Planet Earth: It's all downhill from here

Killjoys! First geologist Peter Ward and astronomer Donald Brownlee demolish E.T.'s home in their best-selling "Rare Earth: Why Complex Life is Uncommon in the Universe," and now they're going after our world, too.

In the compelling "The Life and Death of Planet Earth" (Times Books, $25), they describe a planet in late middle age on a trajectory toward a lifeless end. As the subtitle announces, this book recounts "How the New Science of Astrobiology Charts the Ultimate Fate of Our World."

The death of our world will be neither pretty nor swift. Our planet will suffer the decline of one life-sustaining system after another. Just as a person loses faculties through the ravages of living, so will Earth lose its ecosystems in a predictable step-by-step sequence.

"[H]istory," write the two University of Washington professors, "will begin to run backward as Earth's environment eventually slips toward the simple ecology of hundreds of millions of years ago. This decline, we assert, ... has already started. Biologically, Earth has already peaked — perhaps as long as 300 million years ago."

The time scales of geology are far longer than those of human history. Civilization has arisen in a brief interglacial period between ice ages. Global warming, though a genuine concern in the short run of the next few centuries, will be a brief interlude. When we exhaust our reserves of fossil fuel, the decline of atmospheric carbon dioxide will resume. Ocean ice and glaciers will creep toward the equator from the poles.

Eventually, the Earth and sun will conspire to melt the ice, setting the stage for planetary death by fire. A quarter of a billion years from now, thanks to continental drift, a supercontinent will form. It will be larger than the ancient Gondwanaland whose appearance a quarter of a billion years ago produced stagnation of ocean currents and may have led to Earth's greatest mass extinction.

Meanwhile, the aging sun will gradually increase its output. In another billion years, if an "Accidental Armageddon" has not sterilized the planet first, solar heat will lead to first the loss of plants, then animals, then Earth's oceans. The planetary corpse would be host for (at best) heat-loving bacteria in subterranean niches of a Venus-like wasteland.

Several billion years after that, the sun will exhaust the hydrogen fuel in its core and begin burning helium, becoming a red giant that will either consume Earth or turn it into a cinder. The existence of humanity will be documented only by a few scattered spacecraft — unless we can somehow engineer "The Great Escape." Though not ruling out that possibility, the authors close with more practical advice: "[T]his moment on this Earth truly is a precious gift to be savored and appreciated. ... Another obvious lesson is that we tinker with our atmosphere and oceans at great risk."

Fred Bortz is a physicist and author who runs a children's science Web site at www.fredbortz.com. His book reviews are archived at www.scienceshelf.com.