If humanity wishes to preserve a planet similar to that on which civilisation developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm [parts per million] to at most 350 ppm… If the present overshoot of this target CO₂ is not brief, there is a possibility of seeding irreversible catastrophic effects.
~ James Hansen (born: 1948-02-04 age: 64) along with 14 other scientists in Science 2005-06-03

Analogously, you can’t do it until you need glasses, because by the time you need glasses, you have set in motion an irreversible process that leads not only to blindness but to death. Because we did nothing in 2005, the action we need to take is now ever more drastic.