In 1980, after college, I joined the crew of a sailboat partway through a circumnavigation of the globe. Becalmed and roasting one day during a 21-day crossing of the western Indian Ocean, several of us dived over the side. Within a few swimming strokes, the bobbing hull seemed a toy over my shoulder as I glanced back through my diving mask. Below me, my shadow and the boat’s dwindled to the vanishing point in the two-mile-deep water. Human activity seemed nothing when set against the sea itself.
Just a few weeks later, on an uninhabited island in a remote part of the Red Sea, I was proved wrong. The shore above the tide line was covered with old light bulbs, apparently tossed from the endless parade of ships over the years.
Now scientists are building the first worldwide portrait of such dispersed human impacts on the oceans, revealing a planet-spanning mix of depleted resources, degraded ecosystems and disruptive biological blending as species are moved around the globe by accident and intent.
A paper in the Feb. 15 issue of the journal Science is the first effort to map 17 kinds of human ocean impacts like organic pollution, including agricultural runoff and sewage; damage from bottom-scraping trawls; and intensive traditional fishing along coral reefs.
About 40 percent of ocean areas are strongly affected, and just 4 percent pristine, according to the review. Polar seas are in the pristine category, but poised for change. Some human impacts are familiar, like damage to coral reefs and mangrove forests through direct actions like construction and subtler ones like the loss of certain fish that shape ecosystems.
Others were a surprise, said Benjamin S. Halpern, the lead author and a scientist at the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. He said continental shelves and slopes proved to be the most heavily affected areas, particularly along densely populated coasts.
The most widespread human fingerprint is a slow drop in the pH of surface waters around the world as a portion of the billions of tons of carbon dioxide added to the atmosphere from fuel and forest burning each year is absorbed in water, where it forms carbonic acid.
That progressive shift in ocean chemistry could eventually disrupt shell-forming plankton and reef-building species, particularly where other impacts, including rising temperatures from human-caused global warming, create simultaneous stresses, many marine biologists say.
“I study this stuff all the time and didn’t expect the impacts to be as pervasive as we found,” Dr. Halpern said.
The review provides a baseline necessary for tracking further shifts, he said. It also identifies some unanticipated trouble spots, similar to terrestrial biodiversity “hot spots” that environmental groups have identified over the years.
Such an analysis is long overdue, many marine biologists said in interviews. People’s conservation concerns have mainly focused on land, even though the seas cover two-thirds of the planet and are a vital source of food and pleasure.
Sylvia Earle, an oceanographer and National Geographic Society “explorer in residence,” said people care only about what they know. A big question now is whether such surveys are providing too little knowledge, too late.
“We learned more about the nature of the ocean in the latter part of the 20th century than during all preceding human history,” Dr. Earle said. “But we also lost more.”
A separate mapping effort published this month focused on introduced invasive species and found that 84 percent of the world’s coastal waters were affected, with Arctic waters next in line as shipping there grows in a warming world.
More than half the introduced species that take hold are having deleterious effects, said Jennifer Molnar, a conservation scientist at the Nature Conservancy who led that study, which was published in the journal Frontiers in Ecology and the Environment.
The House of Representatives is considering legislation aimed at tightening controls on the ballast water that stabilizes freighters when they are not full. Ballast water and organisms clinging to hulls and anchors have been the source of many costly marine invasions, including the introduction of zebra mussels to American waters and the comb jelly, a small jellyfish, to the Black Sea.
That species exploded after its accidental introduction in 1993, vacuuming up plankton until it made up 90 percent of the sea’s life by weight, causing fisheries to collapse. Its population there has since crashed, partly because of the arrival of a species of jellyfish that eats the established invader.
In May, invasive species will be a significant subject at the meeting of the world’s nations to assess the progress of the Convention on Biological Diversity.
Even as efforts to gauge human effects intensify, other scientists are simply trying to survey marine species large and small, an enormous task given how little is known about the oceans.
The hub for this work is the Census of Marine Life, a 10-year project initiated under the Alfred P. Sloan Foundation that is scheduled to produce a first synthesis report on marine species in 2010.
More than 2,000 scientists from 81 countries have chipped in, said Michael Feldman of the Consortium for Ocean Leadership, a group in Washington running the global project.
Since 2003, the project has discovered more than 5,300 species, Mr. Feldman said, adding: “We’ve only been able to formally describe a few hundred so far. They’re still discovering things at a rate we don’t even know what to do with.”
There is a growing sense of urgency among marine researchers in cataloging what is there, what is being threatened and what is already a fading memory.
Recent books, including “The Unnatural History of the Sea” by the marine scientist Callum Roberts, have painted vivid portraits of how much more abundant marine resources were a few generations ago, a situation well known to anyone who has worked in a fishery.
In the 1970s, I worked summers for the Rhode Island marine fisheries agency. At one point, I was tagging lobsters as part of an effort to find ways to revive depleted populations. A crusty old custodian in the laboratory, Jim Pimentel, reminisced about how different things had been a few decades earlier.
“We used lobsters for cod bait,” Mr. Pimentel said.
Looking ahead, Jane Lubchenco, a marine biologist at Oregon State University, said a wide array of efforts is required to sustain productive, if altered, seas. Among the needed steps, Dr. Lubchenco said, are expanding protected marine areas and curbing pollution, including carbon dioxide.
“We cannot go back in time to some past system,” Dr. Lubchenco said. “But we can protect and restore the functioning of today’s ecosystems so they can be as healthy, productive and resilient as possible.”