The Permian/Triassic event, approximately 251 million years ago, was probably triggered by an outpouring of volcanic CO2, and caused extinction of 96% of all marine species and 70% of vertebrate species, see Permian–Triassic extinction event. There was wide-spread desertification on land, and anoxia in the oceans, coupled with acidification. The global temperature increased by 6 degrees, according to some models. Within the last 100 years, greenhouse gas levels have nearly doubled. If we reach a tipping point, and the current global warming gets out of control, we could well see a temperature increase of 6 degrees, with catastrophic results, akin to the Permian/Triassic event. There are signs that the tipping point has been reached for the Greenland ice sheet, where ice loss has doubled in less than a decade. There is a strong positive feedback effect as polar ice is replaced by water, which absorbs more sunlight, causing further melting.