Just as we all die, all species eventually go extinct. However, the rate of extinction varies dramatically, and a new estimate suggests we are currently running at 1000 times the normal rate. This rate of extinction is only seen in the fossil record after incredibly dramatic and unusual occurrences, such as huge asteroid strikes or supervolcano eruptions.

In order to calculate the effect humans are having we need to know two things – how many species are disappearing each year, and how many vanish as part of the normal background.

Estimating the number of current extinctions is hard enough, since some species disappear without us ever knowing they were there in the first place. It has been said we are having trouble even “counting the books while the library burns” . However, finding out what is normal is harder still. The fossil record preserves some species much better than others, and the fact that we can't find a species after a particular point may indicate it disappeared entirely, or just became a fair bit rarer.

Professor Stuart Pimm of Duke University has published a paper in Science in which he and his coauthors, “Document what we know, how it likely differs from what we do not, and how these differences affect biodiversity statistics.” The authors say, 

“We start by asking how many species are known and how many remain undescribed. We then consider by how much human actions inflate extinction rates. Much depends on where species are, because different biomes contain different numbers of species of different susceptibilities. Biomes also suffer different levels of damage and have unequal levels of protection. How extinction rates will change depends on how and where threats expand and whether greater protection counters them.”

As the paper notes, the species we are most familiar with are not typical. “The species we know best have large geographical ranges and are often common within them. Most known species have small ranges, however, and such species are typically newer discoveries.”

Pimm's new estimate is that the background rate is 0.1 extinction per million species per year. This is a tenth the figure produced in 1995, in what had been considered the definitive paper on the topic. However, there will be no pushback from the authors of the higher estimate – Pimm was one of the authors of the 1995 paper as well.

The rate today is between 100 and 1000 extinctions per million species per year. In other words our way of life may be 10,000 times more deadly than all the threats faced by animals at other times. Climate change, hunting and invasive species are all playing a part, but Pimm says habitat loss is the largest factor.

The world has experienced five mass extinctions over the last half a billion years. In each of these, most of the animal and plant species on the planet disappeared. The asteroid that wiped out the dinosaurs is of course the most famous. It wasn't, however, the most destructive mass extinction ever recorded. The Permian-Triassic extinction event occurred approximately 252 million years ago and wiped out an astonishing 96% of all marine species and 70% of all terrestrial vertebrate species. In between these major events there have been smaller spikes in the death rates, often driven by climatic changes. While we have already lost more species than in many of the more minor events, Pimm believes a combination of habitat protection, captive breeding and action on climate change can avoid a sixth mass extinction.