Conflicts tend to scatter people, and ideas, in unexpected ways. After the American Civil War, a flood of so-called Confederados fled the devastated South and set up farms in the Brazilian Amazon. They planted rice and sugar cane and tobacco, and they prospered. But the lands they settled - primarily high bluffs along rivers - weren't any more pristine than Alabama or the Carolinas had been. As they plowed, the settlers unearthed vast quantities of potsherds that showed the land had been inhabited before. And the ceramics weren't the only sign of previous human cultivation: The deep black earth itself, very different from the pale, nutrient-poor soils of much of the Amazon, quickly revealed that people had been indispensable in creating its fertility.

"The rich terra preta, 'black land,'" of one settlement was "the best on the Amazon. ... a fine, dark loam, a foot, and often two feet thick," wrote an American naturalist named Herbert Smith in 1879. "Strewn over it everywhere we find fragments of Indian pottery. ... The bluff-land owes its richness to the refuse of a thousand kitchens for maybe a thousand years."

Though they have always been prized by farmers, the dark soils of the Amazon were largely forgotten by science for a century after their discovery. They are now re-emerging as an important topic of study, not because they're an ethnographic or historical curiosity, but because they show an exceptional ability to store carbon, which in the form of carbon dioxide has rapidly turned into one of humanity's most pernicious waste products. As a result, they're joining the rapidly growing roster of tactics that might be used to combat climate change. Researchers around the world are considering whether people may, by engineering soils specifically to maximize carbon storage, be able to absorb substantial amounts of our emissions, increase the fertility of agricultural areas and dampen some of the effects of climate change.

Sound utopian? Maybe. But as the long aftermath of the Civil War shows, solutions to deeply ingrained social problems often do emerge - though not always quickly and certainly not without enormous and sustained effort.

"We could gear up for this with something like the Manhattan Project," says William Woods, a University of Kansas geographer and expert on terra preta. "Imagine all the organic stuff that comes into a city - and then imagine putting all that carbon into the soil. It works, though we aren't there yet. So far no one seems to have the will do it."

Carbon is the essential building block of all life, the bustling captain of industry, the stuff at the core of diamonds. Carbon has long starred quietly in virtually everything that goes on in human lives, but now its blandly essential air has been eclipsed by a new role: that of villain in the long-running drama of climate change. As the key component of carbon dioxide, element 12 has now firmly moved in the public mindset from good guy to a problem that threatens the future of the very lives it has made possible.

Carbon dioxide isn't the only greenhouse gas out there - methane, the nitrogen trifluoride used in the manufacture of flat-panel televisions, and others contribute to global climate change, too - but it is the most widespread and the one most directly associated with the industrial revolution. Combustion begets CO2, simply, and as that extra gas accumulates in the atmosphere, it causes the Earth to retain more heat. The litany of effects that result from that warming is becoming increasingly well known: rising oceans, more severe heat waves, irregular precipitation, greater threat of drought. So is the precise concentration of carbon dioxide in the atmosphere, which has been rising steadily since humans started burning a lot of coal in the 19th century - and which is currently rising at a rate faster than anticipated by most of the predictions made by the Intergovernmental Panel on Climate Change.

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