The Intergovernmental Panel on Climate Change, which pools the efforts of scientists around the globe, has begun releasing draft chapters from its latest assessment, and, for the most part, the reading is as grim as you might expect. We are still on the road to catastrophe without major policy changes.

But there is one piece of the assessment that is surprisingly, if conditionally, upbeat: Its take on the economics of mitigation. Even as the report calls for drastic action to limit emissions of greenhouse gases, it asserts that the economic impact of such drastic action would be surprisingly small. In fact, even under the most ambitious goals the assessment considers, the estimated reduction in economic growth would basically amount to a rounding error, around 0.06 percent per year.

What’s behind this economic optimism? To a large extent, it reflects a technological revolution many people don’t know about, the incredible recent decline in the cost of renewable energy, solar power in particular.

Before I get to that revolution, however, let’s talk for a minute about the overall relationship between economic growth and the environment.

Other things equal, more G.D.P. tends to mean more pollution. What transformed China into the world’s largest emitter of greenhouse gases? […]