I used to work in an office with eight postgraduate students. The office was overly hot. Everyone complained about this heat. One day I persuaded everyone to turn off their computers before they went home if they were not running a simulation. The next day the office was not overly hot until about 1pm. This gave us four hours of pleasant temperature at a cost of two minutes of booting up in the morning. Every one of these smart people agreed the cause of the heating problem and they agreed the solution. And a week later six of them were leaving their computers on overnight.
They were not running simulations but they felt that their computer could be left on as it was not adding much to the heat problem. So a small change by a group of individuals who all recognised the problem and the solution failed because individuals did not take responsibility for their slight effect on the environment. We had failed to control warming in a room by reducing heat emissions.
Now take global warming. 6 billion people will have to change. They do not agree on the cause of the problem. Their sacrifices will benefit their grandchildren rather then themselves. They will have to sacrifice foreign holidays, car transport, imported food and much more. This is much greater then having to boot your computer in the morning.
If eight smart people who agree on a simple fix to a present warming problem fail what chance is there for billions of disagreeing people on a complicated and difficult series of changes to benefit the future?