Globally, oceans are thought to add around 15 Tg CH4/year to the atmosphere, with parts of the surface of the world's oceans having relatively high concentrations of dissolved CH4. CH4 is not very soluble in water and so is often emitted as bubbles rising up from aquatic and oceanic sediments (like those seen by Volta), rather than via diffusion between the water and air. As in wetlands, oceanic CH4 is largely produced by methanogenic bacteria, which, because they need anaerobic conditions, are usually found to be producing CH4 either within sinking particles or in sediments.
Humans have an impact on oceanic CH4 emissions, primarily through the effect on oceanic nutrient inputs via rivers and estuaries. The high nutrient loads of many rivers, produced largely by sewage input and agricultural runoff, lead to eutrophic conditions in estuaries and coastal waters. Such nutrient-rich waters and sediments are ideal for methanogenesis, with oxygen levels in the water often very low and with plenty of organic carbon available that the methanogenic bacteria are able to utilize.
for big surges in atmospheric CH4 concentrations during the last 500 years. As such, CH4 hydrates represent a potentially huge positive feedback mechanism to global warming. With increasing ocean temperatures, large CH4 hydrate deposits may become unstable and thus lead to very rapid CH4 emissions.
The hard-to-predict, but potentially catastrophic, consequences of global warming on the deposits of CH4 hydrate underline the potential threat posed by feedbacks within the global climate system. Limiting future global warming may be crucial to prevent such a runaway scenario of CH4 release leading to even greater warming.
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