The 2015 Paris climate deal lacked hard commitments, but did encourage a number of follow up actions. In the most important step to date, nearly 200 countries agreed to amend the Montreal Protocol, the 1989 treaty on ozone-depleting substances, to call for the phase-out of hydrofluorocarbons (HFCs). As the go-to alternative to ozone-depleting chlorofluorocarbons (CFCs), HFC use in such applications as refrigeration and air conditioning has skyrocketed over the past quarter century. Although this has been good for the ozone layer, it has been bad for the climate: HFCs have become a significant greenhouse gas, with a molecule-for-molecule potency more than a thousand times greater than CO2. Under the agreement, HFC use will peak in 2028 for all countries (earlier for developed countries), with reductions of at least 80 percent from their respective baselines by the late 2040s. The deal will prevent a half degree of warming, the largest reduction ever achieved by a single agreement, equivalent to eliminating global fossil-fueled CO2 emissions for at least two years. Of course, we must move urgently toward stopping all CO2 emissions permanently, especially from fossil fuels. So as we celebrate this victory for a livable future, there’s no time to rest.