25 November 2020
Global warming could enlarge world’s largest dead zone
Published online 29 May 2019
Climate change is depleting a region in the Arabian Sea of its scarce oxygen supply limiting its replenishment by Arabian Gulf waters.
Climate modelling experts ran computer experiments to predict how the world’s largest oxygen minimum zone (OMZ), situated in the Arabian Sea, will react to future warming scenarios.1 First documented in the 1960s, the dimensions of the Arabian Sea’s dead zone, an area the size of Scotland, were only formally established a year ago by marine biologists who dispatched underwater robots.2
The most recent simulation, led by Zouhair Lachkar from New York University Abu Dhabi’s Center for Prototype Climate Modelling, showed that an additional warming of 2°C to 4°C is bound to intensify the OMZ. This will further reduce marine habitats for fish species intolerant to hypoxic conditions, strain commercially important fisheries, and accelerate the release of toxic heat-trapping gas into the atmosphere.
Naturally occurring oxygen minimum zones exist in three other locations worldwide. They result from an imbalance between oxygen supply and loss. In any water body, atmospheric oxygen is injected and mixed by waves and eddies, and consumed by bacteria feeding on sinking and decomposing organic matter. Because the Arabian Sea is one of the most productive marine ecosystems, it generates large amounts of organic matter that is decomposed by bacteria, tipping the fragile oxygen balance towards anoxia.
Lachkar and his colleagues are showing that climate change is further depleting the Arabian Sea of what little oxygen it has, by limiting its replenishment by the Arabian Gulf waters. When a fluid warms, it becomes lighter. With increased warming, the waters from the Arabian Gulf that typically sunk in the Arabian Sea and ventilated its OMZ are less prone to do so.
“The simulations where the Arabian Gulf experiences additional warming of 2°C to 4°C clearly show the intensification of the OMZ, as well as enhanced denitrification and nitrous oxide production,” says Lachkar. Denitrification refers to a situation where bacteria ‘breathe’ nitrate instead of oxygen and as a result produce nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide.
Bastien Queste, a research fellow at the University of East Anglia’s Centre for Ocean and Atmospheric Sciences, UK, who was not involved in the study, says the work shows this environment is very sensitive to human impacts on a climate scale, and is not a result of local mismanagement or pollution.
The intensification of the oxygen minimum zone, especially nearer the surface, has huge impacts for local communities, Queste says. “When the low oxygen boundary gets shallower, fish communities are squeezed into a thinner layer. This puts a lot of stress on fisheries.”
- Lachkar, Z. et al. Strong intensification of the Arabian Sea oxygen minimum zone in response to Arabian Gulf warming. Geophys. Res. Lett. https://doi.org/10.1029/2018GL081631 (2019).
- Queste, B. et al. Physical controls on oxygen distribution and denitrification potential in the north west Arabian Sea. Geophys. Res. Lett. http://dx.doi.org/10.1029/2017GL076666 (2018).