Extinction
John Baez
April 8, 2006
Phillip and Donald Levin estimate that right now one species is going extinct every 20 minutes, and that half of bird and mammal species will be gone in 200 to 300 years.
Richard Leakey estimates a loss of between 50,000 and 100,000 species a year, and says that only during the Big Five mass extinctions was the rate comparably high. E. O. Wilson gives a similar estimate. In his book, Michael Benton reviews the sources of uncertainty and makes an estimate of his own: given that there are probably somewhere between 20 and 100 million species in total, he estimates an extinction rate of between 5,000 and 25,000 species per year. This means between 14 and 70 species wiped out per day.
Skeptics find these numbers alarmist. For example, in Chapter 23 of this book:
- Bjørn Lomborg, The Skeptical Environmentalist: Measuring the Real State of the World, Cambridge U. Press, Cambridge, 2001.
the author does his best to tear apart Leakey and Wilson's estimates. Wilson has issued a convincing
rebuttal. However, the really interesting thing is that Lomborg's
own estimates
also point to a high extinction rate! He estimates that over the next 50 years, about 0.7 percent of all species will go extinct. This may not sound like much until you realize how short 50 years is on a geological time scale. To put things in perspective, note that given Lomborg's estimate that there are between 10 and 80 million species total, a loss of .7 percent of all species would mean between 70,000 and 560,000 extinctions in the next 50 years. This amounts to 1,200 and 10,000 per year, or between 4 and 30 a day - the same order of magnitude as what Benton suggests! Perhaps more to the point, Lomborg says the current extinction rate is about 1500 times the natural background rate.
In short, despite plenty of bickering, there seems to be agreement that humans are causing a vastly elevated extinction rate.
And there's also lots of
other data pointing to a massive human-caused disruption of the biosphere. One in eight plant species are in danger of extinction within the next 30 years, according to the IUCN
Red List of threatened species, along with one in eight bird species and a quarter of all mammals. The
Audubon Society reports that 30% of North American songbird species are in significant decline. Worldwide populations of frogs and other amphibians have been
declining drastically, and a recent
detailed study shows that of 5743 known species of amphibians recorded in the last couple of centuries, 34 are now extinct, while another 122 are probably extinct: they can no longer be found. Even worse, of these 122 missing species, 113 have disappeared since 1980!
In the oceans, 90% of all
large fish have disappeared in the last half century, thanks to overfishing. We see the spread of
dead zones near the mouths of rivers, where nutrients from fertilizer create blooms of plankton leading to low-oxygen water where few organisms can survive. Coral reefs are becoming unhealthy around the world, with a strong upswing in the
bleaching of reefs since the 1970s. "Bleaching" is the loss of algae called
zooxanthellae which live in coral and give it its color. It seems to be caused by higher water temperatures due to global warming.
extinction
