Scientists know from the geological record that the Earth's climate can change rapidly. They have identified a number of potential tipping points, where relatively small amounts of global warming caused by human activities could cause large changes in climate. Some tipping points, like the losses to the Amazon forest, involve positive feedback loops and could lead to runaway climate change.

Arctic icecap: The white icecap is good at reflecting the Sun's warming light back into space but, when it melts, the dark ocean uncovered absorbs this heat. This leads to more melting.

Tundra: The high north is warming particularly fast, melting the permafrost that has locked up vast amounts of carbon in soils for thousands of years. Bacteria digesting the unfrozen soils generate methane, a potent greenhouse gas, leading to more warming.

Gas hydrates: Also involving methane, this tipping point involves huge reservoirs of methane frozen on or just below the ocean floor. The methane-water crystals are close to their melting point and highly unstable. A huge release could be triggered by a little warming.

West Antarctic ice sheet: Some scientists think this enormous ice sheet, much of which is below sea level, is vulnerable to small amounts of warming. If it all eventually melted, the sea level would rise by six metres.— © Guardian Newspapers Limited, 2011

Keywords: climate change

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