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John Schellnhuber says twelve "tipping points" for climate change have been identified so far, according to this report. They are the Sahara desert; the Amazon rainforest; the stratospheric Ozone hole; the Greenland ice sheet; the Tibetan plateau; salinity valves ("pinch" points between adjacent seas); the North Atlantic current; El Nino; the West Antarctic ice sheet; methane clathrates; the South Asian monsoon; and the [Southern] circumpolar current.