text: Around 150 countries are meeting in Kigali to try and agree a speedy ban on hydrofluorocarbon (HFC) gases.
  HFCs were introduced to limit damage to the ozone layer, but cause much greater levels of global warming than CO2.
  However nations are divided over the speed and timing of any phase-out.
  Concern over a growing hole in the ozone layer above Antarctica saw the Montreal Protocol agreed back in 1987.
  The key aim was the removal of gases called chlorofluorocarbons (CFCs) which scientists had determined were causing the destruction of ozone, which protects people and animals from the dangerous impacts of ultraviolet radiation.
  Found in hairsprays, refrigeration and air conditioning, CFCs were ultimately replaced by factory-made hydrofluorocarbons, which essentially do the same job but without the damage to the Earth's protective layer.
  The substitution worked. Earlier this year, scientists said that the ozone hole is showing "the first fingerprints of healing."
  There has been just one unfortunate side effect caused by the solution.
  HFCs are several thousand times better at retaining heat in the atmosphere than carbon dioxide. HFCs have helped the ozone layer, but exacerbated global warming.
  As well as being destructive, they are also the fastest growing greenhouse gases - increasing demand for air conditioning in emerging economies has seen the use of HFCs up by 10-15% per year.
  Scientists, through the Intergovernmental Panel on Climate Change (IPCC), have warned about the warming dangers of HFCs.
  Unusually, governments took heed and have sought an international approach to phase out all these chemicals.
  This move has been given added urgency in the wake of the Paris climate agreement, which aims to keep temperature rises this century well below 2C and as close as possible to 1.5C.
  The scale of HFC growth is adding greater urgency say experts. After a year of negotiations, an amendment to the Montreal Protocol to phase out these chemicals is expected to be agreed at this meeting in Kigali.
  "It's a big piece, these are the fastest growing greenhouse gases right now, although they are still a small percentage," said Durwood Zaelke, from the Institute for Government and Sustainable Development (IGSD).
  "But an amendment could bend the curve down quickly and take out 100 billion tonnes of CO2 equivalent by mid century, and by the end of the century you'll avoid up to half a degree of warming."
  There are dozens of replacement gases emerging including natural alternatives like ammonia, hydrocarbons and ironically, CO2. Refrigerators based on these coolants are already available in some developed countries.
  A new generation of short-lived refrigerant chemicals called Hydrofluoroolefins (HFOs) are also coming on stream.
  However countries meeting here in Kigali are divided over the speed at which existing HFCs should be phased out.
  Around 100 nations including the US, EU, African and island states are pushing for a peak in their use by 2021. India, a large manufacturer of the gases, favours a much later date of 2031.
  "The Montreal Protocol has a good track record of getting things done quickly and efficiently," said Gaby Drinkwater from Christian Aid.
  "We would hope that there would be an ambitious baseline and an early freeze date for all parties concerned. The earlier the freeze date the better for the planet."
  There is a lot to play for. An early peak means a far greater impact on temperatures  - but it will cost a lot more in funding to help poorer nations adapt. The hope is that by having an early phase down, emerging economies will not take intermediate steps but go for the most advanced and sustainable options.
  Unusually, industry and environmental campaigners are fairly well aligned on the need for an early phase out. Governments and private donors are willing to step into the breach and last month offered $80m to speed the transition.
  There is also a hope that newer coolants will also spark more efficient cooling devices.
  "If you increase the efficiency of your room air conditioner, you can double the climate benefits of HFC phase down," said Durwood Zaelke.
  "So private funders have said this is a very good opportunity, and they have put together a fund that is designed to be a bridge to greater sources of funding."
  Ministers arrive in Rwanda on Thursday to lead the negotiations to a conclusion. There is still much detail to be agreed. However, in the light of the imminent ratification of the Paris agreement, and a new deal on aviation emissions, there is added pressure for the Kigali talks to succeed.
  "A meeting like this has its ups and downs, we will go through that cycle," said Durwood Zaelke.
  "We just want to land on the up, that's the key."
  Follow Matt on Twitter @mattmcgrathBBC and on Facebook.
target: Cooling chemicals that play a key role in refrigeration and air conditioning are likely to be rapidly phased out if delegates can reach agreement in Rwanda this week.