The Holocene Extinction: Earth's Sixth Mass Extinction in Real Time

The Extinction Rate That Geologists Will Read in the Rock Record

The background extinction rate — the baseline pace at which species disappear through natural evolutionary processes in the absence of a catastrophic event — is approximately 0.1 to 1 extinction per million species per year, according to paleontological estimates synthesized in a 2015 paper by Ceballos et al. in Science Advances. The current observed extinction rate for vertebrate species is 100 to 1,000 times that background rate. The same analysis found that vertebrate species losses over the past century would have required 800 to 10,000 years to occur under natural background conditions. Earth is losing species in a geological instant.

The sixth mass extinction is not a future prediction. It is an ongoing event.

The Five Previous Mass Extinctions

Earth's fossil record documents five major mass extinction events, each defined as a period when at least 75% of species went extinct within a geologically brief interval:

Event	Approximate Date	Estimated Species Lost	Primary Cause
Ordovician-Silurian	~444 million years ago	~85%	Glaciation, sea level fall
Late Devonian	~375–360 million years ago	~75%	Anoxic oceans, climate change
Permian-Triassic ("The Great Dying")	~252 million years ago	~96%	Volcanic eruptions, ocean acidification
Triassic-Jurassic	~201 million years ago	~80%	Volcanism, climate disruption
Cretaceous-Paleogene (K-Pg)	~66 million years ago	~76%	Asteroid impact + volcanism

The current event differs from all five predecessors in one critical respect: a single species is the causative agent.

Scale of Current Losses

The International Union for Conservation of Nature (IUCN) Red List, as of 2024, has assessed approximately 157,000 species across all taxa. Of those assessed, 44,000 (28%) are threatened with extinction — a category encompassing Critically Endangered, Endangered, and Vulnerable designations. Among specific groups:

Amphibians: 41% of assessed species are threatened — the most imperiled major vertebrate class.
Sharks and rays: 37% threatened, driven by overfishing and habitat degradation.
Mammals: 27% threatened.
Reptiles: 21% threatened.
Birds: 13% threatened.
Plants: More than 40% of assessed plant species are estimated to be threatened, according to the Royal Botanic Gardens, Kew's 2020 State of the World's Plants and Fungi report.

The IUCN Red List is understood to be a significant undercount — only a fraction of Earth's estimated 8–10 million species have been formally assessed.

The Five Drivers of the Holocene Extinction

The Convention on Biological Diversity and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) have synthesized five primary drivers of current biodiversity loss, ordered by their relative contribution:

1. Habitat Loss and Degradation

Land use change — primarily conversion of native habitat to agriculture — is the dominant driver. Approximately 75% of Earth's ice-free land surface has been significantly altered by human activity, according to the 2019 IPBES Global Assessment. Tropical rainforest destruction is particularly consequential: the Amazon, Congo Basin, and Southeast Asian forests contain an estimated 50–70% of all terrestrial species within roughly 6–8% of Earth's land surface.

2. Overexploitation

Direct killing for food, medicine, and trade is the second major driver. Commercial fishing has depleted more than 34% of global fish stocks to biologically unsustainable levels, according to the FAO's 2022 State of World Fisheries report. The illegal wildlife trade, estimated at $23 billion annually, applies heavy pressure on target species including rhinoceroses, pangolins, tigers, and sea turtles.

3. Invasive Species

Species introduced outside their native ranges — intentionally or accidentally — have driven extinctions on islands with particular severity. Of bird extinctions since 1500 CE, introduced rats, cats, and mongoose are implicated in more than half. The brown tree snake, introduced to Guam after World War II, eliminated 12 of 22 native forest bird species within decades.

4. Pollution

Agricultural chemical runoff, plastic pollution, heavy metal contamination, light pollution, and noise pollution each impact specific taxonomic groups. Neonicotinoid pesticides have been linked to widespread declines in wild bee populations across Europe and North America. Microplastic ingestion has been documented in over 800 marine species.

5. Climate Change

Currently ranked fifth by contribution to existing extinctions, climate change is projected to become the dominant driver in coming decades as temperatures rise beyond the adaptive range of temperature-sensitive species. A 2004 study in Nature (Thomas et al.) projected that 15–37% of species sampled would be "committed to extinction" by 2050 under mid-range climate change scenarios — a projection that remains a reference point in extinction literature despite subsequent debates over methodology.

Population Loss vs. Species Extinction

A 2017 study in PNAS (Ceballos et al., "Biological annihilation via the ongoing sixth mass extinction") introduced the concept of "biological annihilation" — distinguishing between species-level extinction (which moves slowly enough to appear manageable in news cycles) and population-level collapse (which is occurring at a far more rapid pace). The study found that 32% of all vertebrate species assessed had experienced declining populations and range contractions in recent decades. Even species not yet categorized as endangered have lost most of their historical population size and geographic range — a structural hollowing-out of biodiversity that precedes extinction by decades.

African lion populations declined by 43% between 1993 and 2014.
African elephant populations declined from approximately 5 million in 1930 to around 415,000 today.
The global wild vertebrate population has declined by an estimated 69% between 1970 and 2018 (WWF Living Planet Index).

The Deep Time Context

Recovery from previous mass extinctions required 10–30 million years of evolution to restore biodiversity to pre-event levels, based on the fossil record. The K-Pg extinction 66 million years ago, which eliminated non-avian dinosaurs and most large vertebrates, took approximately 10 million years for mammalian diversity to recover to comparable levels. The Permian extinction, which eliminated 96% of species, required 30 million years for marine ecosystem recovery. Against that timescale, the ongoing Holocene event — accelerating over a mere few centuries — will leave a geological signature in the future fossil record as unmistakable as any asteroid impact.