Sure, wind and solar energy kept getting cheaper, and an electric car became America’s best-selling luxury vehicle. But the most important metric of climatic health—the amount of heat-trapping gas entering the atmosphere—got suddenly and shockingly worse.

In the United States, carbon emissions leapt back up, making their largest year-over-year increase since the end of the Great Recession. This matched the trend across the globe. According to two major studies, greenhouse-gas emissions worldwide shot up in 2018—accelerating like a “speeding freight train,” as one scientist put it.

U.S. emissions do remain 11 percent below their 2007 peak…