The World Without Us

The problem is, by tapping the Carboniferous Formation and spewing it up into the sky, we’ve become a volcano that hasn’t stopped erupting since the 1700s.

So next, the Earth must do what it always does when volcanoes throw extra carbon into the system. “The rock cycle kicks in. But it’s much longer.” Silicates such as feldspar and quartz, which comprise most of the Earth’s crust, are gradually weathered by carbonic acid formed by rain and carbon dioxide, and turn to carbonates. Carbonic acid dissolves soil and minerals that release calcium to groundwater. Rivers carry this to the sea, where it precipitates out as seashells. It’s a slow process, sped slightly by the intensified weather in the supercharged atmosphere.

“Eventually,” Volk concludes, “the geologic cycle will take CO₂ back to prehuman levels. That will take about 100,000 years.”

Or longer: One concern is that even as tiny sea creatures are locking carbon up in their armor, increased CO₂ in the oceans’ upper layers may be dissolving their shells. Another is that the more oceans warm, the less CO₂ they absorb, as higher temperatures inhibit growth of CO₂-breathing plankton. Still, Volk believes, with us gone the oceans’ initial 1,000-year turnover could absorb as much as 90 percent of the excess carbon dioxide, leaving the atmosphere with only about 10 to 20 extra parts per million of CO₂ above the 280 ppm preindustrial levels.

The difference between that and today’s 380 ppm, scientists who’ve spent a decade coring the Antarctic ice assure us, means that there will be no encroaching glaciers for at least the next 15,000 years. During the time that the extra carbon is being slowly sopped up, however, palmettos and magnolias may be repopulating New York City faster than oaks and beeches. The moose may have to seek gooseberries and elderberries in Labrador, while Manhattan instead hosts the likes of armadillos and peccaries advancing from the south…

…unless, respond some equally eminent scientists who’ve been eying the Arctic, fresh meltwater from Greenland’s ice cap chills the Gulf Stream to a halt, closing down the great ocean conveyor belt that circulates warm water around the globe. That would bring an ice age back to Europe and the East Coast of North America after all. Maybe not severe enough to trigger massive sheet glaciers, but treeless tundra and permafrost could become the alternative to temperate forest. The berry bushes would be reduced to stunted, colorful spots of ground cover among the reindeer lichen, attracting caribou southward.

In a third, wishful scenario, the two extremes might blunt each other enough to hold temperatures suspended in between. Whichever it is, hot or cold or betwixt, in a world where humans stayed around and pushed atmospheric carbon to 500 or 600 parts per million—or the projected 900 ppm by AD 2100, if we change nothing from the way we do business today—much of what once lay frozen atop Greenland will be sloshing in a swollen Atlantic. Depending on exactly how much of it and Antarctica go, Manhattan might become no more than a few islets, one where the Great Hill once rose above Central Park, another an outcropping of schist in Washington Heights. For a while, clutches of buildings a few miles to the south would vainly scan the surrounding waters like surfacing periscopes, until buffeting waves brought them down.

2. Ice Eden

Had humans never evolved, how might the planet have fared? Or was it inevitable that we did?

And if we disappeared, would—or could—we, or something equally complicated, happen again?

FAR FROM EITHER pole, East Africa’s Lake Tanganyika lies in a crack that, 15 million years ago, began to split Africa in two. The Great African Rift Valley is the continuation of a tectonic parting of the ways that began even earlier in what is now Lebanon’s Beqaa Valley, then ran south to form the course of the Jordan River and the Dead Sea. Then it widened into the Red Sea, and is now branching down two parallel cleavages through the crust of Africa. Lake Tanganyika fills the Rift’s western fork for 420 miles, making it the longest lake in the world.