evidence: NARRATOR: If we continue as we are, combining reduced air pollution with an increase in greenhouse gases, temperatures could rise by a further two or even three degrees Celsius. That's as much as five degrees [...] I cannot imagine that the ice sheets could survive more than a few centuries with a three-degree Celsius warming. So that would mean a sea level rise of several meters per century, and it would just continue. And once that starts, it's out of our control. [...] JAMES HANSEN: I think we have less than a decade to avoid passing what I call "point of no return." I think we have to keep global warming less than one degree Celsius, or we're going to get very bad effects. And the problem is that to achieve...to keep the warming less than one degree Celsius, we have to level off the emissions and get them to decline before the middle of the century, substantially.