A big problem with climate change is that it responds so very slowly. Even if we somehow magically stopped all emissions by 2020, we would still be facing increased global temperatures and rising sea levels for a century or more. As it is, the earliest we could probably hope to significantly cut emissions (at least without addressing population growth) is likely closer to 2050, at which point many of the adverse effects will already be in full swing. So even in the best case we need to start working on our mitigation of the effects today.
Also, while hope might spring eternal, I honestly don't think a deal will ever become politically possible. Nobody is ever going to come out and tell 5 billion poor people that they'll just have to stop trying to better their lives, for everyone's sake. And no matter how many people die from GW-related effects, precious few will give up their steaks, their plane trips, their 19C house temperatures etc. Any politician who tries to force them to will be voted out ASAP.