Climate change will cause the number and size of ocean dead zones to increase across the globe, according to a study published in the journal Global Change Biology.
Commonly found in isolated estuaries and along coastlines, the aptly-named dead zones are areas where the bottom layer of water is extremely low in oxygen. These hypoxic pockets of ocean are uninhabitable to marine life and suffocating to any creature that ventures into it.
Dead zones can form from natural process, though more often they are the result of nitrogen- and phosphorus-rich runoff entering the ocean from nearby farms or sewage systems. The nutrients provide a feast for single-celled algae, which develop into massive – and potentially toxic – algal blooms, which then die and sink to the bottom of the water column. The decomposition of so much algae consumes the dissolved oxygen in the area, turning it into what the National Oceanic and Atmospheric Administration describes as a “biological desert.”
Dead zones primarily occur in shallow areas near the shore, close to where the runoff meets the ocean. Here, the temperature of the water is more closely linked to the temperature of the air. In their study of over 400 dead zones (265 of which are located in the United States), researchers Andrew Altieri of the Smithsonian Tropical Research Institute in Panama and Keryn Gedan of Smithsonian Environmental Research Center predict that rising temperatures will exacerbate existing zones and lead to the formation of new ones.
This is due to the nature of oxygenated water. “Not only does warmer water hold less dissolved oxygen,” NOAA explains, “but it sits at the top of the water column like a balloon that is hard to submerge. Buoyant and stable, the oxygenated surface water resists being mixed to the bottom, where dead zones generally occur.”
NOAA reports that the formation of dead zones throughout the world has doubled since the mid-20th century. In the U.S. they are especially prevalent along the east coast, though they appear throughout the southwest, with the second largest dead zone in the world located in the northern Gulf of Mexico.
The UN’s Intergovernmental Panel on Climate Change has stated that global warming must remain under 3.6 °F (2°C) to prevent the worst impacts of climate change. Using a “middle-of-the-road” greenhouse gas emissions scenario, Altieri and Gedan project temperatures rising 4.1°F (2.3°C) above the 1980-1999 average by the year 2099.
In the map above, the white dots are zones where air temperature is projected to increase by less than 4.1°F. The black dots (the remaining 65 percent) are zones where median warming is projected to increase by at least 4.1°F by the end of the century.
“Climate change will drive expansion of dead zones, and has likely contributed to the observed spread of dead zones over recent decades,” write Altieri and Gedan.
Dead zone formation and spread will also be affected by sea level rise, ocean acidification and changes in weather patterns, with more precipitation leading to more urban and agricultural runoff.