The changes humans are making to the planet are happening over a few centuries, but in geological terms that is still extremely fast. That speed is at the center of one of the most troubling debates in modern biology: another mass extinction may already be beginning.

This time, though, the cause may not be an asteroid or some other natural disaster. The argument is that the cause is us.

We are not biologists, paleontologists, or climate scientists, and nothing here is a verdict on anything. This is a reading of a live scientific debate, and the researchers cited disagree with each other in important ways. Estimates of extinction across the whole living world are still estimates, not settled facts.

What counts as a mass extinction, and why five

The phrase “mass extinction” gets used loosely, but paleontologists have a fairly specific meaning in mind. A mass extinction means losing more than three-quarters of Earth’s species in a relatively short period of geological time.

By that definition, it has happened only five times in roughly the last 540 million years. The first, the Late Ordovician extinction around 445 million years ago, was linked to severe glaciation. The worst, the end-Permian “Great Dying” about 252 million years ago, wiped out an estimated 96 percent of marine species.

The Big Five had natural causes with effects across the planet: rapid climate change, huge volcanic eruptions, major changes in ocean conditions and, 66 million years ago, the Chicxulub asteroid impact. 

How the sixth is different: no asteroid, no supervolcano, just us

The idea that humans might be causing a sixth mass extinction became widely known through Richard Leakey and Roger Lewin’s 1995 book and, later, Elizabeth Kolbert’s Pulitzer-winning The Sixth Extinction.

What makes the modern crisis unusual is the cause. Human activity is changing the atmosphere, oceans and land through habitat destruction, overuse of wildlife, global warming and ocean acidification.

It helps to separate two questions. First: is there a serious, human-caused biodiversity crisis happening now? On that there is broad agreement. Second: have enough species already disappeared for this to count as a true “mass extinction”? That is where researchers disagree.

The rate problem, and why speed is the whole story

One of the most important parts of the argument is not just how many species are disappearing, but how quickly they are disappearing.

In a 2023 study of 5,400 vertebrate genera, excluding fishes, Ceballos and Ehrlich found that 73 genera had gone extinct since 1500. Without human impacts, they estimated, those same losses would have taken about 18,000 years to occur naturally. They estimated that vertebrate genera are now disappearing at roughly 35 times the normal background rate seen over the past million years.

The authors are very clear about what they think this means. They write: “Beyond any doubt, the human-driven sixth mass extinction is more severe than previously assessed and is rapidly accelerating.” That is a strong conclusion, but it is their interpretation rather than a neutral scientific consensus.

Where the scientists disagree

Robert Cowie of the University of Hawaii and his colleagues argue that the sixth extinction is already under way, especially once less-studied animals such as insects and snails are included. Their 2022 review estimated that Earth may have lost between 7.5 and 13 percent of its roughly two million known species since 1500.

John Wiens and Kristen Saban take a more cautious view. In 2025 they argued that less than 0.1 percent of Earth’s known species have documented extinctions over the last 500 years.

A later analysis of plants and animals found 102 genera that had gone extinct since 1500. It concluded that recent extinctions of entire genera were rare, were concentrated especially among birds, mammals and species living on islands, and had not rapidly accelerated toward the present.

That does not mean the biodiversity crisis is not real. Wiens and Saban themselves acknowledge human-driven extinctions and serious threats to species. Their argument is narrower: the losses documented so far may not yet justify calling the current crisis a “mass extinction.”

Closing thoughts 

Speed matters because species do not all respond to environmental change in the same way. Some can evolve quickly or move into new areas. Others cannot.

Problems arise when the climate or habitat changes faster than a species can adapt or move somewhere suitable. Research on climate responses finds that climate change can exceed species’ ability to adapt, increasing the risk of extinction.

Our read is that the sixth mass extinction is not yet a finished fact. Ceballos, Ehrlich and Cowie argue that it is already under way, while Wiens and Saban argue that the documented losses so far do not meet the mass-extinction threshold.

What makes this possible sixth extinction different from the previous five is the proposed cause. This time, the driver is a species capable, at least in principle, of changing what it is doing.