Ward's argument originates with the earliest days of life 3.7 billion years ago, a time when, he says, extensive microbial blooms saturated the planet's atmosphere with methane gas, creating a worldwide blanket of smog that blotted the sun and nearly eradicated incipient life.
But worse, said Ward, was a subsequent mass extinction 2.3 billion years ago when the Earth endured a period of global glaciation lasting 100 million years. The cause, says Ward, was again life itself.
Two hundred million years earlier, photosynthesis had debuted, the biological process by which atmospheric carbon dioxide and sunlight are converted into usable energy, with oxygen generated as a waste product.
Before photosynthesis, atmospheric levels of oxygen were relatively low. The gas was poisonous to early life. But with the emergence of photosynthesis, microbes began sucking so much heat-trapping carbon dioxide out of the atmosphere that the planet froze, said Ward. Virtually everything died, except for the photosynthesizers and microbes that quickly adapted to tolerate higher oxygen levels.
Ward says the rise of multicellular plants produced a similar cataclysmic decline in atmospheric CO2 and a second “snowball Earth” 700 million years ago. And he contends there is strong evidence most of the mass extinctions since the evolution of animals 565 million years ago have been, in some way, driven by adverse microbial activity.
These biologically induced mass extinctions are quite contrary to the idyllic concepts of Gaia, says Ward. And while life on Earth has enjoyed long periods of stability, Ward adds that it also has a nasty streak of self-destructiveness that is bound to rise again. Next time, he said, will be the last time.
The end begins with the sun, according to Ward, which has increased in brightness by about 30 percent over the past 4.5 billion years. That trend will continue, spurring global warming which will, in turn, boost the weathering rate of silicate rocks like granite, a process that chemically removes CO2 from the atmosphere.
At first, Ward speculates, the atmospheric loss of CO2 will benefit life by buffering the rising temperatures caused by a hotter sun. But at some point – perhaps 500 million years from now – there will not be enough CO2 left in the atmosphere to support photosynthesis. Plants will die; animals will follow. The loss of photosynthetic life will lead to a renewed buildup of carbon dioxide in the atmosphere, a runaway greenhouse effect and, in time, a planet too hot to sustain even microbial life. The Earth will return to its original state: a barren, sterile rock in space.
Like Lovelock's Gaia, Ward's Medea hypothesis has skeptics and critics.
Lynn Margulis, a noted biologist at the University of Massachusetts, Amherst, who helped Lovelock shape and promote the Gaia concept, dismisses Ward's work as misguided, describing it as “pseudo-quantitation, a nice example of the fallacy of misplaced concreteness.”
J. David Archibald, a professor of biology and a vertebrate paleontologist at San Diego State University, said he was “underwhelmed” by the Medea idea. “I don't know that it tells us anything we didn't already know.”