Planting trees will save the climate. At least, that’s the claim of many environmental organizations. As a result, massive reforestation efforts are taking place worldwide. But just how effective are these campaigns?

At the beginning of the Triassic Period, with the continents locked together from pole-to-pole in the supercontinent of Pangea, the world is hot, flat, and very, very dry. But then 234 million years ago, the climate suddenly changed for the wetter.

In central Africa a giant slab of carbon-rich peat land, discovered by a team of British and Congolese scientists, is under threat from oil companies and farming. The carbon must be kept in the ground to avoid accelerating climate change, but some plots...