As oxygen-deprived waters increase, they emit more greenhouse gasses into atmosphere
By Bob Berwyn
SUMMIT COUNTY — Growing marine dead zones along the world’s coasts are intensifying global warming effects by releasing nitrous oxide into the atmosphere, University of Maryland researchers concluded in a study published in the March 12 edition of the journal Science.
The research shows how global warming can create a feedback loop that intensifies itself with impacts on various systems.
University of Maryland Center for Environmental Science oceanographer Dr. Lou Codispoti said low-oxygen waters can elevate concentrations of nitrous oxide in the atmosphere, exacerbating the impacts of global warming and contributing to ozone holes that cause an increase in harmful UV radiation.
“As the volume of hypoxic waters move towards the sea surface and expands along our coasts, their ability to produce the greenhouse gas nitrous oxide increases,” said Dr. Codispoti. “With low-oxygen waters currently producing about half of the ocean’s net nitrous oxide, we could see an additional significant atmospheric increase if these dead zones continue to expand.”
The dead zones themselves are caused by a variety of factors, including rising global temperatures and pollution from industry and agriculture.
Nitrous oxide is a highly potent greenhouse gas. It’s present in minute concentrations in Earth’s atmosphere, and is becoming a key factor in stratospheric ozone destruction.
For the past 400,000 years, changes in atmospheric levels of nitrous oxide appear to have roughly paralleled changes in carbon dioxide, and have had modest impacts on climate, but this may change.
Marine nitrous oxide production may also rise substantially as a result of nutrient pollution, warming waters and ocean acidification. Because the marine environment is a net producer of nitrous oxide, much of this production will be lost to the atmosphere, thus further intensifying its climatic impact.
In healthy, well-oxygenated oceans, microbes microbes produce nitrous oxide at low rates. But as oceans warm and grow more acidic and lifeless, they produce more of the greenhouse gas.
Under certain conditions, in shallow waters near shore, nitrous oxide production rates can be 10,000 times higher than the average for the open ocean. Those near-shore areas are the most prone to becoming dead zones.
The future of marine nitrous oxide production depends critically on what happens to about ten percent of the ocean volume that is hypoxic and suboxic, Dr. Codispoti said.