One of the tricky aspects of writing about climate change is that it often involves tossing around tiny numbers with mind-bogglingly large consequences.

If, for instance, global average temperatures rise a mere 1.5°C above pre-industrial levels, we start losing coral reefs. Go up to 2°C and we risk serious damage to our food supply. At 3°C, the ocean swallows even larger chunks of our coastlines. Each of those notches on the global thermometer, when unpacked across this vast planet of ours, has far-reaching impacts on ice melt, drought, heat waves, and extinctions.

So keep that in mind when you hear about an obscure UN conference this week that is trying to reduce global warming by as much as 0.5°C by the end of the century. Half a degree sounds like a pittance. But it’s not. Not at all.

I’m referring to the current UN talks in Rwanda to tackle hydrofluorocarbons (HFCs), an extremely potent greenhouse gas found in air conditioners, refrigerators, and foams. These HFCs were originally developed to replace the CFCs that were famously chewing a hole through the ozone layer. But HFCs have since become a major contributor to global warming, so countries are trying to phase them out under the same treaty that got rid of CFCs, the Montreal Protocol. (Note, this is all separate from the UN’s bigger Paris accord to tackle most other aspects of climate change.)

It won’t be easy to reach an agreement on HFCs. If the phase-out proves too costly, it could make air conditioning unaffordable for millions of people in India and other developing countries who badly need it. But a successful deal would matter greatly for the future of this planet. The stakes are awfully high for such an obscure meeting.

Why HFCs matter so much for global warming

Most discussions of global warming revolve around carbon dioxide, the greenhouse gas produced when we burn oil, gas, and coal. And fair enough: The rise in man-made carbon dioxide has done the most to trap extra heat in the atmosphere and warm the planet over the last century. Shifting away from fossil fuels is, by far, humanity's number one challenge.

But we shouldn't forget that we emit other important greenhouse gases, too. There's methane (CH4), which comes from landfills, livestock, and natural gas leaks. There's nitrous oxide (N2O) from agriculture. And there are the halocarbons such as the CFCs and HFCs in our air conditioners and refrigerators that also trap heat when they leak out of aging or faulty equipment and waft into the atmosphere.

Those halocarbons are responsible for about 8 percent of humanity’s total global warming impact:

So where’d they come from? For much of the 20th century, we relied on chemicals known as CFCs (short for chlorofluorocarbons, also known as Freon) as coolants in our air conditioners and refrigerators. Then, in the 1970s, scientists discovered that CFCs were chewing a hole through our ozone layer.

So the world's nations got together and enacted the Montreal Protocol in 1989 to phase out CFC use over time. It was one of the all-time great environmental success stories, and the ozone layer is now recovering.

Except for one teensy detail. One of the most popular substitutes for CFCs are a class of chemicals known as HFCs (hydrofluorocarbons). These coolants are fairly harmless to the ozone layer, but they turn out to be extremely potent greenhouse gases — up to 10,000 times as effective at trapping heat as carbon dioxide — when they seep out into the atmosphere. And they’re becoming widespread:

In the coming decade, the use of HFCs is expected to soar in China, India, and other developing countries, which are on pace to install some 700 million air conditioners. Total HFC concentrations in the atmosphere could rise 140 percent. Basically, we stopped one environmental problem only to confront another.

The battle to stop HFCs from heating the planet

The good news is that there are many ways to reduce HFC leakage from existing air conditioners and refrigerators. More significantly, new coolants are available that are both harmless to the ozone layer and don't warm the planet significantly. One promising class of alternatives are HFOs, or hydrofluoroolefins, which trap far less heat over their lifespan. (The Center for Climate and Energy Solutions has a good rundown of other options here.)

Some countries are already starting to take action. The European Union has a number of regulations to seal leaks from equipment and reduce HFC usage. In the United States, big refrigerant users such as Dupont, Coca-Cola, and Target have pledged to shift away from using HFCs and toward more benign alternatives.

The not-so-good news is that these alternatives can be more expensive, and poorer countries like India are reluctant to crack down on HFCs aggressively. India, after all, has one of the hottest climates on Earth, and as it gets richer, more and more people would like to install air conditioners in their homes. That’s not just a matter of comfort; India’s sweltering summers can be brutal for health and productivity. Cheap air-conditioning is a boon in many ways:

So that brings us to this conference in Kigali, Rwanda, this week.

Officials from 170 countries have gathered to amend the existing Montreal Protocol to phase out HFCs in favor of more benign alternatives. In theory, this is just a matter of tweaking a treaty that already tackles CFCs. Just add another chemical to the list! But the really tricky stuff is in the details.

In recent months, after extensive discussions with President Barack Obama, Indian leader Narendra Modi has come around to the view that the world needs an "ambitious phasedown schedule" of HFCs under the Montreal Protocol. But he also points out that replacing HFCs with cleaner alternatives could cost India between $15 billion and $38 billion through 2050. So he’s asked for aid from richer countries to help make that transition. Other developing nations, such as Brazil and Pakistan, have made similar arguments.

In response, rich countries have pledged to chip in aid, while various donors and philanthropies have created a $53 million fund to ease the transition costs. The broad outlines of an agreement are there.

Still, translating this into actual text that everyone can agree on will take work. Negotiators still have to wrangle over timetables and exactly how much each country needs to cut, and precisely what sorts of financial aid should be given to developing countries. (The US and India, for instance, each want the other to shoulder a bigger share of the cuts.) There’s no guarantee that they’ll all reach a satisfying agreement by October 14, when the conference ends.

But if they do, the climate impact could be significant. One 2013 study in Atmospheric Chemistry and Physics found that an aggressive phase-out of HFCs could help the world avoid up to 0.5°C of global warming by the end of the century compared with the trajectory we’re currently on. Obviously a less ambitious agreement would reduce global warming by less than that. But even little notches count for a lot.

Further reading