Following Is a List of Possible Climate Change Tipping Points

Following is a list of possible climate change tipping points, except for one.Choose the one that is not a tipping point.

A) atmospheric level of 450 ppm
B) collapse and melting of the Greenland ice sheet
C) expansion of Amazon rainforest
D) massive release of methane from Arctic permafrost
E) sharp drop in the ability of the oceans to absorb CO2