By a bit of luck I found a short discussion of a really “inconvenient” paper by associate-professor Q-B Lu of the University of Waterloo (see personal website) in Lubos Motl’s blog “The Reference Frame“. There are quite a lot of versions of this paper flying around the internet, some pay-walled, some free. The title is: Cosmic-ray driven reaction and greenhouse effect of halogenated molecules: culprits for atmospheric ozone depletion and global climate change.
It is well worth the time to spend an hour or two reading this 24 pages paper. It is well written, and relatively easy to understand. The main conclusion of Lu is that the global warming observed from 1970 to 2002 and the following slight cooling can be explained by the action of cosmic-rays on (anthropogenic) atmospheric halogens; the absorption bands of the usual GHG’s are more or less saturated, so that increasing CO2 for instance does not play a visible role from 1970 to present, and will not in the future.
Variations of total ozone column
The chlorine needed for ozone destruction is created by cosmic rays through a process called DET (dissociative electron transfer, a process running on the tiny ice crystals in Polar Stratospheric Clouds (PSC’s). As the intensity of cosmic rays is modulated by the solar wind (and as such by solar activity), one should observe an 11 year cycle in the Antarctic ozone hole. This is indeed the case:
LU gives a very simple equation that shows that the relative change in total ozone column is proportional to the concentration of halogens (or more precisely the “equivalent effective chlorine” Ci delayed by about 2 years for Antarctica and 10 years for mid-latitudes) and the cosmic ray intensities in the preceding and current year:
This figure shows the equivalent effective chlorine (due to halogen dissociation) peaking around 1995, and the excellent parabolic fit of the relative total ozone column change to time (note spelling error in legend to red curve!). As anthropogenic halogen emissions are supposed to fall during the coming years (in accordance to the Montreal protocol), a total recovery back to the 1980 level is expected for 2050-2060.
LU accepts without discussion that all CFC’s in the atmosphere have a human origin. This might not be absolutely true, as some researchers show that CFC’s also may have a natural origin, like volcanoes (see here). So the conjecture of continuously lower concentrations in the future might not be rock-solid. Nevertheless, if LU is correct up to this point, reducing CFC’s emission through the Montreal protocol was the correct thing to do, and the photo-chemical reactions discovered by Molina and Crutzen are at least broadly correct.
Link to global warming
This second part of the papers has attracted the most vigorous critics (see here and here), often with the argument that a good correlation is not necessarily a sign of a strong causation. As according to LU all absorption bands of GHG’s are saturated (an argument also given by numerous greenhouse “sceptics”), one need only compute the radiative forcing from the halogens, and use this forcing to compute a global temperature change. Doing this he finds a really high correlation between global warming and total halogen concentration, as shown in this figure:
Note that the CFC concentration nicely follows the plateau in global temperature anomaly (see C and D). Extending his conclusions into the near future would suggest a gradual cooling extending at least until 2050, as resumed in this figure:
The wiggles in the green temperature curve represent the influence of the 11 year solar cycle.
According to LU, the only variations in solar activity that have a climatic effect are those that change the intensity of cosmic rays, i.e. the solar wind. Neither changes in total solar nor UV irradiances would do much to the global temperature, nor the (rising) concentrations of greenhouse cases. With these two assumptions, LU squeezes the toes of the alarmists as well of those who think that climate change is a total natural phenomenon. Hypothesizing that cosmic rays mediated changes in halogens is the climate driver does in fact unite both those who see human activity as the evil, and those who think that the sun is the BIG driver of climate change. No wonder that overall reactions are mostly unsympathetic!
Nevertheless this is a quite interesting paper, and as said above, very readable. It does not try to bury the arguments in obscure mathematics or baroque statistics. Hopefully another team of researchers will try its hands on this, so that the scientific debate over the climatic role of halogens would become more intense.