Due to a combination of prudence and morbid curiosity, a great deal of scholarly research (and journalism) about climate change has focused on the worst of all possible worlds. For scientists running climate-economic models, that nightmare scenario has a concrete definition: In 2011, such researchers established four baseline scenarios for the future of greenhouse gas emissions (ranging from the benign to the catastrophic) for the sake of facilitating comparable studies.

The most fearsome — and widely cited — of these baselines, known as “RCP8.5,” imagined a year 2100 in which an overpopulated, technologically underdeveloped humanity is digging up and burning every last piece of coal it can find. Thus, by the turn of the next century, coal — the most carbon-intensive major fuel source…