Envoys from nearly 200 nations meet in Kigali next week to discuss ridding the world of HFCs, gases introduced to save the ozone layer only to unwittingly assail Earth's climate.

Observers are hopeful that after years of talks, countries are poised to commit to phasing out hydrofluorocarbons, rolled out 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 turns 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.

HFCs "are increasing at a rate of 10-15 percent a year," Greenpeace global strategist Paula Carbajal told AFP. "That makes them the fastest-growing greenhouse gas."

This growth was mainly due to demand for air conditioners, expected to treble in use by 2050.

Carbajal said HFCs could add as much as 0.1 degree Celsius (0.18 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 which 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.

- Time running out -

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 phaseout of interim replacement gases called HCFCs (hydrochlorofluorocarbons) -- by 2040 for developing countries, and 2030 for rich ones.

Next week's meeting, from Monday to Friday, will be the 28th of the treaty's 197 country parties.

Negotiators will weigh 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 aircon-reliant countries in very hot parts of the world 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.

For Maxime Beaugrand of the Washington-based Institute for Governance and Sustainable Development, HFCs are the "low-hanging fruit" of efforts to curb global warming.

"It is time to pick them. There are no major obstacles," she told AFP. "There are about a hundred alternatives (to HFCs)."

These included natural refrigerants such as CO2, hydrocarbons, water, air and ammonia, added Carbajal.

"Natural substances are available and technically and economically feasible in almost all cooling applications."

Next 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 were 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 will meet for negotiations on Monday and Tuesday, followed by environment ministers thrashing out a final agreement from Wednesday to Friday.

"It's critical that nations seize this opportunity to reach the most ambitious HFC phasedown agreement possible," said Rachel Cleetus, climate policy manager at the Union of Concerned Scientists, a US-based thinktank.

"Our time to act to limit the worst consequences of climate change is rapidly dwindling."