The US and India just made progress on a little-known (but potent) climate problem

In Washington, DC, today, President Barack Obama and Indian Prime Minister Narendra Modi announced that they'd reached a deal on cutting the use of HFCs — a little-discussed but extremely potent greenhouse gas used in air conditioners — in the decades ahead.

This is (potentially) a big deal for climate change, but to understand why, we have to step back a bit.

Why HFCs matter 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 leaked into the atmosphere.

Those halocarbons are responsible for about 8 percent of our global warming impact:

(IPCC, 2013)

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. The world's nations got together and enacted the Montreal Protocol in 1989 to phase out CFC use. It was a huge environmental success story, and the ozone layer is now recovering.

Except for one teensy thing. One of the popular substitutes for CFCs are a class of chemicals known as HFCs (hydrofluorocarbons). These coolants are 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.

And HFC use is set to soar in China, India, and other developing countries, which are on pace to install some 700 million air conditioners in the next decade. By some estimates, HFC concentrations are set to rise 140 percent by the end of this decade. We inadvertently swapped one problem for another. Oops.

How to stop HFCs from heating the planet

(China Photos/Getty Images)

The good news is that there are other coolants available — like HFO-1234YF — that are both harmless to the ozone layer and don't warm the planet significantly. Many companies in the United States, such as Dupont, Coca-Cola, and Target, have already pledged to shift away from using HFCs as refrigerants and toward more benign alternatives.

That still leaves the developing world, which is still using HFCs heavily. But we don't need a whole new complicated global treaty to deal with this particular problem. Nations can simply make use of the existing Montreal Protocol to phase out HFCs in favor of more benign alternatives.

Countries are currently discussing how best to amend the Montreal Protocol to deal with HFCs. The hope is to get a new agreement later this year. But up until now, poorer countries like India have opposed a complete phaseout of HFCs, arguing that alternatives are more expensive and the transition won't be cheap or easy.

So that brings us to today's news. In their meeting, Obama and Modi agreed to work together in amending the Montreal Protocol so as to phase out HFCs in the coming decades. Andrew Freedman of Mashable has the gritty details, but the basics are that India would agree to an "ambitious phasedown schedule," while the US would support a push to financial aid from wealthier countries to help manage the transition away from HFCs.

This seems like a bit of arcane bureaucratic wrangling, but it's potentially a big step. If countries can revamp the Montreal Protocol to phase out HFCs, some analysts argue that we could avoid as much as 0.5 degrees Celsius worth of global warming by century's end. Given that the Earth has already heated 1 degree Celsius (or 1.8 degrees Fahrenheit) since the dawn of industry, and 2 degrees Celsius is deemed unacceptable by many countries, every bit helps.

This meeting today is just one step on the long road to a HFC phaseout, but it's a significant one. Now, about shifting away from fossil fuels…

Further reading: At the Washington Post, Steven Mufson has more on the Obama-Modi talks, which included a deal to build six nuclear reactors in India. Note that India is the world's third-largest emitter of carbon dioxide, and it will almost certainly burn more coal, oil, and gas in the decades ahead as it lifts itself out of poverty and brings electricity to 400 million more people.

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