Six Mass Extinctions in Earth’s History. Are We on the Brink of a Seventh?
Scientists define a mass extinction as the extinction of about three-quarters of all species over a short geological period of time, which is less than 2.8 million years, according to The Conversation. Right now, humans are at the beginning of the last mass extinction, which is moving much faster than any other. Since 1970, vertebrate species populations have declined by an average of 68%, and more than 35,000 species are currently considered threatened with extinction, according to the International Union for Conservation of Nature (IUCN). During the 20th century alone, about 543 species of land vertebrates became extinct, according to a research paper in the journal PNAS.
Causes of the Permian-Triassic Extinction: Atmospheric Carbon Dioxide Overload VS Deadly Pulse of Ultraviolet Radiation
At the end of the Permian period 252 million years ago, Earth was devastated by a mass extinction that wiped out more than 90% of the planet’s species. Compared to other mass extinctions, the recovery from the Great Dying was slow: it took at least 10 million years for the planet to be repopulated and begin to restore its diversity. The largest mass extinction in Earth’s history may have been triggered by a strong El Niño cycle. A deadly pulse of ultraviolet (UV) radiation may also have played a role in Earth’s largest mass extinction, fossilized pollen grains suggest.