Thought for the day : In recent weeks, two reports have emphasized the grave dangers posed to humanity by accelerating climate change. As a growing body of research over recent decades has made clear, human activities are responsible for alterations in Earth’s environment that will, if not addressed, lead to devastating consequences for humans and all life on this planet. At least five times in the past, severe disruptions in Earth’s climate have caused the extinctions of large portions of the life forms extant at the time. The most well known is the extinction of the dinosaurs, along with many other species, at the end of the Cretaceous Period, approximately 66 million years ago , almost certainly caused by the collision of a large asteroid with our planet, resulting in the equivalent of “nuclear winter.” Indeed, the concept of nuclear winter was developed by Carl Sagan and others once the cause of the dinosaur extinction was understood. The most catastrophic of these events was the devastation that occurred at the end of the Permian Period, 252 million years ago ( mya) , known as “The Great Dying.” Approximately 90% of marine species and 70% of terrestrial species disappeared in what, geologically speaking, was the blink of an eye. So dramatic was the change in life forms that this event defines the boundary between the Paleozoic era (541 to 252 mya) and Mesozoic era (252 to 66 mya). One might say that the course of life on Earth experienced a fundamental “reset” at the end of the Cretaceous and, indeed, at each of the other mass extinctions as well. The world that we know, dominated among the larger animals by birds and mammals, including humans, would not exist had it not been for the devastation caused by the impact of the Chicxulub asteroid that extinguished the dinosaurs . The specific causes of each extinction event appear to differ. As indicated, the end-Cretaceous extinction was caused by the collision of Earth with another celestial object. The extinction at the end of the Ordovician (485 to 443 mya) appears to have been caused by extensive glaciation. The Great Dying is thought to have been the result of massive volcanic eruptions in what is now Siberia, resulting in the emission of huge amounts of gases (including methane from the burning of organic materials) that triggered a runaway “greenhouse effect.”
Posted on: Mon, 02 Jun 2014 14:32:10 +0000
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