Is the rate of species extinctions accelerating?

My Quillette article, Climate Change—Assessing the Worst Case Scenario, has a section near the end dealing with biodiversity and species extinctions. I’ve received a couple of critical comments on it, one accusing me of “hand-waving”. I can understand that reaction, and I see that it’s partly a result of my editing the article for length. When shortening the section, I lost parts of the explanation for why I think the way I do. So let me expand on it.

Strictly speaking, Gregory Wrightstone’s excellent analysis is sufficient to explain why I say I’m unimpressed with the UN IPBES report and answers the question in the title of this blog post. The answer is “no”:

A closer review of the most recent information dating back to 1870 reveals that, instead of a frightening increase, extinctions are actually in a significant decline.

His graphs make this crystal clear. But anyone who didn’t look at those graphs (although I did link to the blog post), might not get it.

That’s not all, though. If there is no acceleration, why does the IPBES report unequivocally state the opposite in section 2.2.5.2.4?

The global rate of species extinction is already at least tens to hundreds of times higher than the average rate over the past 10 million years, and is accelerating (Barnosky et al. 2010, Pimm et al. 2014, Ceballos et al. 2015).

Let’s look at those references, then, shall we? Is the above statement justified and balanced given what they contain?

Barnosky et al. 2011

Barnosky et al. 2010 is not in their list of references and appears to be non-existent. I believe they are referring to Has the Earth’s sixth mass extinction already arrived? which is by Barnosky et al. but from 2011. This chapter of the report is still in draft form, but since this is a key claim in the report, you might have expected them to be more careful with the citations.

More importantly, does this paper support the quoted statement? The first part, about the extinction rate being higher than the average over the past 10 million years, is not my concern here. On the other claim, that it is currently accelerating, the paper is silent. However, they explore the question of whether it will accelerate in the future, and they admit that they don’t know whether it will accelerate, stay the same, or decelerate:

Among major unknowns are (1) whether ‘critically endangered’, ‘endangered’ and ‘vulnerable’ species will go extinct, (2) whether the current rates we used in our calculations will continue, increase or decrease; and (3) how reliably extinction rates in well-studied taxa can be extrapolated to other kinds of species in other places.

Pimm et al. 2014

The second one, The biodiversity of species and their rates of extinction, distribution, and protection, states that the rate of extinction is “poised to increase”:

Current rates of extinction are about 1000 times the likely background rate of extinction. Future rates depend on many factors and are poised to increase.

But what does “poised” mean? That it’s possible? Plausible? Likely? I find no explicit clarifications anywhere in the paper. This, though, is telling:

Pereira et al.’s review of projected future extinctions classified and compared various models. Strikingly, the six sets of projections predicted a hundred-fold range of extinction rates.

A hundred-fold range makes it hard to reach firm conclusions, obviously.

So to summarize these two papers: Neither of them makes any claims that the extinction rate is currently accelerating, and they seem to agree that we don’t know whether extinctions will accelerate in the future.

In contrast, the IPBES report seems confident that it will accelerate:

These two very different lines of evidence both point to a further sharp acceleration in the global rate of species extinction – already at least tens to hundreds of times higher than the average rate over the past 10 million years.

This, however, comes from models which are based on the number of species listed as threatened. But that in turn depends on the first of the “major unknowns” cited by Barnosky et al.: “whether ‘critically endangered’, ‘endangered’ and ‘vulnerable’ species will go extinct”.

Ceballos et al. 2015

As suggested by the title of this paper, Accelerated Modern Human-Induced Species Losses: Entering the Sixth Mass Extinction, here we have an explicit claim that extinctions are accelerating. What is that based on? Primarily the same graph as the one Wrightstone debunks, and the accompanying statement:

Modern extinction rates have increased sharply over the past 200 years (corresponding to the rise of industrial society) and are considerably higher than background rates.

This seems to be a misleading way to say that there have been more extinctions during the past 200 years than before. Which is true, but does not contradict the falling trend from around 1900 until today.

But there is one more statement about acceleration.

For example, amphibians, comprising of ~7300 species, show an accelerating rate of extinction: only 34 extinctions have been documented with a high level of certainty since 1500, yet >100 species have likely disappeared since 1980.

This is bizarre. The apples to oranges comparison between documented extinctions and “likely” recent extinctions (derived theoretically) is meaningless. It turns out that this is a distorted summary of a paper called Status and Trends of Amphibian Declines and Extinctions Worldwide. The main message of this paper is that amphibians (as of 2005) are in a particularly perilous situation compared to mammals and birds. This means that citing it “for example”, as if it were randomly chosen, leaves a misleading impression that it’s typical and might apply equally to other animal groups.

The amphibian paper also has a more reasonable version of the comparison between current and earlier rates:

Only 34 species of amphibian are reported to have become extinct since 1500, compared with 129 birds and 74 mammals, but there is strong evidence that this situation is worsening because nine of these extinctions took place since 1980 (compared with five birds and no mammals).

But is the estimate of 100 extinctions since 1980 supported by the amphibian paper? Not quite; it’s actually 9–122:

The GAA estimates that between nine and 122 amphibian species have therefore become extinct since 1980, and extensive fieldwork is needed to produce a more precise number.

And Paul Ehrlich is part of it

Also notable is the fact that one of the co-authors of Ceballos et al. paper is Paul Ehrlich, known for his spectacularly failed predictions of environmental catastrophe. It’s intriguing that Ehrlich is one of the authors of the paper that is the main source for the IPBES claim of accelerating extinctions. I’m not implying that he is the mastermind behind the report’s alarmist bent, but there is no doubt he has contributed to it.

In summary, it’s pretty clear that the references don’t support the statement about acceleration. I said before that I was unimpressed. But after digging into those references, “appalled” is probably more appropriate.