Unchecked emission of carbon dioxide into the atmosphere would lead to a tenfold expansion of low-oxygen areas in the global ocean that will remain for thousands of years to come, adversely affecting fisheries and ocean ecosystems far into the future.
Mississippi Dead Zone image courtesy NASA
This prediction is made by Danish scientists in a paper “Long-term ocean oxygen depletion in response to carbon dioxide emissions from fossil fuels,” published online today in the journal Nature Geoscience.
Also known as “dead zones,” low-oxygen areas in the ocean are where fish, crabs and clams are not able to live. In shallow coastal regions, these zones can be caused by runoff of human waste or excess fertilizers from farming.
Oxygen-starved areas in bays and coastal waters have been expanding since the 1960s, according to the United Nations Environment Programme (see sidebar). There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, National Geographic News reported in August last year.
Gary Shaffer, of the Niels Bohr Institute, University of Copenhagen, who is the leader of the research team at the Danish Center for Earth System Science (DCESS), says in a news release about the paper published in Nature Geoscience today that expansion of low-oxygen zones “would lead to increased frequency and severity of fish and shellfish mortality events, for example off the west coasts of the continents, like off Oregon and Chile.”
Working with senior scientists Steffen Olsen, oceanographer at the Danish Meteorological Institute, and Jens Olaf Pepke Pedersen, physicist at National Space Institute, Technical University of Denmark, Shaffer used a computer model to make projections 100,000 years into the future.
“If, as in many climate model simulations, the overturning circulation of the ocean would greatly weaken in response to global warming, these oxygen-minimum zones would expand much more still and invade the deep ocean,” Shaffer said.
“Extreme events of ocean oxygen depletion leading to anoxia [absence of oxygen] are thought to be prime candidates for explaining some of the large extinction events in Earth history, including the largest such event at the end of the Permian 250 million years ago,” he said.
As oxygen-deprived zones expand, essential nutrients are stripped from the ocean by the process of denitrification, Shaffer added. “This in turn would shift biological production in the lighted surface layers of the ocean toward plankton species that are able to fix free dissolved nitrogen.
“This would then lead to large, unpredictable changes in ocean ecosystem structure and productivity, on top of other large unpredictable changes to be expected from ocean acidification, the other great oceanic consequence of high atmospheric carbon dioxide concentrations from fossil fuel burning.”
As a result, Shaffer said, “the future of the ocean as a large food reserve would be more uncertain.”
Reduced fossil fuel emissions are needed over the next few generations to limit ongoing ocean oxygen depletion and acidification and their long-term adverse effects, he added.
News story about this research:
Study predicts ocean ‘dead zones’ (Sydney Morning Herald, Australia)
NatGeo News Watch:
Mass Extinctions Loom in Ocean Habitats, Scientist Warns
Vast Tracts of the Pacific Saved for Conservation
Coral Triangle Rescue Plan Gathers Momentum
National Geographic News:
“Dead Zones” Multiplying Fast, Coastal Water Study Says
Gulf of Mexico “Dead Zone” Is Size of New Jersey
“Dead Zone” off Oregon Coast Is Growing, Study Says
Seafood May Be Gone by 2048, Study Says