The costs of mitigating climate change are unobtainable, not insignificant. To put it simply, major carbon emission reduction is not obtainable without a central world government. Any major solutions can and will likely be mitigated by an application of the tragedy of the commons - a country or many countries, realizing it can 'get ahead' by ignoring carbon reductions. The only options to prevent this are centralized world governments with fangs or an authoritarian push by a world power to control the world, both unobtainable.
This is why I'm focused on mitigation by technology. Crush the cost/KWH of carbon by creating technologies that are better, develop structures that can adapt to the change, develop seawall tech to prevent change. If we look at how fast humans have adapted in the past, I am confident that we will be able to adjust in the future.