THAT IT TOOK almost a quarter of a century of diplomatic wrangling, setbacks and innovation for the world to put Paris’s numbers and procedures to Rio’s remarkable ambition makes it unsurprising that, as yet, little has been done to meet its goals. In 1992, according to figures from Our World in Data, 78% of the world’s primary energy—the stuff used to produce electricity, drive movement and provide heat both for industrial purposes and to warm buildings—came from fossil fuels. By 2019 the total amount of primary energy used had risen by 60%. And the proportion provided by fossil fuels was now 79%.