From the book Rare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald Brownlee

In this fantastic book about how life formed on Earth, in the chapter Mass Extinctions and the Rare Earth Hypothesis, in the section The history of Mass Extinctions on Earth: Ten Events, the authors state that over the last 500 million years, there have been 15 events officially classified as mass extinctions.

At least one mass extinction was caused by an object striking the planet, which is what wiped out the dinosaurs 65 million years ago. The causes of some of those mass extinctions are unknown.

They measure the degree of extinction by counting the number of species in the fossil record before and after the extinction. The worst mass extinction was the Permo-Triassic, which took place 250 million years ago, in which estimates from 80% to 90% of species went extinct.

Intensive research into the cause of this extinction ... has yielded a clearer view. ... it has long been recognized as the most catastrophic mass extinction ... although several causes contributed to the event, the most important derived from a short-term degassing of carbon dioxide from sediments sequestered on the ocean floor and from volcanic gas emitted during unusually severe volcanic eruptions occurring about 250 million years ago. A sudden release of huge volumes of carbon dioxide directly killed marine organisms by carbon dioxide poisoning and indirectly decimated terrestrial life via sudden and intense global heating. The excess carbon dioxide released into the atmosphere greatly enhanced the greenhouse effect, thus increasing the amount of heat trapped in the atmosphere. A heat spike of perhaps 5 to 10 degrees from 10,000 to 100,000 years in duration probably caused the terrestrial extinctions.

When you read books like this one, you are astounded by the amount of things people have figured out. When these same people tell you about global warming, you should listen. They know what they are talking about.