Are We in a Sixth Mass Extinction? What the Evidence Actually Shows
Short answer: it depends on what you measure. By the fossil-record definition, a mass extinction is the loss of more than three-quarters of species in a geologically short time, and today's recorded losses are nowhere near that share. By speed, however, the picture is alarming: a 2015 study using deliberately cautious assumptions found vertebrate species disappearing over the past century at up to 100 times the normal background rate. Many researchers therefore say a sixth mass extinction is under way, meaning that the rate is on that path, not that it has already happened. Others dispute the claim, and the debate turns largely on which species have actually been counted.
This page sets out the definition, the main measurements behind the claim, the strongest objections, and what the numbers can and cannot show. Every figure comes from the peer-reviewed papers listed at the end.
What counts as a mass extinction
A 2011 review in Nature led by Anthony Barnosky sets out how palaeontologists use the term: times when Earth loses more than three-quarters of its species in a geologically short interval. That has happened only five times in roughly the past 540 million years. The review compared fossil data with modern records, taking into account the differences between the two kinds of evidence, and concluded that current extinction rates are higher than would be expected from the fossil record.
The definition matters because it contains two different ideas. One is how much is lost: three-quarters of species. The other is how fast: a short interval in geological terms, which can still mean many thousands of years. Claims about a sixth mass extinction today rest mainly on the second idea, the rate.
Measuring the rate: the "background" comparison
Species go extinct all the time, even without people. The normal pace between mass extinctions is called the background rate. Scientists express it in extinctions per million species-years (E/MSY): how many species out of a million would be expected to disappear each year.
A 2015 study in Science Advances by Gerardo Ceballos and colleagues set out to test the claim with assumptions that would, if anything, understate the crisis:
- It used a background rate of 2 E/MSY, which the authors describe as twice as high as widely used earlier estimates. In mammal terms, that is 2 extinctions per 10,000 species per 100 years.
- It counted only species formally listed as extinct, which requires meeting stringent criteria, so the modern count is conservatively low.
Even so, the study found that the average rate of vertebrate species loss over the last century was up to 100 times higher than that background rate. Put another way, the species lost in the past century would, at the background rate, have taken between 800 and 10,000 years to disappear, depending on the group of vertebrates. The authors concluded that "a sixth mass extinction is already under way", while adding that intensified conservation could still avert a dramatic loss, though that window is "rapidly closing".
| Study | What it measured | Key figure |
|---|---|---|
| Barnosky et al., Nature 2011 | Definition and fossil comparison | Mass extinction: >75% of species lost; five in ~540 million years; current rates above fossil expectation |
| Ceballos et al., Science Advances 2015 | Vertebrate species extinction rate vs 2 E/MSY background | Up to 100× background over the last century; 800–10,000 years' worth of losses in one century |
| Ceballos, Ehrlich & Dirzo, PNAS 2017 | Population declines in 27,600 vertebrate species | 32% (8,851) decreasing; all 177 mammals studied lost ≥30% of range |
| Cowie, Bouchet & Fontaine, Biological Reviews 2022 | Bias in the IUCN Red List; mollusc extrapolation | Possibly 7.5–13% of ~2 million known species extinct since about 1500, vs 882 (0.04%) on the Red List |
Sources: the four papers listed at the end of this article, as stated in their abstracts.
Beyond species: shrinking populations
A species is only counted as extinct when its last individual is gone, so species counts lag behind what is happening on the ground. A 2017 paper in the Proceedings of the National Academy of Sciences by Ceballos, Paul Ehrlich and Rodolfo Dirzo looked instead at populations and ranges.
Using a sample of 27,600 vertebrate species, nearly half of those known, the authors found that 32% (8,851 species) are decreasing in population size and range. They report that the rate of population loss in land vertebrates is extremely high, even among species listed as of low concern. For 177 mammal species with detailed data covering 1900 to 2015, every one had lost 30% or more of its geographic range, and more than 40% had suffered severe declines, defined as more than 80% range shrinkage. The authors called this a "biological annihilation" and argued that the extinction episode is more severe than species counts alone suggest.
