To the regular Joe on the street, it doesn't seem like we're in the midst of a mass extinction. Even experts admit that only 1 or 2 percent of all species have gone extinct in the past 200 years [source: Pappas]. That's a long way from the 75 percent needed to join the mass-extinction club. So what's gotten scientists worked up?
If you'll recall from earlier, a mass extinction can occur when plants and animals start dying off a lot faster than the normal, or background, rate. So a great way to see whether we're headed toward such an event is by looking at the current extinction rate versus the background extinction rate. And sure enough, a number of studies have done just that.
One of the more pessimistic findings estimates that the background rate of extinction for all species is 0.1 extinctions per million species per year (E/MSY), while the current rate is more like 100 E/MSY. That would mean we're losing species 1,000 times faster than normal [source: Orenstein]. Yikes! A more optimistic study, which looked only at mammals, pegged the background rate of extinction at 1.8 E/MSY, and the current rate at 50 to 75 E/MSY. But even in that supposedly rosy scenario, the current rate is at least 27 times too high [source: Simons].
What, then, is causing all of this? One problem is habitat loss. As the global population expands, more land is being cleared for farming, leaving less room for the creatures that lived there before. Another big issue is that many species are being driven to near extinction for short-term economic gain (think poaching and overfishing).
The explanation receiving the most attention, however, is human-caused climate change: When we burn fossil fuels, carbon dioxide is released into the atmosphere where it traps heat from the sun and causes the planet to warm. Some plants and animals just can't adapt fast enough to the changing environment and are dying off as a result [source: Barnosky].
If we are indeed experiencing a mass extinction, that doesn't mean we're goners. Plenty of creatures have survived before.