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Could CFCs be causing global warming?

Posted on 27 December 2009 by John Cook

A new paper Cosmic-ray-driven electron-induced reactions of halogenated molecules adsorbed on ice surfaces: Implications for atmospheric ozone depletion (Lu 2009) examines the link between CFCs, cosmic rays and global warming. The bulk of the paper concentrates on the link between cosmic rays and depletion of ozone in the Arctic and Antarctic regions. This is interesting work that warrants a closer look (perhaps the subject of a future post) and the role of cosmic rays on climate are address elsewhere. However, the strongest statement is found towards the end of the paper, stating that "these data strongly indicate that global temperature has been dominantly controlled by the level of CFCs, modulated by the cosmic ray-driven ozone depletion over the past century." Lu is saying that the increased greenhouse effect from CFCs is the main contributor to global warming in recent decades. What is this based on?

The IPCC AR4 estimate that the radiative forcing from CFCs is 0.33 W/m². This is about 13% of the total radiative forcing from increased greenhouse gases (with carbon dioxide being the main contributor). However, Lu dismisses this value, arguing that CFC radiative forcing is calculated from climate models and not from direct observations. To examine the potential relationship between temperature and CFCs, Lu compares global temperature to Equivalent Effective Stratospheric Chlorine (EESC), a measure of atmospheric CFCs. There is no EESC data before 1970 so the data is extrapolated backwards assuming an identical growth rate until EESC levels hit 0 in the 1940s.

Figure 1: Global surface temperature (HadCRUT3). Solid black line is original observed data. Solid red line is 3-point smoothed average of global temperature. Blue is Equivalent effective stratospheric chlorine (EESC), a measure of atmospheric CFCs (Lu 2009).

Lu argues that the correlation between global temperature and CFCs is evidence that CFCs have been the dominant driver of climate over the past century. While there were large short-term fluctuations between 1850 to 1950, global temperature did not show significant overall rise over this period when CO2 was rising. Temperatures started increasing around 1950, when EESC levels started to become significant. Finally, EESC peaked around 2000 and has fallen in the last few years while the HadCRUT3 temperature record shows cooling in the same period. Based on this correlation, Lu concludes that "these data strongly indicate that global temperature has been dominantly controlled by the level of CFCs... over the past century".

There are several problems with this analysis. The notion that global cooling has been occuring over the last few years is not borne out when one peruses the full range of empirical data. Lu uses HadCRUT data which does not cover the entire globe - the regions where most warming has occured are excluded from the HadCRUT record. Consequently, HadCRUT underestimates recent warming. When one considers the energy building in the entire climate system (especially the oceans where most heat resides), we see that the planet is still accumulating heat through to 2009 (Murphy 2009von Schuckmann 2009). In recent years while the radiative forcing from CFCs was falling, the planet has still been in positive energy balance.

The physics of how CFCs might impose such a strong radiative forcing are not addressed. Lu mentions that the radiative forcing from CFCs haven't been directly measured, then moves onto statistical correlations. In fact, the greenhouse effect from CFCs have been quantified from surface observations of the infrared radiation spectrum (Evans 2006). The observed results are broadly consistent with model predictions of greenhouse forcing (although observations show slightly higher forcing than model results). The proportion of CFC forcing compared to total greenhouse forcing is still around 14%, a close match to the IPCC estimate of 13%.

Figure 2: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapor is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

So we see that CFCs play only a small part in driving global temperatures. Unfortunately, this means that the recent drop in CFC levels will only have a small impact on global temperatures (if only it could be that easy). Of more concern is the increasing radiative forcing from CO2. The infrared spectrum analysis in Evans 2006 measures the extra heat trapped by rising carbon dioxide, finding a CO2 radiative forcing of 2.1 W/m² (again slightly higher than model predictions). This begs the question to those that argue that CFC (or any other mechanism) is meant to be causing global warming. If so, what's happening to all the heat trapped by CO2?

Thanks to David Brown who initially informed me of Lu 2009. Thanks also to David and John Cross for their support in writing this post.

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Comments 1 to 14:

  1. Hofmann et al. (2006) might be of interest here. They show (from direct observations) that radiative forcing from both CFC-11 and CFC-12 has decreased since 1980's.
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  2. "When one considers the energy building in the entire climate system (especially the oceans where most heat resides), we see that the planet is still accumulating heat through to 2009 (Murphy 2009, von Schuckmann 2009)." These two papers don't outweigh the Argo data which indicate flat or decreasing upper ocean temperatures. Since this is consistent with land and atmospheric measurements, we should conclude the earth is not accumulating heat unless it is in the deep oceans. I am not aware of a plausible mechanism for this.
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    Response: von Schuckmann 2009 does use Argo data. Other recent papers on ocean heat trends focused on upper ocean heat which shows more variability than measurements down to 2000 metres which is what von Schuckmann does, showing a less noisy signal:

