Also known as laughing gas, nitrous oxide seems less funny since scientists branded it as the third most important greenhouse gas emitted through human activities.
 | A woman sprays fertilizer on her field in Nakornsawan, Thailand. Fertilizers are the single most important source for man-made nitrous oxide emissions (Photo: Reuters) |
Contribution to Human-Induced Climate Change: 7 percent
Global Warming Potential (100 years): 298 times stronger than CO2
Like most other greenhouse gases, nitrous oxide is neither toxic nor destructive but a fundamental part of the mechanisms that keep our planet healthy and green. Produced by digesting bacteria, nitrous oxides are part of the nitrogen cycle, one of the most important chemical reactions on Earth.
Nitrogen is the chemical basis for proteins and DNA; plants need it for photosynthesis and growth. While the gas is the most abundant element of our atmosphere, it cannot be used. Higher organisms have to rely on tiny bacteria to turn it into ammonia or nitrates. Once a plant dies, other bacteria feed on the leftovers and turn nitrates back into gases like nitrous oxide or nitrogen.
This benevolent cycle went on for millions of years until chemists and farmers realized that nitrogen fertilizers greatly increase crop yields. Since then, more and more nitrogen has been added to the cycle. Farmers around the globe use more than 70 million tons of nitrogen fertilizers annually. According to a study conducted by Nobel prize-winning chemist Paul Crutzen in 2007, some 3 to 5 percent of this nitrogen is converted directly into nitrous oxide, twice the amount previously thought.
Fertilizer application will increase with a growing world population. Over the next three decades food production will need to increase by about 60 percent, estimates the UN’s Food and Agricultural Organization. In the same period, nitrous oxide emissions could double, especially in developing countries.
 | Animation (click on the image to enlarge)Greenhouse gases will be around for a long time. Find out more about our global warming legacy (Animation: Allianz) |
Developing more effective ways of adding nitrogen to the soil will be a key challenge. Today, fertilizers are often washed away by rain into lakes and seas where algae feed on them, bloom uncontrollably, and starve the water of oxygen. Every summer, nitrogen-rich river flow from the Mississippi River creates a “dead zone” in the Gulf of Mexico the size of Massachusetts.
Higher emissions from nitrogen fertilizers also shed new light on the carbon footprint of biofuels. Stanford University ecologist Peter Vitousek believes that “there’s a great danger in over-fertilizing a cornfield to boost biofuel production, where the carbon benefits are far outweighed by the nitrogen damage.”
If Paul Crutzen’s calculations are correct, rapeseed biodiesel, which accounts for about 80 percent of the biofuel production in Europe, would have a N2O-induced warming effect 1 to 1.7 times larger than its cooling effect due to saved CO2 emissions. For corn bioethanol, dominant in the US, the warming effect would be 0.9 to 1.5 times larger.
Mankind’s binge on fertilizers and fossil fuels, another source of the gas, has increased nitrous oxide levels in the atmosphere by about 18 percent since 1750. Fortunately, nitrous oxide appears only in scarce quantities; it is about a thousand times rarer than carbon dioxide. But its long atmospheric lifetime of about 150 years and its strong warming potential, more than 300 times stronger than CO2, make it an effective agent of global warming.
But the picture is not all bleak. Emissions from transportation, once a major source of nitrous oxide, have decreased recently due to the use of catalytic converters in modern cars. Reducing nitrous oxide emissions is also cheaper than reducing CO2 or methane emissions, according to the U.S. Environmental Protection Agency. A study published by British farming associations calculates that more efficient use of nitrogen fertilizers could reduce nitrous oxide emissions by up to 20 percent.
Carbon Dioxide - Endless Warming
editor: Thilo Kunzemann
publishing date: November 27, 2008
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