Positive feedbacks

As the atmosphere warms, its capacity to hold water vapour (in the form of clouds and humidity) increases; water vapour is itself a greenhouse gas, and so the warmer the atmosphere gets, the greater its potential to continue doing so. This is one of several ‘positive feedback’ processes associated with climate change. Ice and snow reflect sunlight, so in areas where they are melting, solar energy is absorbed instead by the sea or land beneath, causing the earth to heat up further. Permafrost, the deep frozen soil of the arctic and subarctic regions which locks away carbon dioxide and methane in the form of long-dead vegetation, is also melting as the earth warms, releasing more and more of the greenhouse gases into the atmosphere. Rising sea temperatures are threatening to wipe out plankton, microscopic organisms which act as a vital carbon ‘sink’ by ingesting carbon dioxide. This would leave yet more of the greenhouse gas in the atmosphere. Some scientists also fear that rising sea temperatures could cause a sudden release of methane from clathrate compounds on ocean floors. These are vast deposits of solid water crystals containing methane which, if melted, could create a runaway greenhouse effect. Such an occurrence is believed to have caused the Permian-Triassic event, or the ‘mother of all mass extinctions’, around 250 million years ago.