[ak217]

The cost is not entirely relevant, even putting aside the need to take out the carbon that we have already emitted. There is a power law distribution of wealth, and people on top have just about all of the financial and technological means to combat global warning, but limited means to convince the people on the bottom to use their tools. Deforestation, automotive and agricultural emissions are just not going to stop fast enough. It's faster, easier and probably more morally sound to build something like a Gigafactory for carbon sequestration modules than to wage (even economic) war on countries that emit too much greenhouse gas.