Twenty thousand years ago, the last great continental ice sheet plowed the northern Midwest, planing the land down to bedrock, grinding stone to dust, mixing sediment and then dropping it as the ice melted. For a few hundred years, the boreal forest replaced the glacier, but as the weather grew warmer and drier, spruce and fir gave way to oak and hickory, then to bluestem and indiangrass.

For thousands of years, generations of tallgrass prairie plants lived and died, penetrating eight to 10 feet below the surface with extensive root systems and building a thatch of rotting stems at the surface. Slowly and steadily, the organic material turned into humus, mixing with the minerals below to form as much as eight feet of topsoil.

Soil is black for the same reason oil and coal are black -- carbon. Plants take in carbon dioxide and use the energy of the sun to forge the carbon into long-chain organic molecules. Plant physiologists call the process photosynthesis, but a new generation of climate specialists and entrepreneurs has another name for it: carbon sequestration, the long-term storage of carbon in its solid form. Carbon sequestration has become one of the hottest topics in discussions about climate change and wetland conservation.

Given the chance, the planet does a pretty good job of locking up carbon. The oil, coal and natural gas we burn by the billions of tons are, after all, the result of natural carbon sequestration that occurred as much as 360 million years ago. Natural vegetation continues the process today. A walnut tree on the northern plains might absorb more than a ton of carbon per year over the first 80 years of its life, much of which is right where we can see it, in that exquisite dark wood we use in our gun stocks.

The carbon uptake in prairies and wetlands is harder to recognize because so much of it happens underground. The taproot of a prairie rose might grow 20 feet straight down, and the main roots of compassplant and leadplant aren't far behind. Most prairie plants have huge systems of roots and underground stems -- more than three-quarters of the plants are below ground, which makes sense for vegetation that has evolved to handle the extremes of prairie weather and the not-so-tender attention of millions of grazing animals.

When one of these plants dies, much of the carbon in its roots is left below the surface, out of reach of many of the creatures, large and small, that might eat it and release it into the atmosphere. Little wonder prairie soils are exceptionally rich in carbon. An acre of tallgrass prairie might have more than 50 tons of organic carbon bound in the soil underneath. Marshes in the pothole country of the north-central U.S. bind about the same amount of carbon per acre.

When the prairie sod was first broken, the soil immediately started leaking carbon.

Because nobody was taking careful soil samples in the last half of the 19th century, it is hard to know exactly how much carbon was lost to the plow. One study in the prairie potholes estimates that 16 percent of the carbon in wetland basins is lost when they're plowed. Data from a long-term study plot at the University of Illinois suggests the loss there was well over 30 percent, and a similar study at the University of Missouri suggests it can be as much as 96 percent.

The conversion of native prairie to cropland, combined with the clearing of American forests and wetlands, released unimaginable amounts of carbon into the atmosphere.

Scientists at the Woods Hole Research Center have estimated the amount of carbon released as the United States was settled. According to their data, the loss reached a broad peak in 1881 and began to decline slowly afterward. In 1881 alone, more than 300 million tons of carbon was liberated from America's soil and timber.