CBA. Costs of the disruption* that could be expected to occur are higher than costs of mitigation. And the fact that technically and (in theory) politically, we can act as a global entity to apply this mitigation.
Population though, remains the more difficult issue - as Hardin pointed out - The population problem has no technical solution.
*sea level rise not good for Miami, Singapore, Bangladesh etc etc. Water problems, mass population movements, possible shift in ocean currents and so on.