Well, from a purely scientific perspective, the only solution to the anthropogenic global warming is to completely eliminate greenhouse gas emissions. The residence time of carbon dioxide is so long that at some point we have to stop using fossil fuels, but of course that's not feasible at all.
So in terms of the targets we set, it's not a scientific question. It's really a social-technological-political question. They need to be as aggressive as possible, as aggressive as our technologies and our infrastructure and our political will.