William Fletcher tells Metta about five tipping points -- hypothetical moments when a linear rate of change breaks and a grave irreversible trend begins, such as a movement of the Gulf Stream, which changes the climate of Europe, or the breaking off of ice shelves in Antarctica, which enables ice on the continent to slip off into the sea and raise its level. Bill and Metta disagree as to whether the reduction of greenhouse gas emission can suffice to bring climate change under control.