Context

In the last 500 million years, 75 to more than 90 percent of all species on Earth have disappeared in mass extinctions.

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What is mass extinction?

Mass extinctions are defined as any substantial increase in the amount of extinction (lineage termination) suffered by more than one geographically wide-spread higher taxon during a relatively short interval of geologic time, resulting in an at least temporary decline in their standing diversity.

Major mass extinction events in the geological history of Earth:

• Ordovician-Silurian extinction 485 to 444 million years ago:
• During this period massive glaciation locked up huge amounts of water in an ice cap that covered parts of a large south polar landmass. This may have been triggered by the rise of North America’s Appalachian Mountains.
• The large-scale weathering of these freshly uplifted rocks sucked carbon dioxide out of the atmosphere and drastically cooled the planet.
• As a result, sea levels plummeted by hundreds of feet. Creatures living in shallow waters would have seen their habitats cool and shrink dramatically, dealing a major blow.
  • Late Devonian extinction - 383-359 million years ago:
• Starting 383 million years ago, this extinction event eliminated about 75 percent of all species on Earth over a span of roughly 20 million years.
• Volcanism could be a possible trigger for this extinction.
• Within a couple million years of the Kellwasser event, a large igneous province called the Viluy Traps erupted 240,000 cubic miles of lava in what is now Siberia. The eruption would have spewed greenhouse gases and sulfur dioxide, which can cause acid rain.
• Asteroids may also have contributed. Sweden’s 32-mile-wide Siljan crater, one of Earth’s biggest surviving impact craters, formed about 377 million years ago.
• During the Devonian, plants hit on several winning adaptations, including the stem-strengthening compound lignin and a full-fledged vascular structure. These traits allowed plants to get bigger and for their roots to get deeper than ever before, which would have increased the rate of rock weathering.
• The faster rocks weathered, the more excess nutrients flowed from land into the oceans. The influx would have triggered algae growth, and when these algae died, their decay removed oxygen from the oceans to form what are known as dead zones. In addition, the spread of trees would have sucked CO2 out of the atmosphere, potentially ushering in global cooling.

• Permian-Triassic extinction - 252 million years ago:
  • Of the five mass extinctions, the Permian-Triassic is the only one that wiped out large numbers of insect species. Marine ecosystems took four to eight million years to recover.
  • The extinction’s single biggest cause is the Siberian Traps, an immense volcanic complex that erupted more than 720,000 cubic miles of lava across what is now Siberia. The eruption triggered the release of at least 14.5 trillion tons of carbon.
  • Magma from the Siberian Traps infiltrated coal basins on its way toward the surface, probably releasing even more greenhouse gases such as methane.
  • In the million years after the event, seawater and soil temperatures rose between 25 to 34 degrees Fahrenheit.
  • As temperatures rose, rocks on land weathered more rapidly, hastened by acid rain that formed from volcanic sulfur. Just as in the late Devonian, increased weathering would have brought on anoxia that suffocated the oceans. Climate models suggest that at the time, the oceans lost an estimated 76 percent of their oxygen inventory. These models also suggest that the warming and oxygen loss account for most of the extinction’s species losses.
    • Triassic-Jurassic extinction - 201 million years ago:
      • This mass extinction caused the extinction of 80 percent of all land and marine species.
      • At the end of the Triassic, Earth warmed an average of between 5 and 11 degrees Fahrenheit, driven by a quadrupling of atmospheric CO2 levels. This was probably triggered by huge amounts of greenhouse gases from the Central Atlantic Magmatic Province, a large igneous province in central Pangaea.
      • Remnants of those ancient lava flows are now split across eastern South America, eastern North America, and West Africa.
      • The Central Atlantic Magmatic Province was enormous. Its lava volume could cover the continental U.S. in a quarter-mile of rock.
      • The uptick in CO2 acidified the Triassic oceans, making it more difficult for marine creatures to build their shells from calcium carbonate.
      • On land, the dominant vertebrates had been the crocodilians, which were bigger and far more diverse than they are today. Many of them died out. In their wake, the earliest dinosaurs—small, nimble creatures on the ecological periphery—rapidly diversified.
    • Cretaceous-Paleogene extinction - 66 million years ago:
      • The Cretaceous-Paleogene extinction event is the most recent mass extinction and the only one definitively connected to a major asteroid impact.
      • Some 76 percent of all species on the planet, including all nonavian dinosaurs, went extinct.
      • About 66 million years ago, an asteroid roughly 7.5 miles across slammed into the waters off of Mexico’s Yucatán Peninsula at 45,000 miles an hour. The massive impact left a crater more than 120 miles wide flung huge volumes of dust, debris, and sulfur into the atmosphere, bringing on severe global cooling.
      • Wildfires ignited any land within 900 miles of the impact, and a huge tsunami rippled outward from the impact. Overnight, the ecosystems that supported nonavian dinosaurs began to collapse.
      • Global warming fueled by volcanic eruptions at the Deccan Flats in India may have aggravated the event. Some scientists even argue that some of the Deccan Flats eruptions could have been triggered by the impact.
        • Extinction today
          • Earth is currently experiencing a biodiversity crisis. Recent estimates suggest that extinction threatens up to a million species of plants and animals, in large part because of human activities such as deforestation, hunting, and overfishing.
          • Other serious threats include the spread of invasive species and diseases from human trade, as well as pollution and human-caused climate change.
          • Today, extinctions are occurring hundreds of times faster than they would naturally. If all species currently designated as critically endangered, endangered, or vulnerable go extinct in the next century, and if that rate of extinction continues without slowing down, we could approach the level of a mass extinction in as soon as 240 to 540 years.