msnbc.com staff and news service reports
updated 3/29/2004 1:12:51 PM ET 2004-03-29T18:12:51

The number of oxygen-deprived "dead zones" in the world's oceans has been increasing since the 1970s and is now nearly 150, threatening fisheries as well as humans who depend on fish, the U.N. Environment Program announced Monday in unveiling its first-ever Global Environment Outlook Year Book.

These "dead zones" are caused by an excess of nitrogen from farm fertilizers, sewage and emissions from vehicles and factories. In what experts call a “nitrogen cascade,” the chemical flows untreated into oceans and triggers the proliferation of plankton, which in turn depletes oxygen in the water.

While fish might flee this suffocation, slow moving, bottom-dwelling creatures like clams, lobsters and oysters are less able to escape.

“Humankind is engaged in a gigantic, global experiment as a result of the inefficient and often overuse of fertilizers, the discharge of untreated sewage and rising emissions from vehicles and factories,” program executive director Klaus Toepfer said in a statement accompanying the report.

From small to vast zones
Toepfer noted that 146 dead zones — most in Europe and the U.S. East Coast — range from under a square mile to up to 45,000 square miles. "Unless urgent action is taken to tackle the sources of the problem," he said, "it is likely to escalate rapidly."

Compare your thoughts with U.S. surveyThe program noted that some of the earliest recorded dead zones were in Chesapeake Bay, the Baltic Sea, Scandinavia's Kattegat Strait, the Black Sea and the northern Adriatic Sea.

The most infamous zone is in the Gulf of Mexico, where the Mississippi River dumps fertilizer runoff from the Midwest.

Others have appeared off South America, China, Japan, southeast Australia and New Zealand, the program said.

Preventive measures
The report was released in Jeju, South Korea, where governments from around the world are sending officials this week for a Global Ministerial Environment Forum.

The program noted preventive steps can be taken, citing these examples:

  • European nations along the Rhine agreed to halve discharged nitrogen levels, reducing the discharge into the North Sea.
  • Planting new forests and grasslands will help soak up excess nitrogen, keeping it out of waterways.
  • Requiring vehicles to reduce nitrogen emissions.
  • Fostering alternative energy sources that are not based on burning fossil fuels.
  • Better sewage treatment would reduce nutrient discharges to coastal waters.

Global warming warning
Guide lists seafood by abundance and scarcityBut the report also noted new research that indicates global warming could aggravate the problem. Should humans double emissions of carbon dioxide, a key gas that many scientists fear is warming the Earth, that could change rainfall patterns, according to the research.

"In some areas, this in turn could lead to a marked increase in the levels of run-off from rivers into the seas," the U.N. program said. "They calculate that dissolved oxygen levels in the northern Gulf of Mexico, triggered by an increased discharge from the Mississippi River basin of 20 percent and a climb in temperature of up to four degrees Centigrade, could fall by 30 to 60 percent."

The U.N. report is online at www.unep.org/geo/yearbook.

© 2013 msnbc.com

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