The book’s main theme is just how urgent a crisis climate change poses. We must act—and fast. Gates makes this clear from the start as he tells how he came to be so passionate about the subject. When he first started paying attention to climate change in the early 2000s, he writes, he thought that we could mitigate and manage it by getting serious about reducing greenhouse gas emissions. However, the more he learned and talked with experts, the more he realized that what climate scientists were predicting was correct: that we will never stop increasing the Earth’s temperature without cutting emissions to zero. Given that the world emits 51 billion tons of greenhouse gases every year, that’s a tall order.

Moreover, most experts think we need to get to zero by the year 2050 to avert a major climate disaster. In Chapter 2, Gates frankly discusses why this will be so difficult: because of our dependence on fossil fuels and the processes that emit greenhouse gases. The word “fuels” can be misleading, as it evokes only of transportation—but we use fossil fuels for much more. From the concrete in apartment buildings, to plastic bags at the supermarket, to the electricity that powers our appliances and computers, the products of fossil fuels surround us.