Is this a mass extinction?

A 2015 study used a deliberately conservative background rate, two extinctions per 10,000 species per hundred years, twice the figure then in wide use, and counted only extinctions recorded on the IUCN Red List.

Even so, vertebrate losses since 1900 came out 22 times the background rate counting only species verified extinct, and 53 times counting those also listed as extinct in the wild or possibly extinct. Amphibians reach a hundred on the second count. The authors’ point was that the conclusion survives every assumption chosen against it.

The five earlier events are measured mostly from the fossils of marine invertebrates. Shells and bones last; most other animals vanish without trace. Modern assessment runs the other way: the Red List has evaluated almost every described bird and mammal and only a minute fraction of invertebrates, and the present wave of losses is largely on land and in fresh water. The two datasets are not samples of the same thing.

A 2011 analysis worked through those biases. Palaeontologists count a mass extinction as the loss of more than three quarters of species in a geologically short interval, which has happened five times in 540 million years, and the losses so far are well short of that while the rates are elevated.

Whether an elevated rate makes a mass extinction is itself argued. John Wiens and Kristen Saban argued in 2025 that extinction rates can exceed background outside mass extinctions and that recent losses were mostly of island species, and their own count found rates rising across five centuries but generally falling over the last hundred years. Robert Cowie, Philippe Bouchet and Benoît Fontaine replied that figures like these rest largely on vertebrates and plants, which are far better known than the rest of life and not representative of it.

A species is declared extinct only after exhaustive searches fail, which can take decades. Most described species have never been assessed, and on the estimate now treated as the default, most species have never been described at all.

So the documented count is a floor by construction, and estimates of the true figure depend on extrapolating from the groups that are well studied to the invertebrates and plants that are not. One such extrapolation, from a random sample of land snails, puts losses since 1500 at 7.5 to 13 per cent of all known species, against well under one per cent on the Red List; its authors call it bold.

Nature’s summary of the 2011 analysis put the direction plainly: the loss of the species now classed as critically endangered “would soon propel the world into its sixth mass extinction”. A 2022 review, citing that analysis, says a mass extinction could arrive within one or a few centuries if nothing slows the loss.

The five previous events all had natural causes, mostly indifferent to what was alive, though not wholly: one hypothesis ties the Devonian crisis in part to land plants spreading, enriching the seas and drawing carbon dioxide down. This one has a cause that can in principle stop.

That is a genuine difference and not a reassurance: it means the outcome depends on decisions rather than on physics, which makes it unpredictable in a different way.