Are Humans Causing the Sixth Mass Extinction Event?
Dinosaurs were famously driven to extinction by an asteroid, the Great Dying (the largest mass extinction event in history) was likely caused by volcanic activity, and humans may be the catalyst for the next apocalypse, which may already be underway.
There have been five mass extinction events throughout history, in which at least half of all species on Earth were wiped out of existence. There has always been some kind of catalyst, whether it's an asteroid, volcanism, marine regression, or ocean anoxia (deficiency of oxygen). Human activity has easily had enough of an impact on the world's biodiversity to be a prime contender for the cause of the next mass extinction event; our effects have been so profound geologists are informally calling our current era the Anthropocene. We have significantly changed the world's climate, our civilization uses more than half of the unfrozen land in the world, and we constantly destroy the habitats of other species for the sake of expansion and the harvesting of food and fuel.
Although we cannot consider this time period a mass extinction yet, scientists estimate that we are well on our way. According to biologists and paleoecologists, approximately 1,000 species have been driven to extinction by humans in our 200,000 years of existence, 322 of which were wiped out within the last five hundred years. Biologists are uncertain as to the exact rate of extinction, as the extinction of many different kinds of insects are not well-studied, but estimates put the extinction rate at 1,000 times faster than the average rate in geologic history. As a result, even though it is not technically a mass extinction according to the above definition, it is already the fastest extinction event in history. Scientists are calling this extinction event the Anthropocene Defaunation. (Americans are also more responsible than most, as environmental destruction tends to be directly proportional to consumerism.)
As long as we don't change our behaviors (which we certainly haven't so far), the picture looks bleak. The invertebrate population has decreased by half in the last 35 years, while the human population has doubled. In addition to the species that are actually extinct, another estimated 20,000 species are threatened or endangered. The populations of individual species are, on average, 28% smaller as a direct result of the ravages of humanity.
However, the damage is not irrevocable just yet. Without major changes, this era will qualify as a mass extinction event in a century or two. Until then, biologists recommend that we not only stop destructive behaviors, such as destroying other species' natural habitats and hunting endangered species for consumer products, but also taking various actions to prevent extinctions, such as assisted migration. According to an article in Science, assisted migration has so far saved 424 species from extinction. Widespread vegetarianism would go a long way as well, partially to stymie the killing of animals but also as a result of the fact that the meat industry causes 18% of our greenhouse emissions. There is also some hope that we will be able to resuscitate species that have already become extinct through synthetic biology.