As serious as the threat from shrimp farming is to the world's remaining mangroves, there looms a potentially more disastrous problem: rising sea levels. Standing as they do at the land's frontiers, mangroves will be the first terrestrial forests to face the encroaching tides.

Loss of mangrove forests could prove catastrophic in ways only now becoming apparent. For more than 25 years Jin Eong Ong, a retired professor of marine and coastal studies in Penang, Malaysia, has been exploring a less obvious mangrove contribution: What role might these forests play in climate change? Ong and his colleagues have been studying the carbon budget of mangroves—the balance sheet that compares all the carbon inputs and outputs of the mangrove ecosystem—and they've found that these forests are highly effective carbon sinks. They absorb carbon dioxide, taking carbon out of circulation and reducing the amount of greenhouse gas.

By measuring photosynthesis, sap flow, and other processes in the leaves of the forest canopy, Ong and his team can tell how much carbon is assimilated into mangrove leaves, how much is stored in living trees, and how much eventually makes its way into nearby waterways. The measurements suggest that mangroves may have the highest net productivity of carbon of any natural ecosystem (about a hundred pounds per acre [45 kilograms per 0.4 hectares] per day) and that as much as a third of this may be exported in the form of organic compounds to mudflats. Mangroves, it seems, are carbon factories, and their demolition robs the marine environment of a vital element.

Ong's team has also shown that a significant portion of the carbon ends up in forest sediments, remaining sequestered there for thousands of years. Conversion of a mangrove forest to a shrimp pond changes a carbon sink into a carbon source, liberating the accumulated carbon back into the atmosphere—but 50 times faster than it was sequestered.

If mangroves were to become recognized as carbon-storage assets, that could radically alter the way these forests are valued, says Ong. If carbon trading becomes a reality—that is, if forest-rich, carbon-absorbing countries are able to sell so-called emissions credits to more industrialized, carbon-emitting countries—it could, at the least, provide a stay of execution for mangroves.

But Ong notes that the financial incentives have to be great enough to make forest preservation economically viable. "Take Indonesia, which has the largest total area of mangroves of any country in the world. It can't afford to save them for nothing," he says. "But if the Indonesians could trade the carbon-storage potential of their mangroves as a commodity, that would create a great incentive to stop bulldozing them for shrimp ponds or chipping them for the production of rayon."

Countries that have squandered their mangroves could also replant them, gaining both a tradable asset and coastline protection. At Ong's research site small boys stuff their pockets with cigarillo-shaped mangrove seeds, or propagules. The boys will sell them for a few cents. Ong says that throughout Asia there's a run on propagules, as countries replant their mangrove defenses in the wake of the 2004 tsunami.

On the east coast of Africa, a very different kind of mangrove experimentation is going on. In Hirgigo, Eritrea, a few miles down the coast from the port of Massawa, two men sit on planks on the hot desert sand. With a knife for a chisel and a rock for a hammer, they knock the bottoms out of empty tomato sauce cans—discards from the Eritrean Navy. Nearby, on the shores of the Red Sea, a group of women push the hollow cans into the soft sediment, forming long alleys on the mudflats. Into each can, the women press mangrove propagules.

This is the planting of the Red Sea, the brainchild of cell biologist, cancer-drug pioneer, and humanitarian Gordon Sato. In the early 1980s, Sato's laboratory at the University of California at San Diego developed Erbitux, a breakthrough drug for colorectal cancer. These days 79-year-old Sato works to cure a different disease—poverty—attacking the problem not by culturing cells but by cultivating mangroves.

Eritrea was reeling from war and famine when Sato first traveled there in the mid-1980s. Since water is such a scarce resource in this arid country, Sato wondered if he could develop some form of salt water–based agriculture on Eritrea's long coastline, to help provide food for the hungry. Mangroves seemed a logical, if unconventional, choice. They occurred naturally, though patchily, along the Red Sea shore, they flourished in salt water, and camels were known to eat the leaves. If camels ate them, why not feed the foliage to sheep and goats? Grow enough mangroves, Sato reasoned, and you could provide food security for thousands.

So, like a maritime Johnny Appleseed, he began planting—and failed. All the saplings died. Undaunted, Sato looked closely at places on the Eritrean coast where mangroves were growing naturally, and he noticed they occurred only where fresh water was channeled during the brief rains that fall on this desert coast. Sato reasoned it was not fresh water the trees needed but minerals the water was bringing from inland—specifically nitrogen, phosphorus, and iron, elements in which seawater is deficient.