Envoys from nearly 200 nations meet in Kigali on Thursday to discuss ridding the world of HFCs, gases introduced to save the ozone layer only to unwittingly assail Earth's climate.
Representatives of 197 countries — among them 40 ministers including US Secretary of State John Kerry — are attending the summit.
Delegates are hopeful that after years of talks, countries are now poised to commit to phasing out hydrofluorocarbons (HFCs), introduced in the 1990s to replace chlorofluorocarbons (CFCs) in refrigerators, aerosols, air conditioners and foam insulation.
CFCs were discontinued under the ozone-protecting Montreal Protocol when scientists realised they were responsible for the growing hole in the ozone layer, which protects Earth from the Sun's dangerous ultraviolet rays.
But it turned out that HFCs — while safe for the now-healing ozone — are thousands of times more potent at trapping heat in the atmosphere than carbon dioxide, the main greenhouse gas.
"We are meeting here in Kigali with unity of purpose: to pass an ambitious amendment to the Protocol that would phase out the use of hydrofluorocarbons," said Rwanda's natural resources minister Vincent Biruta.
"We have a unique opportunity to harness the goodwill and commitment to protect our climate and secure the bright future our citizens deserve," he said.
'FASTEST-GROWING GREENHOUSE GAS'
HFCs "are increasing at a rate of 10 to 15 percent a year", said Greenpeace global strategist Paula Carbajal, making them "the fastest-growing greenhouse gas".
Carbajal said HFCs could add as much as 0.1 degrees Celsius (0.18 degrees Fahrenheit) to average global temperatures by mid-century, and 0.5 C (0.9 F) by 2100.
The higher figure represents more than a quarter of the "well under" 2 C ceiling that 195 nations agreed in the French capital in December for warming over pre-industrial levels.
The goal was enshrined in the so-called Paris Agreement on curbing dangerous climate change.
"If HFC growth is not stopped, it becomes virtually impossible to meet the Paris goals," said David Doniger of the Natural Resources Defense Council, an environmental advocacy group.
Wael Hmaidan, international director of Climate Action Network, a coalition of NGOs, called for "an ambitious deal" to be signed in Kigali.
HFCs — though a greenhouse gas like carbon dioxide, methane and nitrous oxide — are not dealt with under the Paris Agreement but under the Montreal Protocol, adopted in 1987.
The protocol also provided for the phase-out of interim replacement gases called HCFCs (hydrochlorofluorocarbons) -- by 2040 for developing countries, and 2030 for rich ones.
This week's meeting, which kicked off on Monday, is the 28th of the treaty's 197 country parties.
'TIME TO ACT'
Negotiators are weighing various proposals for amending the protocol to freeze HFC production and use-by dates ranging from right away to 2031.
India, which is a major HFC producer along with China, backs the later date, while countries in very hot parts of the world where HFC-using air conditioners are in high demand want temporary exemptions.
Last month, a group of developed countries and companies offered $80 million (72 million euros) to help developing countries make the switch away from HFCs.
This week's meeting follows hot on the heels of an aviation industry agreement to cap CO2 emissions at 2020 levels by 2035, and the Paris Agreement obtaining the required signatures to enter into legal force from November 4.
The climate threat posed by HFCs was recognised by a UN sustainable development summit in 2012, when countries said they "support a gradual phase-down".
When that should happen, and how fast, has been the subject of negotiations ever since.
Bureaucrats have been meeting in Kigali since Monday with senior officials arriving to try to thrash out a final agreement on Thursday and Friday.
"It's critical that nations seize this opportunity to reach the most ambitious HFC phase-down agreement possible," said Rachel Cleetus, climate policy manager at the Union of Concerned Scientists, a US-based think tank.
"Our time to act to limit the worst consequences of climate change is rapidly dwindling."