Part Two / How our Planet is Changing

2.5 Clouds

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  1. The climate impact of removing all clouds The impact of clouds on the Earth’s radiation budget, known as ‘cloud-radiative effect’, is estimated at –19.6 W m–2, Loeb et al. (2020), ‘Toward a consistent definition between satellite and model clear-sky radiative fluxes.’ J. Clim. 33, 61–75, doi: 10.1175/JCLI-D-19-0381.1.

    This means that removing all clouds would be equivalent to a radiative forcing of +19.6 W m–2. By contrast, the forcing of a doubling of atmospheric carbon dioxide is estimated at +3.93 W m–2, Forster, P., T. Storelvmo, K. Armour, W. Collins, J.-L. Dufresne, D. Frame, D. J. Lunt, T. Mauritsen, M. D. Palmer, M. Watanabe, M. Wild, and H. Zhang, 2021: ‘The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity.’ In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 923–1054, doi:10.1017/9781009157896.009, approximately five times smaller.

    Cloud feedback has been a major uncertainty The Charney Report (1979), which was the first model-based assessment of the climate impact of human carbon dioxide emissions, had already noted the large uncertainty in how clouds would affect climate change. National Research Council, 1979. Carbon Dioxide and Climate: A Scientific Assessment. Washington, DC: The National Academies Press, https://doi.org/10.17226/12181.

  2. implying an enhanced blanket effect The greenhouse (or ‘blanket’) effect of clouds is proportional to their altitude: higher clouds are colder, and therefore emit less heat radiation. Hence, rising clouds (while all other things remain equal) mean less loss of heat by radiation to space – a warming effect.

    the amplifying effect of clouds becomes stronger Bjordal et al. argue that cloud feedback in polar regions becomes more amplifying at higher temperatures, because the feedback depends on the ratio of ice to liquid water in the cloud: Bjordal, J., et al., ‘Equilibrium climate sensitivity above 5°C plausible due to state-dependent cloud feedback’, Nature Geoscience, 13 (11), 2020: 718–21, https://doi.org/10.1038/s41561-020-00649-1. Tentative evidence of clouds acting as a tipping point was provided by Schneider, T., et al., ‘Possible climate transitions from breakup of stratocumulus decks under greenhouse warming’, Nature Geoscience, 12 (3), 2019: 163–7, https://doi.org/10.1038/s41561-019-0310-1, with an abrupt reduction in the amount of tropical low cloud once atmospheric carbon dioxide concentration exceeds 1,200 parts per million – but note that this is several times higher than present-day levels.