Photo: Plastic and birds are seen on a Laysan beach.

Plastic litter washes up on a remote beach in Laysan, one of the northwestern Hawaiian Islands.

Photograph by Jon Brack

Brian Handwerk

For National Geographic News

More than half the people on Earth live within 120 miles (193 kilometers) of the ocean, but even those who live nowhere near the sea are dependent on the massive saltwater ecosystem that covers nearly three-fourths of our planet.

The ocean helps create and regulate weather around the globe and produces many of life's essentials, including water, food, and even the oxygen we breathe every day. But scientists warn that the sea is changing rapidly and that our many uses of its bounty must be managed far more sustainably.

"If I were speaking to all the leaders at Rio+20 I'd say this is not [a choice between] the ocean or ourselves," said marine ecologist Enric Sala, a National Geographic explorer-in-residence. "A healthy ocean means healthier humans, more food on our tables, more jobs, and a healthier economy."

The sea's biodiversity is astounding. Scientists have identified some 200,000 marine species but suspect that millions more actually call the oceans home. Some seafloor ecosystems are so remote and inaccessible that we know less about them than we do the surface of Mars.

Yet the ocean is very familiar to more than three billion people whose livelihoods depend directly on coastal and marine ecosystems—about 8 percent of the world's people are fishermen. And three billion people count on marine species as their primary sources of protein.

Until recent decades, most people considered the oceans to be an inexhaustible resource so vast and so plentiful that it was beyond humanity's ability to deplete it of fish or seriously pollute its waters. Today we know that is far from the truth.

A Raft of Challenges

Since the mid-20th century, advances in technology have given rise to massive industrial fishing operations that can rapidly empty waters of species like bluefin tuna or Atlantic cod while satisfying an ever-increasing demand for seafood. UN-tracked fisheries have shown steady declines in catches since 1988—even as more fishers take to the water with ever more efficient gear. Some studies estimate that populations of large ocean fish are only 10 percent as big as their preindustrial levels.

Today's ocean managers are challenged to cooperate internationally and use scientific knowledge of fish stocks to replace loosely regulated fisheries with well-managed, sustainable resources. This can be accomplished by implementing tools such as marine reserves, protected areas, and strict catch limits.

Aquaculture can also play a major part—in fact, farmed fish already constitute half of the world's supply—but it must be done more sustainably. Aquaculture must consume fewer marine resources, like the ground-up seafood used to make fish feed, and it needs to be managed to reduce genetic dilution of wild stocks, destruction of mangroves, and other impacts on sensitive coastal areas.

Other ocean impacts have inland sources. "Most people don't know that every eight months the drops of oil that leak from U.S. cars and other machines and run downstream into the ocean equal the amount of oil [11 million gallons] that was spilled by the Exxon Valdez," Sala said.

Other runoff pollution, like nitrogen-rich fertilizers, has created oxygen-poor, algae-choked "dead zones," including the New Jersey-size swath found each summer in the Mississippi River Delta. Carelessly discarded trash has formed a massive "garbage patch" the size of Texas swirling in the northern Pacific. Pesticides washed into ocean waters may be consumed by small organisms and retained by larger predators, with unknown consequences to species up and down the food chain.

"We know how to fix these problems," Sala said. "Fishing in most cases just requires national action to restore populations to sustainable levels. We need to implement science-based quotas and reduce fishing capacity—there are just too many fishing boats in the world right now—so we can reach a point where we have a globally sustainable catch. Right now we're above that for too many species."

Other problems present bigger challenges and will require even greater concerted action around the world.

The Climate Connection

Ocean water and air share an enormous interface, stretching around the globe, and the two constantly interact. The sea absorbs some 30 percent of all the world's CO2 emissions, which helps mitigate the impacts of climate change caused by greenhouse gases. But the absorption of all that CO2 is changing water chemistry, creating acidic seawater and altering marine ecosystems at their core through base-of-the-food-chain animals such as plankton and corals. These shifts are happening so quickly that some species may not have time to adapt.

Scientists say that oceans are sensitive to even small changes in temperature—and Earth's temperature continues to trend upward. Sea temperatures over the past century have risen only about 0.18ºF (0.1ºC), and most of that occurred between the sunny surface and depths of about 2,300 feet (700 meters).

But some impacts may be evident. Too-warm waters can push coral reefs, also stressed by acidic water and pollution, toward die-off, and threaten the many species that dwell in these "rainforests of the sea." One in five coral reefs is already damaged beyond repair. And krill, the foundation of the Antarctic ecosystem, can't reproduce as efficiently in warming water.

The most visibly obvious result of warming water is already under way—rising sea levels that swamp coastline habitats and human dwellings alike. Due to expanding water volume and melting ice, the global mean sea level has risen about 0.12 inches (3 millimeters) a year during the past two decades, according to the IPCC, which is about twice the rise rate during the previous 80 years.

Though beset with challenges, the sea is resilient and has shown the ability to regenerate resources—if human beings give it a chance. The Rio+20 conference will seek to establish guidelines for sustainable management of the oceans and conservation of their priceless resources through a "blue economy" plan for the future.

Sala said progress has been stalled by what he calls an "artificial dichotomy" between economic development and environmental conservation. That view must be dispelled, he said. "In the long term there is no prosperity without sustainable use of natural resources," Sala said. "A blue economy is a smarter economy than the current one of overexploiting one resource and then simply going on to the next."

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