Climate change will never get solved in a single flourish. If the world’s nations are ever going to stop the planet from warming unbearably, they’ll have do so step by step, pushing down emissions across a dizzying variety of sectors and sources.

And on Saturday, the world took one of the biggest, most promising steps yet.

At a United Nations conference in Rwanda, 197 nations agreed to drastically reduce their use of hydrofluorocarbons (HFCs), an obscure but extremely powerful greenhouse gas used in air conditioners, refrigerators, and foams. Without action, HFC use was expected to soar. So, by cutting these pollutants, the world may end up avoiding between 0.2°C and 0.44°C of warming by century’s end, according to an early estimate by the Institute for Governance and Sustainable Development:

A fraction of a degree may not sound like much, but keep in mind that small ticks in average global temperatures can have outsize impacts. The difference between, say, 1.5°C and 2°C of warming could be the difference between a world that still has healthy coral reefs and a world filled with bleached coral graveyards.

The backstory here is that HFCs were developed in the 1990s to replace the CFCs in our fridges and air conditioners that were famously chewing a hole in the ozone layer. Yet HFCs later became a significant contributor to global warming, so nations have decided to phase them out, too, under the same treaty that got rid of CFCs, the 1989 Montreal Protocol.

It wasn’t easy to reach agreement, and negotiators have been wrangling over the details issue for years. The alternatives to HFCs are, after all, a fair bit pricier. Developing countries like India have long worried that a too-aggressive phase-out would make air conditioning unaffordable for millions.

In the end, the various parties struck a compromise. The newly amended Montreal Protocol, which is legally binding once ratified, will require rich countries like the United States to start cutting HFC use by 2019. Countries like China will have to cap their HFC use by 2024, while even poorer countries like India get until 2028 — although they’ll receive more aid if they act earlier. Ultimately, the deal could cut global HFC use up to 85 percent by 2047, the World Resources Institute estimates.

Now, even with this deal, the world is still on pace to blow past the threshold of 2°C of warming that policymakers have pledged to avoid. Countries still have a lot more work to do under the broader Paris climate accord to even come close to that goal; cutting HFCs was just one of the many items on the to-do list set up last year. But it’s a start.

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 manmade CO2 has done the most to trap extra heat in the atmosphere and warm the planet over the past century. Shifting away from fossil fuels is humanity's No. 1 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 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:

For much of the 20th century, we relied on CFCs (short for chlorofluorocarbons, also known as Freon) as refrigerants in our air conditioners and refrigerators. Then in the 1970s, scientists discovered that the chlorine in those CFCs was chewing a hole through our ozone layer.

In response, the world's nations got together and enacted the Montreal Protocol in 1989, a binding treaty 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.

But there was a catch. One of the most popular substitutes for CFCs is a class of chemicals known as HFCs (hydrofluorocarbons). These refrigerants are fairly harmless to the ozone layer, but they still 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:

Without further action, the use of HFCs would have soared in China, India, and other developing countries, which are on pace to install some 700 million air conditioners. Concentrations in the atmosphere were projected to rise 140 percent or more. Basically, we stopped one environmental problem only to confront another.

How to stop HFCs from heating the planet

So that brings us to the new deal reached in Rwanda this week.

The good news is that there are plenty of ways to cut HFC emissions. We could make air conditioners and refrigerators more efficient, so that they use fewer HFCs to begin with. We could seal up leaks, preventing those HFCs from wafting into the atmosphere.

Or, more significantly, we could switch over to newer refrigerants that are both harmless to the ozone layer and don't warm the planet significantly. Chemical companies are already patenting and producing refrigerants like hydrofluoroolefins (HFOs), which trap far less heat over their life span. (The Center for Climate and Energy Solutions has a rundown of other HFC alternatives here.)

The not-so-good news is that these alternatives can be more expensive, and poorer countries like India had been reluctant to crack down on HFCs too aggressively. India, after all, has one of the hottest climates on Earth, and as the country 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 negotiators in Rwanda had to balance all of these different concerns. The result was a compromise.

Under the final deal, wealthier regions, including the United States and Europe, will take the lead, cutting HFC emissions as much as 10 percent by 2019 and even more thereafter. These countries have already started in earnest — in the US, big refrigerant users such as Dupont, Coca-Cola, and Target have pledged to shift away from HFCs and toward more benign alternatives.

Meanwhile, developing countries such as China and Brazil will get a bit more flexibility, and won’t have to cap their HFC use until 2024. That way, they can wait until newer air conditioners and refrigerators come to market and the price comes down. Even poorer countries with sweltering climates like India and Pakistan get until 2028, although they get financial incentives to start cutting earlier. Rich countries and donors pledged to chip in aid under the treaty to finance the transition, as they did with CFCs. (By some estimates, shifting away from HFCs could cost India between $15 billion and $38 billion through 2050.)

This amendment to the Montreal Protocol, will have to be ratified by individual countries. Once that’s done, the treaty will be legally binding and can be enforced through trade sanctions. The advantage of using an existing treaty is that we already know it works — it demonstrably cut CFC use.*

Ultimately, the new amendment would aim to cut global HFC use 80 percent or more by midcentury. If so, that would have a significant climate impact. One 2013 study in Atmospheric Chemistry and Physics found that an aggressive phaseout of HFCs could help the world avoid up to 0.5°C of global warming by the end of the century compared with a scenario in which HFC use kept rising sharply. Early calculations suggest this treaty gets us most of the way there. And even little notches count for a lot.

Environmental groups hailed the agreement, as did the White House. The Obama administration has been patiently pressing the HFC issue for years, finally persuading Indian Prime Minister Narendra Modi to agree to an aggressive phaseout back in June. And on Saturday, Obama put out a short, triumphant statement on the deal, calling it “an ambitious and far reaching solution to this looming crisis.”

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* Note: In the United States, the treaty will likely require approval by two-thirds vote in the US Senate before it’s ratified and considered the law of the land. Obviously with many Republicans skeptical of climate change, ratification is far from assured. If the deal isn’t ratified, than the US Environmental Protection Agency still has the authority to go through with domestic HFC cuts regardless — something Obama has already been doing. The only catch is that a new president could unilaterally halt the process.

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