This book - or maybe I should say pamphlet - is a concise explanation of the basic forces driving climate change, why to believe it’s caused by humans, why to be concerned about it, and what, at a high level, can be done. I appreciate its focus and its avoidance of political and moral invective. (That its foreword was written by the director of Republican environmentalist group republicEn is a hint that it's making an effort to sidestep political tribalism.)
The greenhouse effect, as I understand it based on the book, works like this:
Unlike water vapor, CO2 stays in the atmosphere a long time, so an increase in it means a lasting increase in the amount of radiation being sent back to the surface. Emanuel notes that way back in 1906 Svante Arrhenius "had calculated that doubling the concentration of CO2 would raise the earth's surface temperature by about 4°C, a number well within contemporary estimates of 2-4.5°C per doubling of CO2". In the subsequent 115 years, CO2 levels and temperatures have risen, and in proportions consistent with that prediction.
Emanuel discusses various obstacles to precisely predicting future temperatures. He says "the largest source of uncertainty" is the difficulty of modeling cloud formation. There's also "chaotic natural variability ... often referred to as 'climate noise.'":
Current estimates of this noise come largely from climate models run for a long time with constant forcing. These estimates suggest that the current global warming trend is clearly distinguishable from climate noise on time scales of around 30 years and longer. Just as a particular week in mid-spring may be colder than a particular week in late winter, there can be stretches as long as 30 years during which, owing to natural chaotic variability, the global mean temperature cools.
Even though computer simulations involve a great deal of uncertainty, Emanuel notes that simulations "using many different climate models" show that you "cannot accurately simulate the evolution of the climate over the last 30 years without accounting for the human input of sulfate aerosols and greenhouse gases."
Options for dealing with climate change fall into three broad categories: curtailing the emissions of greenhouse gases (mitigation), learning to live with the consequences (adaptation), and engineering our way around the problems that greenhouse gases produce (geoengineering).
Like many people commenting on this issue, Emanuel highlights nuclear energy as an excellent and tragically underutilized mitigation option; in addition, he notes that we already have the technology for capturing carbon at the point of emission, though it "is currently estimated to increase the cost of energy produced by 20-90 percent." (This was published in 2018.)
A key but complex issue is the relative costs and benefits of adaptation and mitigation, all of which must be estimated in an environment of considerable uncertainty. An optimal strategy will no doubt involve doing some of both.
In contrast, "most of those whose work focuses on geoengineering regard it as an option to be developed and then kept in our collective back pocket, to be used only if the effects of climate change become catastrophic."