(This will be a continuing series from time to time.)
There have been five Great Extinctions of life on earth in the last 250 million years, where up to 90% of life in the oceans and 70% of life on land was wiped out. Based on evidence from the last one (65 million years ago, death of the dinosaurs) it was thought that all five were caused by what caused the last one—the tremendous impact of a large asteroid a few miles in diameter. However, there were various problems in applying this thesis to the first four of the five great extinctions.
A new hypothesis by scientist Lee Kump, supported by Michael Arthur, Alexander Pavlov, Peter Ward and others, proposes a model for most of the extinctions—four out of the five—that is radically different: that the extinctions were caused by volcanic activity leading to global warming.
The mechanism by which previous episodes of global warming caused these Great Extinctions carries implications for our current time, which has been dubbed The Sixth Great Extinction. The real puzzle was this: Exactly how does global warming cause extinction of life in both the oceans and on land? What is the process?
This is what many scientists now think occurred:
First, the volcanic activity releases large amounts of carbon dioxide and methane into the atmosphere, which beyond a certain point causes rapid global warming. In turn, the global warming heats up the oceans, which means that the oceans can hold less dissolved oxygen.
There is strong evidence that the oceans of the world can basically be divided into two layers—a layer near the bottom where the water holds dissolved hydrogen sulfide, a poisonous gas, and the larger layer above that which holds dissolved oxygen and allows aquatic life to breathe. There appears to be a clear boundary, called the chemocline, between these two layers.
As the oceans warm up and therefore hold less dissolved oxygen, the chemocline boundary can suddenly and rapidly rise to the surface,
releasing large amounts of hydrogen sulfide into both the atmosphere and the upper layers of the ocean.
Since hydrogen sulfide is extremely toxic to both plants and animals and to both aquatic and terrestrial life, massive extinctions of life, both on land and in the seas, begin to occur.
Moreover, the large amounts of hydrogen sulfide released into the atmosphere also destroy the ozone layer, high in the atmosphere, that shields life on land from UV radiation. With the ozone layer gone, intense UV radiation bathes the earth and further decimates terrestrial life, both plants and animals.
The implication for our current situation is potentially ominous. What it means is this: If global warming proceeds beyond a certain point the poisoning process described above could come into action, resulting in potentially sudden and widespread death of both plants and animals, including humans.
The bottom line of this is that if we enter that tipping point of global warming we could find ourselves in a situation that, if past extinctions are a guide, is not only lethal to life but irreversible until hundreds of thousands of years have passed.
In other words, this is one of those situations where an ounce of prevention could be worth many pounds of cure, for there may be no cure once the process begins. In light of the potential consequences, tremendous and concerted global action to decarbonize our energy system is called for, while time is still available to us.
(This is the end of Part 1. Go to Part 2.)
—jim sloman, 10.30.06
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