One of the things we have learned from the international search mission for Malaysia Airlines Flight 370 is that the South China Sea is very, very polluted.

The disappearance is, above all else, a human tragedy for the passengers and crew and their families, and the state of the sea should in no way distract from what happened to them. Nonetheless, the ecological tragedy will have a profound impact on the South China Sea - and the more than 1 billion people living in its coastal areas - in coming decades.

Hours after the first news of the plane's disappearance, the Vietnamese navy reported finding six-mile and nine-mile oil slicks (reports about the size vary), raising hopes. But lab tests revealed that they were diesel fuel characteristic of the ships that ply, and pollute, the South China Sea. Subsequently, fishermen and rescue workers found life rafts, life jackets, a jet's door and plastic oil barrels each initially suspected as originating from Flight 370, vetted in the news media, and then - perhaps literally - tossed overboard as trash.

Where's it all coming from? Data on the pollution in the South China Sea region is scarce, due to the large number of countries involved and the illicit nature of pollution. What data exists isn't reassuring. In 2012, the Chinese government reported that 72 Chinese rivers dumped 17 million tons of pollution into Chinese seas (counting the South China Sea, which China very much claims as its own), including 93,000 tons of oil. That may sound like a lot of oil, but a 2012 study by a consortium of Chinese and European researchers used satellite imagery to determine that land-based discharges and runoff accounted for only 36 percent of the oil in China's seas.

At 45 percent, the bigger contribution was from transportation. This isn't much of a surprise. According to the U.S. Energy Information Administration, roughly one-third of all seaborne oil and more than half of the global natural gas trade passed through the South China Sea. Those tankers weave among some of the heaviest container traffic in the world, and all of those ships carry oil that leaks by accident or on purpose, leaving miles-long slicks that can be seen on satellite images but are mostly ignored except when somebody - such as a search and rescue team - is looking for something.

Professor Chou Loke Ming of the National University of Singapore's Department of Biology has spent much of his career looking at pollution in Asia seas. I called him to ask about the pollution picked up in the course of the Flight 370 search.

"It's not surprising," he told me. "Trash, it's all coming down from land and washing down to the sea. It doesn't get washed away. That's why you have a garbage patch in the Pacific Ocean," he added, referring to the notorious vortex of garbage that swirls with the north Pacific's circular currents. Researchers there have collected as many as 1.9 million plastic fragments per mile; no such studies have been conducted in the South China Sea. The results of large-scale pollution, however, are already evident in the region: degraded habitats, including devastated coral reefs; collapsed fisheries; and negatively impacted tourist sites.

The situation doesn't look likely to improve. "I think it will overall get worse simply because of the sheer scale of the development that's going on in the region," Chou said. "Although there is more awareness about reducing pollution loading, and also some better management being undertaken, the volume is just too great."

In his view, the only hope to reduce the ecological threat is a cooperative effort by the South China Sea's highly competitive coastal neighbors. The unprecedented international effort to find Flight 370 suggests that such an effort is more likely than it might have seemed. But until somebody volunteers to clean up the oil slicks found just last week, there's not much optimism for the South China Sea.

Adam Minter is a regular contributor to Bloomberg View based in Shanghai and the author of "Junkyard Planet," a book on the global recycling industry.