4. Climate Change
STATUS: You've heard all about the earth's water cycle: It rains, rivers flow to the sea, water evaporates and forms cloud vapor, and then it rains again. It always sounded so predictable. Well, hold on to those galoshes, because that stable system is likely to go bonkers, according to climate forecasters. If, as expected, the earth's temperature rises between about three and seven degrees Fahrenheit in this century, the result will be more evaporation and more water vapor, which itself acts as a potent greenhouse gas, accelerating global warming. Essentially, the whole hydrologic cycle may act like it's on speed, sparking fiercer and more frequent storms and flooding, melting snowpack and glaciers, and threatening the water supply of one-sixth of the world's population. Sea levels could rise up to two feet by 2100, drowning cities, wetlands, estuaries, mangroves, and other ecosystems, not to mention a few of your favorite tropical atolls. In turn, the destroyed wetlands would no longer absorb greenhouse gases, feeding an ever more vicious cycle.

SOLUTION: Reduce global emissions of greenhouse gases as soon as possible. "The longer we wait, the graver the risks and the cost of averting them," says Eileen Claussen, president of the Pew Center on Global Climate Change. Groups like Pew are building worldwide coalitions to support treaties and policies like emissions cap-and-trade programs, which set a limit on the amount of CO2 allowed to enter the atmosphere. Such market-based approaches, perhaps combined with a tax on emissions, would provide cash incentives for industries and individuals to adapt more quickly to a low-carbon reality. In January, a coalition of environmental groups and industrial giants like DuPont and Alcoa proposed reducing emissions by 10 to 30 percent over 15 years. And energy guru Amory Lovins insists that clean alternatives, like hybrid cars and low-carbon fuels, are already available or in the pipeline. "Existing efficiency technologies, systematically applied, can save half of our oil and gas, and three-fourths of our electricity," he says. "This by itself would solve nearly half the climate problem."

SOURCE: USGS (courtesy, Deka Research & Development)

5. Excess Dams
STATUS: The Tinkertoy lovers among us may admire their engineering, but dams are the most perilous threat to river systems worldwide. Consider the numbers: 47,655 large dams exist today in 140 countries; they're present on 60 percent of the world's major rivers; and the weight of their water is so immense, it's believed to have altered the speed of the earth's rotation. Dams also supply one-fifth of the world's electricity and help irrigate about one-sixth of the world's food, but the benefits come at a huge cost.

As many as 80 million people have been displaced by dams worldwide, including nearly two million Chinese living above the new Three Gorges Dam, on the Yangtze. Dams prevent fish migration, causing local extinctions; they alter river flows and water temperatures, killing plants and aquatic species; and they destroy habitats and recreational areas. (Paddlers, take note: Not even 2 percent of U.S. rivers today are free-flowing.) Big dams are also a wasteful way to store water—huge reservoirs suffer so much evaporation that they can lose up to 10 percent of their volume each year.

SOLUTION: World leaders and enlightened governments are putting proposed dams—and even existing ones—to the test: Are they worth it? While small hydro projects can be a green boon, since they provide clean, renewable power, most large dams are not. "In many cases, it's clear that the negative impacts of dams outweigh the benefits," says Patrick McCully, director of the International Rivers Network and member of the UN's Dams and Development Project. That new vision is especially noticeable in the U.S., where experts say the era of big-dam building is over. More than 212 dams have been torn down in recent years, with glowing results: When the Edwards Dam came down on Maine's Kennebec River in 1999, schools of migratory alewives returned by the hundreds of thousands. Where knockdowns are not an option, dams can often be better managed—for example, dam operators can release more water during dry seasons, to help fish and downstream wildlife. The Nature Conservancy has partnered with the U.S. Army Corps of Engineers to tweak water releases through 27 dams in nine river basins so far, with more agreements on the way.

6. Lost Habitats
STATUS: As humans pave paradise, expand cities and farms, and suck up more water—and pollute what's left—we leave individual species and entire ecosystems at risk. Topping the endangered list: freshwater ponds, lakes, streams, rivers, and marshes. Already, about half the world's mangrove ecosystems (critical for spawning fish, sheltering birds, and recycling nutrients) and half of its freshwater wetlands—home today to an estimated 40 percent of the earth's species—have been lost to development. Some 75 percent of the globe's fish stocks have been nearly or totally depleted, and more than 20 percent of freshwater fish species have become extinct, endangered, or threatened in recent decades.

Statistics aside, nearly every terrestrial species is in some way dependent on freshwater ecosystems for their survival—humans included. Wetlands and bogs filter and cleanse water, regulate water flows, absorb storm surges, and protect us from hurricanes. These natural services, provided free of charge, can offer more of an economic boon to communities than developing the wetlands would. "We need to recognize that freshwater ecosystems have economic value, and we need to set some boundaries," says Sandra Postel, director of the Global Water Policy Project.

SOLUTION: Rivers and wet-lands can be forgiving: Many will bounce back if you simply keep enough water flowing in them. And U.S. courts are increasingly ruling that "in-stream flows"—water that in the past was often sucked out of rivers for irrigation or urban use—must be protected for the health of fish and other fauna. Conservation groups and citizens' coalitions are also working to take back aquatic zones. Ducks Unlimited is helping preserve wet-lands on two million acres, stretching from Canada and Montana to Minnesota and lowa, by plugging old drainage ditches and letting ponds and marshes refill. In Brazil's Pantanal, the earth's largest wet-land, activists are successfully fighting a massiveshipping waterway. And worldwide, members of the Waterkeeper Alliance, a grassroots group with more than 150 chapters, use patrol boats to monitor polluters, then take the worst of them to court.