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Global Warming, Oxygen Depletion & Dead Zones

A dead zone, or a hypoxic zone, is an area of water with a significantly reduced level of oxygen. Often a dead zone will only be dead during warmer parts of the year, but when in this depleted state they cannot support fish or other marine life. Dead zones can occur naturally, but most are caused by human activities. The use of nitrogen-rich fertilizer is a particularly significant cause. It is thought that global warming will only exacerbate the problem of dead zones, and dead zones are likewise expected to have a negative impact on climate change.

Fertilizer Run-off
- A primary cause of dead zones is run-off from nitrogen-based fertilizers used in industrial agriculture. When nitrogen runs into water it creates a prime environment for microscopic organisms called phytoplankton to flourish. When the nitrogen is used up by the phytoplankton, they sink and are decomposed by bacteria. As the bacteria do this, they consume the oxygen present in the water. Eventually, the oxygen is depleted to the point that the area cannot sustain life and is classified as a dead zone.
Proliferation of Dead Zones
- The number of reported dead zones has increased gradually over the last 100 years, but a sharper increase occurred after the 1970s. Between 1961 and 1970, around 30 dead zones were reported, but this increased to around 90 in the 1980s, 180 in the 1990s, and 415 by 2008. Most of these are near high-population areas or agricultural sites, though some are natural such as in the Bay of Bengal.
Global Warming Affects Dead Zones
- Global warming is likely to exacerbate dead zone proliferation. Hypoxia is more likely at higher temperatures, particularly in the coastal areas that are most susceptible to fertilizer run-off. In areas where global warming increases precipitation, fertilizer run-off will increase, while higher sea levels will increase bottom water volume creating a greater capacity for the decomposing of organic matter. Some global warming effects will actually help prevent hypoxia, such as stronger summer winds, but overall an increase in dead zones is predicted.
Dead Zones Affect Global Warming
- The proliferation of dead zones can also have an impact on global warming. Nitrous oxide is a greenhouse gas and a contributor to ozone destruction. As oxygen in marine areas decreases, microbes produce higher levels of nitrous oxide. Some of this is released from the water, increasing atmospheric concentrations, particularly in shallower waters where the increased sunlight helps the phytoplankton to grow. Dead zones are common in shallower coastal areas.

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