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The Medea Hypothesis: A response to the Gaia hypothesis

A new book, 'The Medea Hypothesis,' looks at the opposite of the Gaia hypothesis and suggests that life on Earth is self-destructive.

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The broader point: Repeatedly throughout Earth's history, organic life, in the form of anaerobic microbes that normally inhabit oxygen-starved nooks and crannies, have emerged to extinguish life. These microbes exhale hydrogen sulfide, a gas poisonous to other life forms.

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The instigator in this process is often oxygen-starved seas. Stagnant, low-oxygen seas are often preceded by rapid atmospheric warming. The tipping point is 1,000 parts per million of carbon dioxide, he says.

(We've written about the end-Permian extinction 251 million years ago, the greatest of all extinctions, and one thought to have occurred in precisely this manner.)

But, this warning aside, Ward's book isn't really about human-caused global warming. It's about the long-term future of life on the planet.

Organic life has repeatedly caused the collapse of the biosphere, and on at least one occasion (snowball earth) has almost extinguished it entirely.

Consider the advent of photosynthesizing organisms more than 3 billion years ago. Oxygen, a byproduct of photosynthesis, was deadly to the other organisms living at the time. As it accumulated, it killed — another mass extinction. (Of course, atmospheric oxygen also allowed for the emergence of multicellular life and, eventually, humans.)

The major long-term problem facing life on Earth is not, Ward says, global warming, but the gradual drawdown of carbon from the atmosphere by photosynthetic organisms.

Depletion of carbon dioxide has, in the past, caused dramatic global cooling. Twice before — once 2.3 billion years ago and again 700 million years ago — photosynthetic organisms used up such a large quantity of greenhouse gases that Earth froze over — what's called "snowball earth" — nearly ending life on the third rock from the sun.

In the future, Ward says, life's hunger for carbon will inevitably lead to a paucity of what's a fundamental building block of life, adding that this is the single greatest challenge facing life on this planet.

With graphs, he illustrates the long-term decline of atmospheric carbon dioxide over the past 3.5 billion years.

In the beginning, CO2 levels were perhaps 10,000 times higher than today's. The atmosphere was maybe one-third CO2. The subsequent trend is one of decreasing CO2 until, just before humans came along, CO2 had dropped to 280 parts per million. (Now, due to human activity, it has risen to 387 ppm.)

If carbon levels fell below 10 ppm, photosynthesis would become impossible for grasses, the most efficient carbon dioxide-utilizers around. (Trees can't handle anything less than 150 ppm.) The food chain will then collapse.

Ward forecasts that, given the aforementioned trends, carbon depletion beyond this critical threshold could happen as soon as 500 million years from now, about 500 million years sooner than the enlarging sun would begin to vaporize the oceans and make Earth uninhabitable anyway.

Then he drops a doozy:

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