The rest comes from ships. Much of it consists of synthetic floats and other gear that is jettisoned illegally to avoid the cost of proper disposal in port.
In addition, thousands of cargo containers fall overboard in stormy seas each year, spilling their contents. One ship heading from Los Angeles to Tacoma, Wash., disgorged 33,000 blue-and-white Nike basketball shoes in 2002. Other loads lost at sea include 34,000 hockey gloves and 29,000 yellow rubber ducks and other bathtub toys.
The debris can spin for decades in one of a dozen or more gigantic gyres around the globe, only to be spat out and carried by currents to distant lands. The U.N. Environment Program estimates that 46,000 pieces of plastic litter are floating on every square mile of the oceans. About 70% will eventually sink.
Albatross are by no means the only victims. An estimated 1 million seabirds choke or get tangled in plastic nets or other debris every year. About 100,000 seals, sea lions, whales, dolphins, other marine mammals and sea turtles suffer the same fate.
The amount of plastic in the oceans has risen sharply since the 1950s. Studies show a tenfold increase every decade in some places. Scientists expect the trend to continue, given the popularity of disposable plastic containers. The average American used 223 pounds of plastic in 2001. The plastics industry expects per-capita usage to increase to 326 pounds by the end of the decade.
The qualities that make plastics so useful are precisely what cause them to persist as trash.
Derived from petroleum, plastics eventually break down into carbon dioxide and water from exposure to heat and the sun's ultraviolet rays.
On land, the process can take decades, even centuries. At sea, it takes even longer, said Anthony L. Andrady, a polymer chemist at the Research Triangle Institute in North Carolina who studies marine debris. Seawater keeps plastics cool while algae, barnacles and other marine growth block ultraviolet rays.
"Every little piece of plastic manufactured in the past 50 years that made it into the ocean is still out there somewhere," Andrady said, "because there is no effective mechanism to break it down."
Oceanographers have counted on beachcombers around the world to help them plot the course of plastic flotsam as it circumnavigates the globe. Ebbesmeyer has found that some debris gets hung up for decades in gyres before being spun out into different currents, flung ashore or picked up by animals.
A piece of plastic found in an albatross stomach last year bore a serial number that was traced to a World War II seaplane shot down in 1944. Computer models re-creating the object's odyssey showed it spent a decade in a gyre known as the Western Garbage Patch, just south of Japan, and then drifted 6,000 miles to the Eastern Garbage Patch off the West Coast of the U.S., where it spun in circles for the next 50 years.
The Hawaiian archipelago, which stretches from the Big Island of Hawaii westward for 1,500 miles to Kure Atoll, acts like 19 unevenly spaced teeth of a giant comb, snagging debris drifting around the Pacific. Most of the archipelago's atolls are awash in plastic junk, as are some beaches on the main islands.
Native Hawaiians, seeking wood for dugout canoes, used to go to Kamilo Beach at the southernmost tip of the Big Island to collect enormous logs that had drifted from the Pacific Northwest. Now, locals like Noni Sanford pick through the debris for novelties to enter in a trash-art show in Hilo every fall.
Sanford, 58, a free-spirited great-grandmother with long gray hair pulled back in a ponytail, once won second place for a mobile fashioned out of fishing line, floats and a colorful palette of plastic toothbrushes.
As a lifelong beachcomber, she is fascinated and horrified by the transformation of Kamilo Beach since she first set foot there in 1959. She was searching for driftwood with her father, a sculptor.
She remembers seeing a few tires back then. Now, plastic debris litters the crescent-shaped beach for more than a mile.
"This is nothing," Sanford said, stepping over a pile of twisted lines and nets. "This used to be 8 and 10 feet high. Of course, that was three or four cleanups ago."