A good answer was given in a recent interview by John Holdren, science advisor to US President Obama:
“Mitigation alone won’t work because the climate is already changing. We’re already experiencing impacts from that. Nothing we can do in the mitigation domain can stop it overnight. And so a mitigation-only strategy would be insanity.The question is whether we have the political and social will to do enough of the former in sufficient time to make the latter actually achievable.
"Adaptation alone won’t work. Adaptation alone won’t work because adaptation gets more difficult, more expensive, and less effective the larger are the changes in climate to which we are trying to adapt. If you live on an island that is one meter above sea level and the sea level goes up two meters, adaptation is no longer the question. You are dealing with evacuation.
"Clearly what we need is enough mitigation to limit changes in climate to a level with which adaptation can largely cope.”
Crucial in this is the role of the somewhat overlooked virtue of prudence, wise consideration of the future. Whether our political and economic systems are designed to encourage this virtue in our leaders remains to be seen.