Why have "ozone-depleting substances" led to a third of all global warming and half of arctic sea ice loss?

Question

According to the Phys.org article "Ozone-depleting substances caused half of late 20th-century Arctic warming, says study" (emphasis added):

A study published today in Nature Climate Change by researchers at Columbia University examines the greenhouse warming effects of ozone-depleting substances and finds that they caused about a third of all global warming from 1955 to 2005, and half of Arctic warming and sea ice loss during that period. They thus acted as a strong supplement to carbon dioxide, the most pervasive greenhouse gas; their effects have since started to fade, as they are no longer produced and slowly dissolve.

Questions:

1. Do these "ozone-depleting substances" also have infrared-absorbing greenhouse impact unrelated to their ozone-depleting chemistry, or is the story more complex?
2. Why the different fractions; 1/3 of all global warming but 1/2 of arctic sea ice loss?

Answer 1

The Global warming potential (GWP) describes how much global warming a particular gas may induce in a particular time period. It is often expressed in terms of CO₂-equivalent. The best known greenhouse gases are carbon dioxide (CO₂), methane (CH₄), and water vapour (H₂O). Water vapour is short lived and therefore its emission is not a primary climate change contributor (but it is a strong secondary contributor through the water vapour feedback). However, there are other greenhouse gases that are, per unit volume, extremely strong. Some of those are CFCs and HFCs.

Wikipedia quotes various IPCC reports and summarises the 20-year global warming potential of some greenhouse gases (emphasis mine):

• CO₂: 1 (by definition)
• Methane: 86
• Nitrous oxide (N₂O): 268
• HFC-134a: 3790
• CFC-11: 7020

The GWP is a result of a combination of the strength of radiative forcing and atmospheric lifetime. The radiative forcing is about 20,000 times stronger, due to those complicated molecules having lots of rotational and vibrational states that can absorb IR radiation. Fortunately, their lifetime is also shorter, resulting in an estimated GWP of around 7000-11000 for commonly used CFCs.

So to answer your first question: yes, ozone-depleting CFCs have a very strong greenhouse gas potential independent of their ozone-depleting properties. Their replacement, HCFCs, are much more gentle to ozone but still very strong greenhouse gases.

I'm not sure why their effect is relatively stronger in the Arctic. The research article you linked provides some ideas:

It is also legitimate to ask whether such a large contribution of ODS to Arctic warming might be an artefact of the CAM5LE model.
(...)
In addition to a larger RF, two factors produce an enhanced Arctic warming with increasing ODS in CAM5LE: (1) a stronger lapse rate feedback (which is positive for the Arctic, confirming previous work) and (2) a weaker negative net cloud feedback (with contributions from both long and short waves).

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Note that the article seems to entirely ignore that HFCs are not phased out at all — HFCs have largely replaces CFCs because they don't destroy ozone, but still have a strong GWP. I find that a rather serious omission because it strongly undermines their conclusion that the phase-out of ODS, which is well under way, will substantially mitigate Arctic warming (the article groups HFCs together with CFCs, which makes sense for GWP but not for ozone depletion).

Answer 2

Do these "ozone-depleting substances" also have infrared-absorbing greenhouse impact unrelated to their ozone-depleting chemistry, or is the story more complex?

Yes, the paper (I have access) actually said that the warming is because of the strong direct radiative forcing of the ozone-depleting agent rather than because of their ability to destroy ozone. They conclude this from findings of running a full (according to historical data) vs. fixed ozone-depletant + stratospheric ozone level (at 1955) vs. fixed ozone-depletant only ensemble simulations.

The paper specifically gives two examples:

Chlorofluorocarbons CFC-11 and CFC-12 are 19,000- and 23,000-fold more radiatively efficient, respectively, than CO2 (in terms of Wm^–2 per parts per billion), resulting in 20-year global warming potentials 7,000- and 11,000-fold larger. So yes, these gases are crazy considering how much we put them into the atmosphere during 1955 - 2005

Why the different fractions; 1/3 of all global warming but 1/2 of arctic sea ice loss?

First thing is you have to know this is just what the model (based on its mechanistic representation) tells you about the relative contributions. The paper tried to explain that ozone-depletant has a higher "warming efficacy" (than CO2, CH4 & N2O) meaning that for the same amount of radiative forcing, it can produce larger temperature differences. So, this discrepancy results from the divergence of radiative forcing and actual temperature impact at a certain location. In particular, the paper said ozone-depletant reinforces the lapse-rate positive feedback and attenuates the negative net cloud feedback in the Arctic. As to how it actually works (and why it doesn't work for CO2, CH4, N2O), I am not an expert in this regard so you would have to look at it yourself. I am guessing this has to do with their molecular weight affecting their movement in the atmosphere (height where they are found relative to the vertical temperature profile and height of clouds at a particular location).

(PS. I also have a feeling that part of it may be just an artifact from parameterization to match Arctic Amplification, meaning that it is more "sensible" to adjust large warming potential gas by a small percentage than to adjust a low warming potential gas by a large percentage. But don't take it too seriously, I am not an expert in this regard.)

Answer 3

The linked article seems poorly written. It refers to unexplained climate models and doesn't link the actual paper. In any case the most intuitive assumption is that the missing ozone doesn't reflect the sun UV rays, in this case it would be the opposite of the greenhouse effect, increasing the incoming of radiation instead of stopping the outgoing radiation.

BTW take care, the article says that the contribution is given to the Arctic warming, not to the global warming. In that region the UV rays come at such an angle that they are much weaker. But I guess that the contribution is so high because the Arctic is so cold that a small increase may have a greater impact.