A large quantity of nitrogen compounds -- emitted into the atmosphere by humans through the burning of fossil fuels and the use of nitrogen fertilizers -- enters the oceans and may lead to the removal of some carbon dioxide from the atmosphere, concluded a team of international scientists led by Texas A&M University Distinguished Professor of Oceanography and Atmospheric Sciences Robert Duce.
The team of 30 experts from institutions around the world presented its conclusions in the current issue of the journal Science.
Human-caused atmospheric nitrogen compounds are carried by wind and deposited into the ocean, where they act as a fertilizer and lead to increased production of marine plant life. The increase in plant life causes more carbon dioxide to be drawn from the atmosphere into the ocean. This process results in the removal of about 10 percent of the human-caused carbon dioxide in the atmosphere, thus potentially reducing the climate warming potential, according to the team's paper.
However, some of the nitrogen deposited in the ocean is re-processed to form another nitrogen compound called nitrous oxide, which is then released back into the atmosphere from the ocean. Nitrous oxide is a powerful greenhouse gas itself -- about 300 times more powerful per molecule than carbon dioxide -- thus cancelling out about two-thirds of the apparent gain from the carbon dioxide removal, Duce explained. "But of course, the whole system is so complex that we're still rather unsure about what some of the other impacts might be within the ocean," he said.
In most areas of the ocean, nitrogen is the nutrient that limits the production of plant life, Duce said. So when all of the nitrogen in an area of the surface ocean is used up, no more plant life forms in that area. The team found that human-caused nitrogen deposits account for up to one-third of the external input of nitrogen into the ocean, and this increase in nitrogen available for the production of plant life causes more plants to form, Duce explained.
Oceanic plant life is produced from marine carbon (bicarbonate) in the ocean, and that amount of bicarbonate is in equilibrium with the carbon dioxide in the atmosphere. So when more bicarbonate is used up to produce marine plant life, it disrupts the equilibrium, and carbon dioxide is drawn down to the ocean from the atmosphere to restore the balance, Duce explained.
Thus, the human-caused nitrogen fertilization of the ocean removes some of the most important greenhouse gas -- carbon dioxide -- from the atmosphere, Duce said. This gain, however, is offset by the nitrogen compound, nitrous oxide, that also forms in the ocean due to the nitrogen fertilization and is re-emitted into the atmosphere as a powerful greenhouse gas, he added.
"If you don't consider the impact of human-caused nitrogen when trying to model climate change, you're missing a possibly significant part of the overall carbon cycle as well as the nitrogen cycle," Duce said. "So nitrogen deposition is potentially a very important factor in the climate change issue."
According to the team's calculations, about 54 million tons of nitrogen produced from human activities entered the ocean from the atmosphere in the year 2000. The team also found that the current nitrogen emissions are about 10 times what they were in 1860, Duce said. He added that the amount of nitrogen entering the atmosphere is expected to rise in the coming decades with the increase in demand for energy and fertilizers, and the team estimates that by the year 2030, human-caused nitrogen emissions will have risen to around 62 million tons per year.
"Clearly, there is much that we do not know about the extent and timescale of the impacts of this nitrogen deposition on the oceans and the subsequent feedbacks to the climate system," Duce said. "The implications are complex and interactive, and this is a very important issue that policy makers need to address and that scientists trying to model and understand the future of climate and climate change need to take into consideration."
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