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Over the next few centuries, Earth could face a mass extinction on par with the one that killed off the dinosaurs 65 million years ago, according to a new study. A mass extinction is when 75 percent or more of the planet's species die out — an event that has happened five times in Earth's history. Now, according to a new report in the scientific journal Nature, we could be on the brink of a new one. Most at risk are amphibians, with 41 percent of species threatened with extinction. For mammals, that number is 26 percent, while 13 percent of bird species are in danger. Less is known about insects, seeing as only 0.5 percent of 1 million known species have been evaluated. (There might be more than 5 million species living on Earth).

The most dire threat comes from what the study calls "exploitation" — hunting, fishing, etc. — while habitat degradation comes in a close second. The big unknown is climate change. If it isn't a huge factor, the sixth extinction could take thousands of years, but if climate change makes things a lot worse, the study says that Earth could lose 75 percent of its species by 2200.



— Keith Wagstaff