Why is humanity still pumping unsustainable levels of greenhouse gases into the atmosphere when all the science points to that being a road that leads to catastrophic consequences which we can't predict?
Second, even if coordination can be solved, by and large the groups/areas that can make a meaningful reduction are not the groups/areas that will bear most of the consequences, there's no such thing as a single, aligned direction of interest/benefit for humanity.
My electrical usage is so low that I don't think installing solar would be a net win in carbon emissions.
Based on various calculators, my carbon footprint is about half that of a typical American. Now can I ask for a 28% decrease in carbon emissions?
Look into how much water and food you need to grow cows.