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D uring a cloudburst in Puerto Rico’s Luquillo Mountains, a threefoot -wide stream sprang up between my partner, Bob Segal, and me. At the time, we were measuring tree diameters during a 1998 survey of Hurricane Georges damage. Although he was only about four feet away, the newly formed stream was moving fast. We each grabbed the nearest tree to keep ourselves from sliding down the steep clay-covered slope while we waited out the quarter-hour downpour. The ephemeral stream between us carried visible leaf material along for the ride, as well as the brownish-orange clay lifted from local soil stores. On another memorable occasion a few weeks later, a heavy rain trapped us on the steep slope of a hill denuded by a landslide, which turned up in one of our plots in Cayey. We were directly upslope from a rushing river, now brown and frothy from the sediment and water washing into it. Several uprooted trees stood as sentinels between the slope and the river, their roots already cleaned of soil by other storms since their fall two months earlier. After about half an hour, when the rains failed to let up, we inched along a trail of fallen trees onto safer ground, taking care not to join the soil particles and debris being pulled off the mountain into the river. Anyone who has observed swiftly Xowing rivers, whether in person or on the news, will recall that the rushing waters churn with shades of local soil—whether muddy brown or dirt red, whether in the Yellow River or the Río Negro. Hurricane Katrina Xoods dropped four to eight inches of mud as the waters settled in Louisiana. A July 2006 Xood in Norwalk, Ohio, similarly left the aptly named Water Street covered with at least an inch of mud after the overXowing water had drifted back into the nearby creek. “It smelled like the river bottom—stinky,” said Edward Christie, a service technician with a local business. Other U.S. Xoods that year brought mud to homes around the Sandusky River, the Delaware, and various eastern rivers. A late July 2006 Xood in Tucson’s Y 7 Z Beneath the Surface Weathering the Warming in Deep Time Sabino Canyon carried so much material that it clogged culverts under roads, then destroyed overlying bridges when it jumped its banks. Mississippi Xoods regularly make the news, with 2008 overXows being among the most recent and the large-scale 1993 event being among the most memorable. These Xoods offer a fast-forward glimpse at an ongoing process that has a slow but deWnite impact on global carbon dioxide levels: weathering . Intense rainfall speeds up the rate of both physical and chemical weathering. For simplicity, I’ll refer to the former as erosion and the latter as weathering. Although both move mountains, weathering and erosion differ slightly. Weathering turns bedrock into soil. Erosion moves the soil off the slopes. Dusty winds can erode soft material such as sandstone , and ice can help split soil and rock. Other than that, both erosion and weathering rely on the presence of liquid water to do their jobs. Weathering plucks carbon dioxide out of the air as it works at the molecular level to divide rock into smaller parts, including chemical compounds such as bicarbonate and calcium. Then it carries some of those compounds off into rivers and eventually the sea. Erosion whisks off soil, along with the organic carbon it contains, as well as leaves and other carbon-rich debris. Rivers then deposit some of that carbon into wetter environments where it is less likely to decay. The organic matter traveling by river that makes it to the seaXoor without being eaten or dissolved can put former carbon dioxide into deep storage. Ocean sediments hold several times the amount of carbon in the atmosphere. The ocean itself holds about Wfty times more carbon than the air, counting dissolved bicarbonates and carbonates. Coal and oil deposits of the world, meanwhile, hold at least several times more carbon than the amount in the air, while the mostly inaccessible but somewhat unpredictable natural gas of the sea, methane hydrates, contains more than ten times the amount of carbon residing in the air. The biggest storehouse of marine carbon, though, resides in solid carbonates—limestone , dolomite, and chalk, mainly—many of which owe their existence to coral reefs and to microscopic shell-forming creatures known fondly as forams and coccoliths. By land and by...

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