There have been five mass extinction events in Earth’s history. In the worst one, 250 million years ago, 96 percent of marine species and 70 percent of land species died off. It took millions of years to recover.
Nowadays, many scientists are predicting that we’re on track for a sixth mass extinction. The world’s species already seem to be vanishing at an unnaturally rapid rate. And humans are altering the Earth’s landscape in far-reaching ways: We’ve hunted animals like the great auk to extinction. We’ve cleared away broad swaths of rain forest. We’ve transported species from their natural habitats to new continents. We’ve pumped billions of tons of carbon-dioxide into the atmosphere and oceans, transforming the climate.