Plastic supermarket bags may be headed for extinction, with the head of the United Nations Environment Program calling this week for a worldwide ban.

“Thin-film single-use plastic bags, which choke marine life, should be banned or phased out rapidly everywhere,” Achim Steiner, UN under-secretary-general and UNEP executive director, said. “There is simply zero justification for manufacturing them anymore, anywhere.

In the United States, an estimated 90 billion thin plastic bags a year wind up as unrecycled trash and litter, even though voluntary recycling of the bags has increased.

Plastic bags are a big part of “a growing tide of marine litter” that is “harming oceans and beaches worldwide,” says a new report by the UN agency and the Ocean Conservancy. The report, the first-ever attempt to take stock of the marine litter situation in the 12 major regional seas around the world, was issued by the two groups on World Oceans Day, June 8.

Plastics and cigarettes top the ‘Top 10’ of marine debris, the UN agency says.

Plastic — especially plastic bags and bottles — is the most pervasive type of marine litter around the world, accounting for over 80 percent of all rubbish collected in several of the regional seas assessed in the report.

Plastic debris is accumulating worldwide. It slowly breaks down into tinier and tinier pieces that can be consumed by the smallest marine life at the base of our food chain. Plastics collect toxic compounds that then can get into the bodies of the organisms that eat the plastic — and ultimately that winds up in you and me.

According to the Ocean Conservancy, plastic bags were the second most common form of coastal litter after cigarette butts in the 2008 International Coastal Cleanup Day.

‘Marine litter is symptomatic of a wider malaise: namely the wasteful use and persistent poor management of natural resources,” Steiner said. “The plastic bags, bottles and other debris piling up in the oceans and seas could be dramatically reduced by improved waste reduction, waste management and recycling initiatives.’

China has already taken a lead on this.

A year ago, on June 1, 2008, China prohibited supermarkets, shops and open markets from supplying free plastic bags to customers, in a drive to protect environment and cut waste. Sellers giving out thin bags can be fined up to $1,464.

Since the ban, China’s National Development and Reform Commission says the number of plastic bags used in supermarkets has dropped by 40 billion, or 66 percent, Xinhua news agency reports. The commission says Wal-Mart’s 106 stores in China reduced plastic bag usage by 80 percent in the past year.

Xie Zhenhua, NDRC deputy director, said plastic bags take about 200 years to decompose and greatly contaminate soil and water sources. If they end up in rivers and seas, they may lead to the deaths of fish, animals and plants, he said.

Likewise, Ireland has cut single-use plastic bag consumption 90 percent by levying a fee on each bag that consumers use.

The United States is lagging on the issue. San Francisco has banned plastic bags and Los Angeles will do so in 2010. The Washington, D.C., City Council will vote on a 5-cent-a-bag tax later this month. But similar measures have failed in New York and Philadelphia.

The new report’s findings indicate that alarming quantities of rubbish thrown out to sea continue to endanger people’s safety and health, entrap wildlife, damage nautical equipment and deface coastal areas around the world.

‘This report is a reminder that carelessness and indifference is proving deadly for our oceans and its inhabitants,’ said Philippe Cousteau, CEO of EarthEcho International and an Ocean Conservancy board member. ‘The time for action is now, and true change will require taking a bold and courageous stand.”

Shi Pengxiang, a project manager of Greenpeace China, told Xinhua it’s hard work to persuade 1.3 billion people to give up plastic bags.

‘But more important, we have made a start,’ said Shi.