DELHI — A thrill goes down Lane 12, C Block, Kamalpur every time another working-class family brings home its first air-conditioner. Switched on for a few hours, usually to cool a room where the whole family sleeps, it transforms life in this suffocating concrete labyrinth where the heat reached 117 degrees in May.

“You wake up totally fresh,” exulted Kaushilya Devi, a housewife, whose husband bought a unit in May. “I wouldn’t say we are middle class,” she said. “But we are closer.”

But 3,700 miles away, in Kigali, Rwanda, negotiators from more than 170 countries gathered this week to complete an accord that would phase out the use of heat-trapping hydrofluorocarbons, or HFCs, worldwide, and with them the cheapest air-conditioners that are just coming within reach of people like Ms. Devi. Millions of Indians might mark the transition from poverty with the purchase of their first air-conditioner, but as those purchases ease suffering in one of the planet’s hottest countries, they are contributing profoundly to the heating of the planet.

HFCs function as a sort of supergreenhouse gas, with 1,000 times the heat-trapping potency of carbon dioxide. While they account for just a small percentage of greenhouse gases in the atmosphere, scientists say a surge in the use of HFC-fueled air-conditioners would alone contribute to nearly a full degree Fahrenheit of atmospheric warming over the coming century — in an environment where just three degrees of warming could be enough to tip the planet into an irreversible future of rising sea levels, more powerful storms and deluges, extreme drought, food shortages and other devastating impacts.