the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jan 2026
Indirect climate forcing from ozone depleting substances
Abstract. Many halocarbons are powerful greenhouse gases and also influence climate indirectly through depletion of stratospheric ozone which opposes their direct greenhouse effect. Changes in effective radiative forcing (ERF) from historical ozone depletion have been diagnosed from model experiments with perturbed halocarbons run under the sixth Coupled Model Intercomparison Project. This is more negative than the offline stratospheric-temperature-adjusted radiative forcing (SARF). Including effects of ozone depletion on the methane lifetime makes the historical net ERF of ozone depleting substances consistent with zero. The Integrated Ozone Depletion (IOD) metric has been used to apportion this ERF between the halocarbon species and thereby derive indirect 100-year Global Warming Potentials (GWP100s) for a suite of halocarbons. The indirect GWP100 for CFC-11 is enough to make the net GWP100 likely negative, whereas the indirect contribution for CFC-12 is smaller due to a combination of longer stratospheric lifetime and fewer chlorine atoms. use of the online ERF, rather than the offline SARF, allows the model physics to account for changes in stratospheric temperature (as well as tropospheric temperature, water vapour and clouds) rather than estimating stratosphere temperature changes using fixed dynamical heating. This online calculation of radiative forcing rather than offline leads to approximately double the indirect GWPs compared to World Meteorological Organization assessments. This formalism can be used with other estimates of ozone ERF, as the indirect GWPs scale linearly with this quantity.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 28 Feb 2026)
- RC1: 'Comment on egusphere-2025-6033', Anonymous Referee #1, 05 Feb 2026 reply
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General: This is a very nice multi-model experiment on ERFs from halocarbons. The effort that has gone into this manuscript is commendable. Could the authors include some evidence that the models are all capable of representing key parameters such as stratospheric distributions of O3 and ODSs (e.g., by comparisons to observations)? Considering the large spread in ozone depletion that these models produce (e.g., from looking at Fig. 2 and 4), one gets the impression that they might however not be very suitable for assessing any effects from ODSs. Should that perhaps be the main message of the paper? Certainly, a better description of, and justification for, the calculation of uncertainties is needed. Here a few more detailed comments:
l51-54: Not sure why halons are not mentioned here. Why mention two substance classes that are not covered but not all that are part of the focus.
l145-146/fig 1: This slope is probably dependent on the location in the stratosphere, most commonly approximated by the mean age of air. Can the authors provide evidence that it (i.e., the slope) can be generalised throughout the extra-polar stratosphere? Also, what is the uncertainty range of this slope, and how was that taken into account in the analysis?
l152 pp: This is an outdated definition of EESC. In the two most recent WMO Ozone Assessments, improved EESC calculations have been used that include, e.g., the shape of the age spectrum, as well as time-independent fractional release. It is unclear why the authors have chosen to ignore these developments here (although they appear to be aware of some of them as Engel et al. (2018) is cited and partly used later).
l159 pp: Have the authors assessed the uncertainties or biases introduced if concentrations are not constant (as they are in reality)?
l180-293 Clearly, the model-setups are already very different, which might explain the massively different levels of ozone depletion in the runs (as shown in Fig. 2 and 4). A table might enable the reader to get a better overview of the differences in Cl and Br concentrations and how they evolve over time. For instance, the Cl and Br catalytic ozone depletion cycles are known to enhance each other, so getting both of them right is crucial (as are many other things, e.g., heterogenous chemistry, vortex dynamics,..). Looking, e.g., at line 344, the chemical mechanism could therefore be very similar but still produce different O3 depletion results. More generally, the current descriptions are lacking an overview/comparison of such key parameters which prevents a proper assessment of the performance of the models.
l333-334: Using a "fill factor" from a 1997 publication seems highly inappropriate here. These factors will have changed drastically over time, depending on the temporal evolution of the tropospheric concentrations of the individual halocarbons. They therefore cannot be applied as a blanket solution. This oversimplified approach is all the more puzzling since one of the great advantages of models is the wealth of data they provide. It should well be possible (if a bit onerous) to derive time-dependent global-scale fill factors from these data, which could serve as a quality check of the models as well if compared with existing observational data sets such as the one from Volk et al. (1997).
l401 pp.: Could the observed O3 column difference (including uncertainty envelope) be added to this figure? It would enable a better perspective on which model might be closest to reality (as clearly not all of them can be very close).
l440-442: This is partly misleading. For example, while many HCFCs appear to have negative GWP100 (at least if one believes the results and especially uncertainties of this model experiment), two of the three main ones (22 and 142b) have substantially positive ones. A total contribution from each substance class weighted by some (current) atmospheric concentration or EESC contribution would help shedding a more balanced light on the matter.
l449-450: How does this assumption from 30 years ago compare to reality?
l468: How are the uncertainty ranges in this table calculated? Looking at Fig. 4, where the polar O3 depletion varies by more than a factor of 2 between models, this does not seem to be reflected in these GWP100 uncertainties.
l472-473 Are there any other references to support this statement?
l490 Could you elucidate why you used the emergent constraint of Morgenstern et al. (2021)? Using results from a different study if your own ones have too large uncertainties does not seem very convincing.
l492-494: The main author of this study is one of the three authors of Western et al., which is apparently coming to a different conclusion (although this cannot be confirmed by the reader as the latter paper is still in review). What does this mean? Which study is right or at least better? Or is the lead author of this study really questioning his own work?