What if the only thing life has to fear is life itself?

At a lecture Monday evening at the American Museum of Natural History in New York City, paleontologist Peter D. Ward laid out the argument that life as we know it serves to make Earth less habitable—a downward spiral that might spell the eventual end of life on the planet. Ward, a professor at the University of Washington, calls this the Medea hypothesis, named for the murderous mother of Greek mythology. It is a direct challenge to scientist and futurist James Lovelock's Gaia hypothesis, which asserts that life constantly tweaks the dials on Earth's control systems to keep the planet in a nice, habitable homeostasis.

Ward has a recent book on the subject, The Medea Hypothesis: Is Life on Earth Ultimately Self-Destructive? (Princeton University Press, 2009). To illustrate the difference between his theory and Lovelock's, the traveling Ward, in town to make the media rounds for his book, used a hotel analogy for Earth. Gaians, Ward says, think that hotel guests are likely to repaint their rooms and leave fresh flowers before checking out, whereas Medeans think that guests are liable to throw furniture out the window, trashing the room like Keith Moon in his prime.

At the lecture, moderator Neil deGrasse Tyson, an astronomer and the director of the museum's Hayden Planetarium, struggled to define the work of the polymath Ward, finally settling on "paleobiogeoastronomer."

And indeed, Ward's dour claim rests on analyses of carbon isotopes, paleofossils, asteroid impact rates and geologic formations. Most of the mass extinctions in history, Ward says, were caused by microorganisms, not by asteroid or comet impacts. Here is how: When Earth warms to the point that it no longer has cold poles and warm tropics, as the result of geologically released greenhouse gases, the oceans stop mixing. Without mixing, only the uppermost layer of the ocean remains oxygenated, and anaerobic bacteria that produce poisonous hydrogen sulfide gas thrive. Before long, the level of hydrogen sulfide in the atmosphere becomes lethal, simultaneously poisoning living creatures and shredding the ozone layer. "This is life killing itself off," Ward says.

As with today's climate crisis, carbon dioxide is the culprit in the ultimately catastrophic warming. Of course, the ultimate source of Earth's massive die-offs wasn't anthropogenic or even the fault of life—Ward points to volcanic floods that churned out enough CO2 to shut down ocean mixing driven by temperature differentials. But thanks to the actions of humankind, the delicate balance that keeps Earth habitable is once again in danger. "All you need is enough [warming] to reduce the temperature difference between the poles and the equator, and the whole system goes down," Ward says.

Thankfully, perhaps, such dire predictions for climate change—not displacement, war or even famine but a nearly wholesale elimination of life of Earth—rest on equally dire forecasts for CO2 levels. Whereas many experts set 350 parts per million as the maximum acceptable level for atmospheric CO2 (today's atmosphere is at about 390 ppm), Ward says that these warming-driven catastrophes arise at about 1,000 ppm. That's not to say that things won't get ugly along the way, with arable land disappearing and rising seas rewriting maps of the world, but there may at least be some air to breathe for another two centuries or so.

As for fixes, Ward did not have any ready answers other than hoping that currently iffy technologies can take off. Practicable nuclear fusion would help a lot, as would advances in bioengineering. "We can convince microbes to do some very interesting things," he says, pointing specifically to their promise in systems to produce food and fuel. Ward is aware that betting on currently untenable technologies as the way out may seem like pie-in-the sky dreaming. "Look, if you don't have hope, you don't do anything," he says. "You go out and get a drink."

Cover image: Princeton University Press