The strongest objection: who gets counted
Some scientists do not accept that the current crisis amounts to a sixth mass extinction. According to a 2022 review in Biological Reviews by Robert Cowie, Philippe Bouchet and Benoît Fontaine, critics often point to the IUCN Red List, the main global register of species' conservation status, to argue that the rate of species loss does not differ from the background rate.
Cowie and colleagues argue that this reading is distorted by what the Red List covers. Almost all birds and mammals have been assessed against its criteria, but only a minute fraction of invertebrates. Focusing on molluscs, the second-largest animal phylum by number of known species, and "extrapolating boldly" in their own words, they estimate that since around 1500 possibly 7.5% to 13% of all roughly 2 million known species, 150,000 to 260,000 species, have already gone extinct. That is orders of magnitude more than the 882 extinctions, 0.04%, recorded on the Red List at the time of their review.
The same review adds important differences between environments:
- Island species have suffered far higher extinction rates than continental ones.
- Marine species face significant threats, but the authors find no evidence that marine life has reached the same crisis as life on land and in fresh water, even though past mass extinctions were largely defined by marine invertebrates.
- Plants face assessment biases similar to invertebrates, with hints that they may have had lower extinction rates.
What the evidence does and does not show
- Rates are well above background. This is the common ground. The Barnosky review and the Ceballos studies both find modern rates above what the fossil record would predict, using different methods.
- The three-quarters threshold has not been reached. Even the boldest estimate cited here, up to 13% of known species since about 1500, is far below the share lost in the five past mass extinctions. The case for a sixth is about trajectory and speed.
- Counts depend on effort. Recorded extinctions reflect which groups have been studied. The gap between 882 recorded extinctions and the 2022 review's extrapolation shows how much the answer depends on assumptions, and the authors themselves describe their extrapolation as bold.
- Different measures tell different parts of the story. Species extinctions, population declines and range losses are separate measurements. The 2017 PNAS paper argues that population losses are the earlier and larger signal.
Common questions
What were the five previous mass extinctions?
The 2011 Nature review describes them as the five times in roughly the past 540 million years when Earth lost more than three-quarters of its species in a geologically short interval. The 2022 review notes that all five were caused by natural phenomena.
What does "100 times the background rate" mean?
It means vertebrate species vanished over the past century up to 100 times faster than the normal pace between mass extinctions, using a background rate of 2 extinctions per million species-years, according to the 2015 Science Advances study.
Why do some scientists say there is no sixth mass extinction?
According to the 2022 Biological Reviews paper, critics often rely on the IUCN Red List, which shows relatively few recorded extinctions. That review argues the Red List mostly covers birds and mammals and misses most invertebrate losses.
Is the ocean affected the same way?
The 2022 review found no evidence that marine life has reached the same crisis as life on land, although marine species face significant threats.
Can the losses still be slowed?
The 2015 study states that a dramatic decay of biodiversity can still be averted through intensified conservation, but that the window of opportunity is closing quickly.
Sources and how to check
- Barnosky A. D. et al. "Has the Earth's sixth mass extinction already arrived?" Nature 471, 51–57 (2011). DOI: 10.1038/nature09678 · PMID 21368823
- Ceballos G. et al. "Accelerated modern human–induced species losses: Entering the sixth mass extinction." Science Advances 1, e1400253 (2015). DOI: 10.1126/sciadv.1400253 · PMID 26601195
- Ceballos G., Ehrlich P. R., Dirzo R. "Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines." PNAS 114, E6089–E6096 (2017). DOI: 10.1073/pnas.1704949114 · PMID 28696295
- Cowie R. H., Bouchet P., Fontaine B. "The Sixth Mass Extinction: fact, fiction or speculation?" Biological Reviews 97, 640–663 (2022). DOI: 10.1111/brv.12816 · PMID 35014169
- IUCN Red List of Threatened Species: iucnredlist.org, for current assessment totals.
- Abstracts and bibliographic details checked through Europe PMC on 23 October 2026.