Mississippi Dead Zone, off U.S. coast. Reds and oranges represent low oxygen concentrations. (Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio)
Dead zones are low-oxygen areas in the ocean where higher life forms such as fish, crabs and clams are not able to live.
"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." Fish could vanish from huge stretches of the ocean for tens of thousands of years unless we drastically reduce our carbon emissions.
Dramatic Expansion Of Dead Zones In Oceans Likely With Unchecked Global Warming
ScienceDaily 25 Jan 09;
Unchecked global warming would leave ocean dwellers gasping for breath. Dead zones are low-oxygen areas in the ocean where higher life forms such as fish, crabs and clams are not able to live. In shallow coastal regions, these zones can be caused by runoff of excess fertilizers from farming. A team of Danish researchers have now shown that unchecked global warming would lead to a dramatic expansion of low-oxygen areas zones in the global ocean by a factor of 10 or more.
Whereas some coastal dead zones could be recovered by control of fertilizer usage, expanded low-oxygen areas caused by global warming will remain for thousands of years to come, adversely affecting fisheries and ocean ecosystems far into the future.
Professor 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), explains that "such expansion 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".
Large extinction events
Together with senior scientists Steffen Olsen oceanographer at Danish Meteorological Institute and Jens Olaf Pepke Pedersen, physicist at National Space Institute, Technical University of Denmark, Professor Shaffer has performed projections with the newly-developed DCESS Earth System Model, projections that extend 100,000 years into the future.
He adds that "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." Extreme events of ocean oxygen depletion leading to anoxia 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.
Series of changes
Furthermore, as suboxic zones expand, essential nutrients are stripped from the ocean by the process of denitrification. 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.
Professor Shaffer warns that as a result, "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".
Global warming could suffocate the sea
Andy Coghlan, New Scientist 25 Jan 09;
Fish could vanish from huge stretches of the ocean for tens of thousands of years unless we drastically reduce our carbon emissions.
Gary Shaffer of the University of Copenhagen, Denmark, and his colleagues used computer models to analyse the long-term impact of global warming on the oceans, looking up to 100,000 years into the future. This is important because less oxygen dissolves in warmer water, affecting the amount of life the oceans can support.
To estimate just how much oxygen will be lost, the team used two existing scenarios of future fossil fuel burning published by the Intergovernmental Panel on Climate Change: a worst case scenario in which we burn three quarters of the Earth's fossil fuel reserves over the coming century, and a mid-range scenario in which we burn only a quarter of reserves. In both cases it is assumed that burning then stops.
Under the worst-case scenario, average ocean oxygen levels will fall by up to 40%, and there will be a 20-fold expansion in the area of "dead zones", like those already discovered in the eastern Pacific and northern Indian Ocean, where there is too little oxygen for fish to survive. Even in the mid-range scenario, dead zones would expand by a factor of 3 or 4. Cold, deep waters will also be affected if warming stifles the currents that deliver oxygen to greater depths.
Shaffer's projections suggest that the oxygen content in surface layers will dip to its lowest levels during the 22nd century, and in deep water a thousand years later. Recovery to pre-industrial levels will be very slow: "Even after 100,000 years, oxygen levels will only have recovered by around 90%," he says.
Journal reference: Nature Geoscience, DOI: 10.1038/NGEO420