  3. Well, the abstract is interesting and suggests that a broad range of material will be presented. It surprised me that the paper has just one author, but then I read that Lu is cross appointed in three departments.... Anyway, I would have liked to read the paper, but it ain't free. One potential triviality that caught my interest is the secondary axis in Figure 1: Normalized EESC. Is this standard normal deviates? I wouldn't think so, but why not present the data in ppm or something? At a broader level of response, I'm thinking of ozone holes being polar phenomena and wondering about Lu's choice of a temperature record that mostly ignores these regions. I'd like to hear about the distribution of CFCs in relation to ozone holes and whether this is relevant to interpretation of Lu's results. And now a borderline off-topic comment: I think we have a lot to learn from the CFC-ozone story (or at least I do), including topics of denial (support for inaction), public education, innovation and solution (Montreal Protocol). This paper seems to stray from several standard assertions of AGW-deniers (e.g., , ). Lu's paper is at odds with much of the science on climate change (as the post shows nicely), but I think its deviation from common talking points could be more fatal to its eventual popularity (and ranking among the hottest skeptical arguments). I'm assuming here from the abstract that Lu finds support for anthropogenic ozone depletion. Is that assumption correct?
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  4. Here's a paper from RC from last year that seems to put this into a bit better perspective;
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  5. So I take it that this theory is that as the sun settles down, and more CR's gain entry into the stratosphere, they react with the CFC's and drive more O3 loss, right? With less O3 in the strat, more of the sun's rays can now reach the earth, but on the other hand, more of the earth's emmisions can now travel out to space "easier". So is he saying that with less ozone more energy escaped the earth compared to the extra amount coming from the sun?
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  6. Couldn't the same line of lousy reasoning (not sure how a reviewer could miss that) be used to declare "global temperature has been dominantly controlled by the level of methane...", which rose raipdly through the 80's and 90's and has been mostly flat over the past decade (increasing only recently)? But...both can't be "dominant".
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  7. Thanks for the update, and I also appreciate the tone of this site.
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  8. I agree with the concept of looking to the physics of CFC radiation forcing and that concept certainly supports the idea that Lu's conclusion is at least premature if not totally wrong. However, we also have to realize that the physics of CO2 radiation forcing doesn't support the results in the IPCC's models either. The majority of the forcing is due to a presumed, but unproven, amplification from water vapor/clouds. It seems the issue of these secondary effects of CO2 lie at the heart of IPCC model credibility. Other than Spencer's latest, I don't see recent evidence of anyone really getting to the meat of this issue.
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  9. Doug Cannon, it's hardly unproven that water vapour content depends on temperature. But you know, scientist always like to go back and check in any different situation. Here you'll find some general discussion on water vapour feedback while here you can find a list of scientific papers.
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  10. Riccardo, Yes, I'm aware of the water vapor/temperature relationship. I'm also familiar with Dessler's work to which you referred. By "unproven" I refer to the direct cause and effect relationship to CO2 in the models and it's magnitude. I found it interesting that Dessler invited the Spencer paper at the AGU conference in SF. It's good that those on both sides of the issue agree it's an important one to debate.
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  11. Doug Cannon, the water vapour cause/effect relation is not with CO2 but with temperature and is well established from both basic physics and observations in the atmosphere. The magnitude of the effects is "just" radiation physics, not much uncertainty on this. On the contrary, clouds are still a weak point both for basic physics and observations. And this was the topic of Spencer talk at the AGU meeting. What i found most interesting is the (tentative) use of correlation to separate the "internal radiative forcing" (in Spencer words) from the feedback; but then it's necesessary to discriminate between the different types of clouds which has not been done. Anyway, good science and new ideas from anyone are always welcome.
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  12. Riccardo, Agreed. Correlation is great for initiating an investigation, but not for demonstrating a cause and effect. The increase in Scotch whiskey consumption in Pennsylvania increased by 3.2% in 2009. The increase in teachers pay in Pennsylvania increased by 3.2% in 2009. Ergo..... Maybe a bad example; that probably is a cause and effect.
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  13. I currently am not able to access my institution's library, so I can't go through the paywall to read Lu's paper. I'm curious though to know exactly what technique he employed that permitted him to establish that "correlation between global temperature and CFCs is evidence that CFCs have been the dominant driver of climate over the past century". When I look at the first graph, I see a 20th century warming trend that commences before the increase in EESC, and I see a period of (aerosol attributed) cooling kicking in at the time that EESC does begin to rise. This would seem to indicate that for a decade or so either side of the point where EESC begins to increase, there is no relationship with temperature at all. Any 'correlation' seems to occur after 1960, or even later, and given the vast number of human-produced substances that would demonstrate a similar trend, I would suggest that the correlation is a completely spurious one. At first blush, and given the poor relationship implied by the graph, it's certainly no evidence of a (partial, at best) correlation indicating causation. If this truly is Lu's claim, it should have been weeded out during review.
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  14. What I find very relevant and not discussed enough is the actual mechanism by which CFCs would need to act - the denier problem being that the atmospheric concentration of CFCs is 1000000 (one MILLION) times smaller than those of CO2.
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