We have now reached the final chapter on our trip through the problem of modern climate change. In the previous 13 chapters, we have explored the fundamental physics that leads us to confidently conclude that humans are now changing the climate, and that the continuing addition of greenhouse gases to the atmosphere will bring potentially significant changes to our climate over the next century and beyond. We are not certain how bad this climate change will be, but the upper end of the range (global and annual average warming of 5°C or more by the end of the century) includes warming large enough for the experts to consider its impacts to be catastrophic. Even the lower end of the range (∼2°C) will be challenging for the world's poor as well as our most vulnerable ecosystems. We have also explored a number of possible responses to this risk, including mitigation, adaptation, and geoengineering.

The science of climate change plays an important role in helping refine decisions about what mix of these options we should pursue. However, science by itself does not determine policy. Policy decisions also involve value judgments concerning the relevant trade-offs among the options. Some of these trade-offs are economic and involve decisions about spending less money now or more money later, and some are moral, concerning the rights of future generations to inherit an unspoiled Earth versus the imperative to consume now in order to lift the poorest individuals out of poverty.