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# Substantial twentieth-century Arctic warming caused by ozone-depleting substances

## Abstract

The rapid warming of the Arctic, perhaps the most striking evidence of climate change, is believed to have arisen from increases in atmospheric concentrations of GHGs1 since the Industrial Revolution. While the dominant role of carbon dioxide is undisputed, another important set of anthropogenic GHGs was also being emitted over the second half of the twentieth century: ozone-depleting2 substances (ODS). These compounds, in addition to causing the ozone hole over Antarctica, have long been recognized3 as powerful GHGs. However, their contribution to Arctic warming has not been quantified. We do so here by analysing ensembles of climate model integrations specifically designed for this purpose, spanning the period 1955–2005 when atmospheric concentrations of ODS increased rapidly. We show that, when ODS are kept fixed, forced Arctic surface warming and forced sea-ice loss are only half as large as when ODS are allowed to increase. We also demonstrate that the large impact of ODS on the Arctic occurs primarily via direct radiative warming, not via ozone depletion. Our findings reveal a substantial contribution of ODS to recent Arctic warming, and highlight the importance of the Montreal Protocol as a major climate change-mitigation treaty.

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## Data availability

All model output analysed in the study is currently stored on data servers at the National Center for Atmospheric Research in Boulder, CO, USA, and are available from the corresponding author on request. The GISTEMP27 data are available at https://data.giss.nasa.gov/gistemp and the HadISST28 data can be found at https://www.metoffice.gov.uk/hadobs/hadisst2.

## Code availability

All code used to produce the figures are available from the corresponding author on request.

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## Acknowledgements

All computations were performed with resources provided by the Computational and Information Systems Laboratory at the National Center for Atmospheric Research, which is sponsored by the US National Science Foundation. The historical CAM5LE integrations were performed by the Large Ensemble22 Project. This research was funded by two grants from the USNSF to Columbia University. L.M.P. is grateful to J. Kay, and the other organizers, for the opportunity to attend the 2018 CESM Polar Modeling Workshop in Boulder, CO, USA. The authors are indebted to P. Forster and J. Fyfe for suggesting an important clarification.

## Author information

Authors

### Contributions

L.M.P. designed the study, carried out WACCM4 integrations and wrote the first draft of the manuscript. M.P. suggested and performed feedback analysis. M.R.E. carried out CAM5LE integrations and helped with their analysis. G.C. computed radiative forcing with the PORT model. K.L.S. helped with analysis of WACCM4 integrations. All authors contributed to the interpretation of the results and to the drafting of the final manuscript.

### Corresponding author

Correspondence to L. M. Polvani.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Climate Change thanks Francisco Estrada and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 Seasonal changes, 1955-2005.

Monthly, ensemble-mean, 1955-2005 change in (a) global and (b) Arctic surface air temperature ($${{\rm{T}}}_{{\rm{s}}}$$) and (c) Sea Ice Extent (SIE, the total area with sea ice concentration exceeding 15%), in the CAM5LE model. On the curves for the FixODSO3 and FixODS ensembles, small and large circles highlight the months in which the difference with the Historical ensemble is statistically significant at the 95% and 99% level, respectively, (from a two-tailed t-test).

### Extended Data Fig. 2 Arctic amplification, 1955-2005.

Arctic amplification factor for the three CAM5LE ensembles over the period 1955-2005. The factor is defined as the annual mean surface air temperature change over the Arctic (60-90$${}^{\circ }$$ N) divided by the corresponding global mean change. The boxes extend from the lower to upper quartile, with a line at the median, and with whiskers showing the entire range across each ensemble; the individual members are shown by the small black dots. The difference between the means of the FixODSO3 and Historical ensembles is statistically significant at the 95% level.

### Extended Data Fig. 3 Synthetic PDFs of 1955-2005 changes.

Probability distribution functions (PDFs) of annual global surface temperature change (top), Arctic surface temperature change (middle), and September sea ice loss computed from the Historical (red) and FixODSO3 (blue) simulations with the CAM5LE model. These PDFs are constructed by ‘resampling with replacement’ âĂŞ 10,000 times âĂŞ the original set of 10 model simulations.

### Extended Data Fig. 4 Arctic surface temperature and sea ice trends, 1955-2005.

As in Fig. 2, but for the WACCM4 model integrations.

### Extended Data Fig. 5 Arctic feedbacks, 1955-2005.

As in Fig. 4, but for the WACCM4 model integrations.

## Rights and permissions

Reprints and Permissions

Polvani, L.M., Previdi, M., England, M.R. et al. Substantial twentieth-century Arctic warming caused by ozone-depleting substances. Nat. Clim. Chang. 10, 130–133 (2020). https://doi.org/10.1038/s41558-019-0677-4

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