A mass extinction can be broadly defined as the loss of a large share of the world's species in a relatively short period of time. A commonly used and more precise definition is the loss of about 75 percent of the world's species within a brief geological time span of less than 2.8 million years.
The fossil record provides conclusive evidence that at least five great mass extinctions have already occurred. The causes are generally thought to include large asteroids hitting the earth, the eruption of large volcanos, and the consequent extreme changes in temperature and sea levels.
The earth is currently in the midst of a biodiversity crisis and is experiencing what many scientists believe is another mass extinction event, with species disappearing at a rate hundreds of times faster than they would naturally. Recent estimates suggest that extinction threatens up to a million species of plants and animals, many in the next few decades.* This is mostly the result of human activity, especially deforestation, pollution, poaching and overfishing, and the consequent climate change and the spread of invasive species and diseases.
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* "UN Report: Nature’s Dangerous Decline ‘Unprecedented’; Species Extinction Rates ‘Accelerating’", United Nations, May 6, 2019, https://www.un.org /sustainabledevelopment /blog/2019/05 /nature-decline-unprecedented-report/.