What Is the Sixth Mass Extinction and What Is Driving It?

What Is a Mass Extinction?

A mass extinction is a catastrophic, rapid (in geological terms) loss of a large proportion of Earth's species. While extinction is a normal background process — species evolve and disappear continuously over geological time — mass extinctions are qualitatively different: they occur over geologically brief periods (tens of thousands to millions of years) and eliminate at least 75 percent of species on Earth. The fossil record documents five major mass extinctions in the past 540 million years, each of which fundamentally restructured the biosphere.

The most famous is the end-Cretaceous extinction (66 million years ago), which wiped out the non-avian dinosaurs and roughly 75 percent of all species when a 10-kilometer asteroid struck the present-day Yucatan Peninsula. The most severe was the end-Permian extinction (252 million years ago), which eliminated approximately 96 percent of marine species and 70 percent of terrestrial vertebrate species — likely caused by massive volcanic eruptions in what is now Siberia releasing enormous quantities of CO2 and SO2. Each mass extinction was followed by millions of years of recovery and the radiation of new lineages into vacated ecological roles.

The Evidence for a Sixth Mass Extinction

In a landmark 2015 paper in Science Advances, biologists Gerardo Ceballos, Paul Ehrlich, and colleagues analyzed vertebrate extinction rates and concluded that vertebrates are disappearing at rates 15 to 100 times higher than the normal background rate. A 2017 follow-up study focused on population losses rather than just extinctions, finding that billions of populations of animals have been eliminated even among species that have not yet gone extinct — a process the authors called biological annihilation.

Key data points illustrating the scale of the crisis:

• The World Wildlife Fund's 2022 Living Planet Index recorded a 69 percent average decline in monitored vertebrate wildlife populations between 1970 and 2018.
• Approximately 40 percent of amphibian species are threatened with extinction — amphibians, as sensitive indicators of environmental health, are experiencing the most severe crisis of any vertebrate class.
• Insect populations in some intensively studied regions of Europe have declined by 50 to 80 percent over three decades — the so-called insect apocalypse.
• Coral reefs, which support approximately 25 percent of all marine species despite covering less than 0.1 percent of the ocean floor, have declined by 50 percent since the 1950s and face near-total collapse under 2 degrees C of warming.

The Five Previous Mass Extinctions

Putting the current crisis in geological context requires understanding what previous mass extinctions looked like:

• End-Ordovician (444 million years ago): About 86 percent of species lost. Caused by glaciation and subsequent sea level changes.
• Late Devonian (375 million years ago): About 75 percent of species lost over a prolonged period. Causes debated — possibly associated with the spread of land plants altering atmospheric CO2.
• End-Permian (252 million years ago): The "Great Dying" — 96 percent of marine species, 70 percent of terrestrial vertebrate species. Massive Siberian Traps volcanism is the leading cause.
• End-Triassic (201 million years ago): About 80 percent of species lost. Central Atlantic Magmatic Province volcanism is the leading cause.
• End-Cretaceous (66 million years ago): About 75 percent of species lost. Chicxulub asteroid impact, amplified by Deccan Traps volcanism.

What distinguishes the current crisis from all previous mass extinctions is its cause: previous events were driven by extraterrestrial impacts, massive volcanism, or climate changes operating over millions of years. The sixth mass extinction is being driven by a single species — Homo sapiens — acting over decades to centuries.

The Primary Drivers

The IPBES (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services) 2019 global assessment identified the five primary drivers of the current biodiversity crisis, in order of impact:

1. Land-use change: The conversion of natural ecosystems to agriculture, pasture, and urban development is the single largest driver. Approximately 75 percent of the Earth's ice-free land surface has been significantly altered by human activities. Tropical deforestation removes some of the world's most biodiverse habitats at millions of hectares per year.
2. Direct exploitation: Overhunting, overfishing, the legal and illegal wildlife trade, and unsustainable logging directly kill organisms faster than populations can replace themselves. The illegal wildlife trade is worth an estimated $23 billion annually.
3. Climate change: Already affecting species ranges, phenology (timing of seasonal events), and survival, and projected to become the dominant driver later this century. Ocean warming and acidification are particularly threatening to marine ecosystems.
4. Pollution: Pesticides, herbicides, heavy metals, plastic pollution, nutrient runoff, light pollution, and noise pollution all degrade habitats and harm species.
5. Invasive species: Species transported to new environments (often through global trade and travel) can outcompete, prey upon, or transmit diseases to native species that have no evolutionary defenses.

Why the Rate Matters: Tipping Points

The speed of the current extinction wave is critical. Ecosystems are not simply collections of independent species — they are networks of interacting organisms held in dynamic equilibrium by millions of years of co-evolution. When species are removed, their ecological roles — pollination, seed dispersal, predator control, nutrient cycling — may go unfilled. If enough species are lost, ecosystems can cross tipping points and collapse into fundamentally degraded states.

The loss of apex predators (wolves, sharks, big cats) triggers trophic cascades — ripple effects through food webs that alter vegetation, hydrology, and soil chemistry. The decline of pollinators threatens not only wild plants but global food production (about 75 percent of food crops require animal pollination). The loss of soil microbiota and invertebrates undermines soil fertility that agriculture depends on. Each extinguished species may take with it ecological functions, genetic information, and potential medicines that cannot be recovered.

What Can Be Done?

The scientific community is clear that preventing catastrophic biodiversity loss requires urgent, transformative action at scale — not incremental improvement. Key interventions identified in IPBES and other assessments include:

• Protecting and restoring at least 30 percent of the planet's land and oceans (the 30x30 commitment made at the 2022 COP15 biodiversity conference).
• Transforming agricultural systems to reduce land footprint through higher yields, reduced food waste, and dietary shifts away from land-intensive animal products.
• Rapid decarbonization to limit climate change to 1.5 to 2 degrees Celsius, preventing climate change from becoming the dominant extinction driver.
• Eliminating harmful subsidies (estimated at over $500 billion annually) that incentivize deforestation, overfishing, and excessive pesticide use.
• Reforming global trade to reduce deforestation and unsustainable resource extraction embedded in international supply chains.