The Future of Extinctions

Through hunting, habitat destruction, and biological invasions, humans have driven many species to extinction and many more to its brink. But just how bad is today’s extinction crisis and how bad will it get? These are supremely difficult questions. Mahaulepu is almost unique in its record of humanity’s impact on a place. It is not easy, as a result, to gauge just how severe the extinctions of the past 50,000 years have been on a global scale. Making matters worse, scientists don’t even know how many species exist. But the best estimates are grim. We seem to be entering a period of mass extinction on a par with the great die‑offs of the past 600 million years.

Every year about 10,000 new species are reported by a small cadre of zoologists and botanists. These scientists have identified about 1.5 million species so far, and they can only guess at how many wait to be found. Based on the rate at which new species are being discovered, they estimate the total at 7 million species, although some argue the figure is as high as 14 million. That means that at least four out of every five species on Earth haven’t been discovered yet, and at the rate they are currently being discovered, it will take 500 years to find them all. For many, the discovery may not come in time. If you don’t even know that a species exists, you can’t expect to know when its last member disappears.

Stuart Pimm has come up with a way to gauge the current extinction crisis that sidesteps our ignorance about the total number of species: he estimates the rate of extinctions, rather than their raw quantity. He has compiled data on documented extinctions in the most carefully studied groups of animals. Across all these groups–including birds, mussels, butterflies, and mammals–he has discovered the same extinction rate: somewhere in the neighborhood of 100 extinctions per million species per year. Given that all of these different groups are suffering the same rate of extinction, Pimm concludes that it’s the average for all animals and plants.

It’s also a rate far above the normal rate of background extinctions that can be seen in the fossil record. Except during mass extinctions, the background extinction rate has ranged between 0.1 and 1 extinction per million species per year. In other words, species are disappearing 100 to 1,000 times faster today than they did before the arrival of humans.

According to Pimm’s calculations, this rate is going to accelerate in the coming years. Two‑thirds of all species live in tropical forests. Half of the world’s tropical forests have now disappeared, and another million square kilometers are destroyed each decade. Much of the forest that still survives has been fragmented by burning and logging. Without some dramatic conservation, tropical forests will go on disappearing until the only regions left are the ones on protected reserves–about 5 percent of their original extent. That will take only 50 years. Applying the half‑life formula to this scenario, Pimm calculates that the extinction rate will rise as much as tenfold. In less than a century, Pimm estimates, half of the world’s species will disappear.

Pimm’s calculations, as stark as they may be, may actually underestimate the coming extinctions. He uses only deforestation in his estimates. With more planes and cargo ships shuttling between continents, for example, biological invasions are going to accelerate and cause even more extinctions. And even more species may go extinct as we alter the atmosphere.

For the past two centuries, humans have been steadily adding carbon dioxide and other greenhouse gases to the atmosphere. These gases prevent heat from escaping the planet and cause temperatures to rise. Today Earth is now on average 0.5 degrees Celsius warmer than it was in 1860. Some of the difference can be ascribed to changes in the sun and some of it to natural oscillations in the circulation of its oceans and its atmosphere. But most climatologists now agree that man‑made greenhouse gases are responsible for most of the warming.

Scientists are now trying to estimate what the climate will be like in the next few decades. That answer depends in part on how much fuel humans continue burning. Will China stick with coal as its economy booms? Will electric cars become more than a publicity stunt? Adding to the uncertainty is how Earth might respond to rising temperatures. The circulation of the oceans might suddenly shift, unloading its hidden heat. The forests of the north might suck up much of the added carbon dioxide, storing away the greenhouse gases as wood. Or the Amazon might turn to a savanna. Or the melting permafrost in the Arctic might release frozen methane. The list of possibilities can fill a book. But as best as researchers can estimate, the planet will warm between 1.4 and 5.8 degrees Celsius by 2100, with the most warming happening in the far north and south.

There are already signs that life is changing as a result of global warming. The growing season in the Northern Hemisphere now starts a week earlier than it did in 1981. With more carbon dioxide available in the air, trees have been growing faster. In North America and Europe, forests are climbing up the sides of mountains. A 1999 study of 35 species of nonmigrating butterflies in North America and Europe found that 63 percent of them shifted their ranges northward over the twentieth century. Even ticks are responding to the warmer winters and marching toward this pole.

These sorts of migrations aren’t too surprising when you look at the Ice Age history of North America: the retreats and advances of glaciers triggered ecological scrambles north and south. If the warming continues, entire forests will soon be on the move. The U.S. Department of Agriculture has run computer models to see what will happen to plants and animals in the United States if the climate keeps getting warmer. The conifers and hardwoods of New England move into Canada, while oak and hickory forests of the Midwest decline as they get replaced by the pine forests that move out of the South. In the West, the saguaro cactus might move out of the Southwest deserts, heading north all the way into Washington State.

But global warming won’t merely rearrange nature’s furniture. The plants and animals that live in cold climates–either in the far north or high in the mountains–have nowhere to shift their range to. Coral reefs, which are proving to be extremely vulnerable to warming in the ocean, cannot simply uproot themselves and head to cooler waters. As a result, global warming may help destroy most of the world’s coral reefs in the next 20 years.

Even the species that have enough room on the map to shift their range north or south may actually have trouble surviving. Many of them will be trying to colonize land that is now broken up into farms, suburbs, and cities. It is difficult enough to spare land for nature reserves where endangered plants and animals live today; setting aside new space for them in the decades to come may be even harder. But if we don’t, they may simply shift themselves right off an evolutionary cliff.