Could carbon dioxide save our planet?

It's man-made, up to 4,180 times more damaging to the climate than carbon dioxide – and it’s all around us: Hydrofluorocarbon (HFC)

The arrival of much-improved refrigeration, air conditioning, foam packaging and hairspray in aerosol cans in the 1930s was made possible by the invention of synthetic chlorofluorocarbons (CFCs). Up to that point, the most common refrigerant was carbon dioxide. CO₂, however, lacked the stability CFC had to offer as the latter remains in its molecular compound for a lot longer than any other gases known at the time. Therein also lies the danger. CFCs’ lack of reactivity gives it a lifespan of up to over 100 years and the ability to reach the upper stratosphere (above 10km altitude) unharmed. Only there, in the ozone layer to be precise, the gas breaks down because of the Sun’s intense ultraviolet radiation – and destroys ozone (O₃) in the process.

In the 1970s scientists discovered a worrying degree of ozone depletion which meant an increase in harmful ultraviolet radiation. Who doesn’t remember a hole in the ozone shield almost the size of the Antarctic? It was refreshing to see how nations around the world gathered to find a solution in the Montreal Protocol, probably the biggest environmental success story of all time: First, to ban all production of CFCs, and to find a substitute gas that wouldn’t result in the Earth’s surface (and everything on it) being burned to crisp.

That substitute gas is – you guessed it – hydrofluorocarbon. So, how can a gas once hailed to save mankind suddenly turn into one of the biggest threats in our fight for survival? The same as with chlorofluorocarbons: lack of foresight. The solution to one environmental problem became the cause of another.

What is HFC and why is it so bad?

HFCs are exclusively synthetic and made of hydrogen, fluorine, and carbon. All greenhouse gases ‘work’ on the same principles – they are absorbing infrared radiation (which otherwise would escape into space). The potency of greenhouse gases depends on three attributes

  • How much radiation it absorbs
  • It’s ‘lifetime’ – how long it hangs around in the atmosphere
  • Whether the specific wavelength of radiation it absorbs would otherwise be sponged up by something else in the atmosphere (e.g. water)

When these three properties are combined, they can determine the global warming potential for each greenhouse gas. The potency is then measured by its relativity to carbon dioxide (CO₂). That means that the global warming potential of CO₂ is 1. Methane (CH₄), which is widely known as the second most harmful greenhouse gas, has a warming potential of 34 – meaning that 1 tonne of CH₄ traps 34 times more heat than 1 tonne of CO₂.

Hopefully, you are sitting down because the global warming potential for the three most abundant HFCs range from – wait for it – 1,370 to 4,180! HFCs trap thousands of times more heat in our atmosphere than the equivalent volume of CO₂.

Will HFC be replaced?

The answer is yes. Almost 35 years ago, nearly 200 nations and the world’s chemical industries signed the Montreal Protocol that ended the mass production of CFCs and put a stop to the depletion of the Earth’s ozone shield. Most of these signatories also agreed to the Kigali Agreement in 2016 (the Biden Administration pledged in April last year to follow suit), which aims at phasing out HFCs in developed countries since 2019. Developing nations are to jump on the bandwagon between 2024 and 2028. Optimists estimate this step will prevent up to 0.5°C of future warming – certainly an important step towards achieving the envisaged limitation of global warming below 2°C in the Paris Agreement.

Since we can’t do without refrigeration and cooling our immediate environment, what is to replace HFCs?

This is where it gets complicated. There are groups promoting another class of fluorine-containing compounds called hydrofluoroolefins (HFOs). They pose much less of a climate risk due to their short lifetime in the atmosphere. But, of course, there is a BUT: There are major concerns about the toxic chemicals produced when HFOs break down. Should we be messing around with molecular compounds that do not appear naturally? Fluorine is super-reactive with almost all other elements and has a bad track record thus far. And as the CFC and HFC problem highly reflects retrospectively, we cannot for certain predict the chemical reactions these aggregates may bring about.

Scientists were also looking at mixtures of hydrocarbons such as butane. But hydrocarbons pose safety risks – imagine your freezer affecting the air quality in your home or, even worse, going up in flames.

However, there is a candidate you may not expect to make a surprise come-back: carbon dioxide. It was already in use as a refrigerant from the mid-19th century before it was replaced by CFC. Yes, there are technical hurdles to master before we can use CO₂ as a refrigerant on a broader basis, but it is non-toxic, non-flammable, and a much weaker greenhouse gas than the HFCs it would come to substitute. This may sound weird – CO₂ looks like the best of the bad bunch of refrigerants.

Every Yang has its Yin – CO₂ operates at a higher pressure than HFC multiplying the leak potential by 4. Engineers must take special care when recharging systems as carbon dioxide changes from gas to a solid at 4.2bar. And last for now, but certainly not least: water and carbon dioxide don’t get on as well as you may think. Water leaking into the cooling system and mixing with CO₂ may result in the formation of ‘unusual’ compounds. All these factors will contribute to making refrigeration more expensive in the short term.

Despite that, carbon dioxide, at least for now, is our best option. And I remain hopeful that the actions needed to replace HFCs are met with more enthusiasm than, say, the rather lacklustre COP26.

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