In 2007, the IPCC warned: "Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."10 The panel did not say how much or how fast temperatures had to rise in order for that to happen, leaving policy makers and scientists to try to define maximum "safe" levels. In 1996, the European Union's environment ministers recommended that global average temperatures not rise by more than 3.6°F (2°C) above pre-industrial levels and that the atmosphere's carbon dioxide concentration should not go above 550 parts per million (ppm)—about twice the pre-Industrial Revolution level. However, a growing number of scientists believe that those levels are dangerously high. James Hansen argues that the current carbon dioxide concentration,
Above, the impact of global warming on various species and their habitats is illustrated. Many scientists are concerned that global warming may lead to worldwide extinctions, possibly as many as a million species lost by 2050.
385 ppm, is already causing damage to the planet and warns that irreversible consequences will occur if levels stay at or above this level for a sustained period of time.
A team of scientists led by Professor Timothy Lenton of the University of East Anglia in England explored possible "tipping elements," relatively small disturbances that "switch" much of the Earth into a qualitatively different state. Lenton's team concluded that "a variety of tipping elements could reach their critical point within this century under anthropogenic climate change. The greatest threats are tipping the Arctic sea-ice and the Greenland ice sheet, and at least five other elements could surprise us by
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