4.4, Box 3.4}.

Section 3.4.4 has 12 subsections, most of which don't deal with this issue (other things were said in the quoted paragraph so that's unremarkable). The only paragraph I can find in that section which even resembles this claim is in

Tropical coral reefs will reach a very high risk of impact at 2ºC (Figure 3.17; Annex 3.1 3.4.4) with most available evidence suggesting that coral dominated ecosystems will be non-existent at this temperature or higher (e.g., coral abundance near zero in most locations, intensifying storms ‘flattening’ reefs’ 3-dimensional structure; Alvarez-Filip et al., 2009) (high agreement, robust evidence). Impacts at this point (coupled with ocean acidification) are likely to undermine the ability of tropical coral reefs to provide habitat for the current high levels of biodiversity as well as a range of ecosystem services important for millions of people (e.g., food, livelihoods, coastal protection, cultural services) (Burke et al., 2011).

This is somewhat similar in concept to what the two summaries say, but it is clearly different. One of the differences is it specifically refers to tropical coral reefs. Tropical and subtropical coral reefs are very different from cold-water, or deep-water, coral reefs. It is much more difficult to tell how the latter will respond to the planet warming. As a result, many papers focus on the impacts on tropical and subtropical coral reefs.

Why both of the summaries refer to "coral reefs" while relying on literature the report describes as being for only tropical/subtropical coral reefs is a mystery to me. I won't dwell on it any further. Just understand for the rest of this post when I refer to "coral reefs" I am only referring to the types that live in shallower waters.

Leaving aside that error, it is still clear that paragraph doesn't say what the two summaries say. That means Section 3.4.4 doesn't say what the summaries say. Naturally, the next spot to look is Box 3.4, the other cited source. This box is all about coral reefs and the challenges posed to them. It would be natural for all the information we'd expect to be in it. It should answer all our questions.

It doesn't do that. In fact, it raises more questions. For instance, the paragraph I cited from Section 3.4.4 relies entirely upon a Alvarez-Filip et al. paper. This paper is never mentioned in Box 3.4. Why Section 3.4.4 relies on one paper while Box 3.4 doesn't even consider that paper is a mystery. Still, Box 3.4 does say:

Even achieving emission reduction goals consistent with the ambitious goal of 1.5oC under the Paris Agreement will result in the further loss of 90% of reef-building corals compared to today, with 99% of corals being lost under warming of 2oC or more above the pre-industrial period (Frieler et al., 2013; Hoegh-Guldberg, 2014b; Hoegh-Guldberg et al., 2014; Schleussner et al., 2016b; Hughes et al., 2017a).

Which... kind of fits what the two summaries say about coral reefs. But only kind of. Box 3.4 says 90% of reefs will be lost under 1.5 degrees of warming and 99% will be lost under 2 degrees of warming. However, both summaries said 70-90% of reefs would be lost under 1.5 degrees of warming and more than 99% would be lost under 2 degrees of warming.

This means Section 3.4.4 says one thing, Box 3.4 says another thing and two summaries cite both of them as saying a third thing. That's... really weird. It's weird enough when the same report says three different things on the same topic, but it's just bizarre when one part of the report claims a different part of the report says something it doesn't say.

But there is a bit of a clue. Another part of Chapter 3, Box 3.5, is not cited as a source of the claim in the two summaries. However, Box 3.5 says:

Mass coral bleaching and mortality are projected to increase due to interactions between rising ocean temperatures, ocean acidification, and destructive waves from intensifying storms (Section 3.4.4, Box 3.4, Section 5.2.3). At 1.5C, approximately 70–90% of global coral reefs are projected to be at risk of long-term degradation due to coral bleaching, increasing to 99% at 2C (Schleussner et al., 2016b).

This matches what the two summaries say while both of the sources cited by those summaries say other things. One cited source cites All three of these sources cite different sets of papers. Section 3.4.4 cites only the Alvarez-Filip et al paper. Box 3.5 cites only the Schleussner et al. paper. Box 3.4 cites the Schleussner et al. paper as well as half a dozen other papers. But only Box 3.5's conclusions are used in the summaries.

My initial thought was part of these discrepancies could be resolved by this portion of Box 3.4:

The assumptions underpinning these assessments are considered to be highly conservative. In some hypothetical cases, ‘optimistic’ assumptions in models include the rapid thermal adaptation by corals (0.2- 1.0oC per decade and 0.4oC per decade; (Donner et al., 2005; Schleussner et al., 2016b), respectively) as well as very rapid recovery rates from impacts (i.e., 5 years; Schleussner et al., 2016b).

This discusses the same paper as Box 3.5, saying "assumptions underppinning" it "are considered to be highly conservative. My thought was perhaps Schleussner et al. came up with a range of 70-90% coral loss for 1.5 degrees of warming like stated in the SPM but Box 3.4 cut off the lower end of that range for being based upon assumptions the chapter authors felt were unreasonable. Doing so without discussing how they were changing the results would be inappropriate, but it would at least resolve the discrepancy.

But it turns out that isn't true. Schleussner et al. did give results for what they considered to be an "optimistic" set of assumptions. They aren't used by the IPCC though. They're not even mentioned. While the IPCC discusses "optimistic" assumptions in a sentence citing this paper which provides results the authors described as being based on "optimistic" assumptions, the IPCC simply chooses not to disclose what results the authors get with their "optimistic" assumptions.

I'm not going to fill this post with rhetoric about how bizarre and inappropriate all this is. I don't understand how the iPCC could cite a paper's results, refer to it having results based on "optimistic" assumptions but only provide its results based on non-optimistic assumptions. What I do know is how the IPCC managed to get both a 70-90% range and a single 90% value for this paper. You can see it in this table from the paper:

The first line for each section of this table is labeled "Adaptation." It is the "optimstic" scenario considered by the Schleussner et al. It says with 2 degrees of warming by 2050, only 40% of coral reefs would be lost while with 1.5 degrees of warming by 2050, only 9% would be lost. It then continues on to say that by 2100, both scenarios would see coral reefs rebound and recover almost completely.

The IPCC report ignores that. The authors of the IPCC report somehow saw this table, saw those results and decided not to disclose them. They decided not to disclose those results even as they explicitly referred to the assumptions underpinning them, claiming the assumptions were optimistic even while choosing only to show results which relied on different, non-optimistic assumptions.

This is all too bizarre for me to maintain a civil tone if I comment further, so instead I'll just share a bit of good news. That Schleussner et al. does resolve one mystery. Remember how Box 3.5 and both summaries said the loss of coral reefs under 1.5 degrees of warming would be 70-90% while Box 3.4 said it'd be 90%? It turns out this Schleussner et al. table explains that discrepancy. According to that table, 90% would be lost by 2050 but the loss rate would drop to 70% by 2100.

So hey, that's one mystery solved. Now if only the IPCC would publish the draft versions of its report along with the reviewer comments. Then maybe we could see how the IPCC managed to screw all this up.

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