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Fertilizers washed into ocean cause increase in potent greenhouse gas: report |
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Written by Bob Weber, THE CANADIAN PRESS
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Thursday, 15 May 2008 |
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Agricultural fertilizers washed into the ocean are causing an eightfold increase in emissions of one of the worst greenhouse gases, according to new research published Thursday in the journal Science.
"All of these things come together to weave a tapestry of concern," said Tom Pedersen of the University of Victoria
Pedersen and his fellow researchers were examining the effect of increased nitrogen released from land into the ocean. Those increases, mostly due to the vastly expanded use of nitrogen-based fertilizer around the world, have been huge.
In 1860, about 20 million tonnes of nitrogen were released into the oceans. About 30 per cent of that was due to human activity. By 2000, those releases had more than tripled to 67 million tonnes - 80 per cent from people.
Scientists have long been aware of the element's role in creating "dead zones" in oceans. The excess nitrogen fertilizes plankton in the water to the point where all the oxygen is used up as the plankton bloom decomposes.
The dead zone in the Gulf of Mexico now covers more than 13,000 square kilometres during its summer peak. The United Nations has counted 150 such zones of varying size around the world. More are growing.
"There's so much plant material raining down from the surface of the ocean it's like a torrential plant rainfall, consuming all the oxygen," said Pedersen.
But the decomposition doesn't stop once all the oxygen is gone, Pedersen's team found. Microbes in the water use chemicals called nitrates to break down the plankton.
That process releases nitrous oxide.
"Nitrous oxide is an extremely powerful greenhouse gas, and its concentration in the Earth's atmosphere is dramatically increasing," said Pedersen.
"Part of it is coming from soils on the continents...and another part of it is happening from the growth of these dead zones in the ocean."
Nitrous oxide absorbs 270 times more heat per molecule than carbon dioxide.
The paper estimates that the amount of nitrous oxide released from oceans due to human activity has increased eightfold since 1860 to about 1.6 million tonnes a year. Pedersen estimates a similar amount of the gas is released from land.
"There's no doubt the ocean is increasing its delivery of nitrous oxide into the atmosphere. We just don't know how much of the total is coming from (dead zones)."
Environment Canada estimates the atmospheric concentration of nitrous oxide has grown by about 15 per cent since the mid-1700s.
Pedersen said carbon dioxide is still by far the most important greenhouse gas, but concerns over nitrous oxide are increasing.
"It's contributing a small portion of the overall greenhouse gas forcing (climate change)," he said. "It's a small piece of the thermostat, but it's nevertheless growing.
"Every piece is important these days."
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Last Updated ( Wednesday, 08 October 2008 )
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