The spreading zones have doubled over the last decade and pose as big a threat to fish stocks as overfishing, UNEP said its Global Environment Outlook Year Book 2003, released at the opening of the agency's 8th summit for the world's environment ministers.
The new findings tally nearly 150 dead zones around the globe, double the number in 1990, with some stretching 27,000 square miles, about the size of Ireland.
Dead zones have long afflicted the Gulf of Mexico and Chesapeake Bay, but are now spreading to other bodies of water, such as the Baltic Sea, Black Sea, Adriatic Sea, Gulf of Thailand and Yellow Sea, as other regions develop, UNEP said.
They are also appearing off South America, Japan, Australia and New Zealand.
The main cause is excess nitrogen run-off from farm fertilizers, sewage and industrial pollutants. The nitrogen triggers blooms of microscopic algae known as phytoplankton. As the algae die and rot, they consume oxygen, thereby suffocating everything from clams and lobsters to oysters and fish.
"Human kind is engaged in a gigantic, global, experiment as a result of inefficient and often overuse of fertilizers, the discharge of untreated sewage and the ever rising emissions from vehicles and factories," UNEP Executive Director Klaus Toepfer said in a statement. "Unless urgent action is taken to tackle the sources of the problem, it is likely to escalate rapidly."
Dead zones are especially dangerous to fisheries because they afflict coastal waters where many fish spawn and spend most of their lives before moving to deeper water, said UNEP senior environmental affairs officer Marion Cheatle.
"It hasn't been something well known by policy makers," Cheatle said. "But it's been getting noticeably worse."
UNEP urged nations to cooperate in reducing the amount of nitrogen discharged into their coastal waters, in part by cutting back on fertilizer use or planting more forests and grasslands along feeder rivers to soak up the excess nitrogen.
The announcement comes as environment ministers from more than 150 nations gathered on the South Korean resort island of Jeju at UNEP's 8th Special Session of the Governing Council/Global Ministerial Environment Forum.
Increasingly frequent dust and sand storms and impending global water shortages will also headline the three-day summit.
UNEP warns that without concerted effort to improve access to safe drinking water, a third of the world's population is likely to suffer chronic water shortages within a few decades. About 1.1 billion people lacked access to safe drinking water in 2000, while another 2.4 billion lacked access to basic sanitation, UNEP said.
The growing frequency of dust and sand storms is another concern, especially storms caused by land degradation and desertification in Mongolia and northern China.
Scientists have recently linked similar storms, originating in the Sahara, with damage to coral reefs in the Caribbean, UNEP said.
Discussions in Jeju will form a basis for deliberations at the 12th meeting of the U.N. Commission on Sustainable Development to be held in New York next month.
That meeting will assess progress toward United Nations targets of halving the proportion of people with no access to safe drinking water or basic sanitation by 2015.
Those goals were set by the United Nations Millennium Declaration in 2000 and the at the World Summit on Sustainable Development two years later.
The current forum will try to generate a Jeju Initiative that will identify concrete measures to be taken to reach those goals, UNEP spokesman Nick Nuttall said.