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IPCC AR5 WGI: Chapter 7

Clouds & Aerosols

    - A Review

    The objective of this website is to serve as a class project of CU-ATOC7500: Reading IPCC Report. It is a review on Chapter 7 of the Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report (AR5) Working Group I (WGI) [cover page on the right]. In AR5, the radiative forcing of clouds and aerosols was for the first time, extensively assessed in a dedicated IPCC chapter. This arrangement, to some extent, reflects the important roles of clouds and aerosols in climate change. The authorship of Chapter 7 is shown below. 

This chapter should be cited as:

Boucher, O., D. Randall, P. Artaxo, C. Bretherton, G. Feingold, P. Forster, V.-M. Kerminen, Y. Kondo, H. Liao, U. Lohmann, P. Rasch, S.K. Satheesh, S. Sherwood, B. Stevens and X.Y. Zhang, 2013: Clouds and Aerosols. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

  Key Assessments

 

This chapter of IPCC AR5 thoroughly assessed the properties and processes associated with clouds and aerosols, their interactions and radiative effects. Influence of clouds and aerosols on precipitation and advances in solar radiation management were also reviewed.

 

  • Processes/feedbacks now understood with medium to high confidence include:

    • rising of high cloud tops and melting levels;

    • increased polar cloud covers and/or optical thickness;

    • broadening of the Hadley Cell and poleward migration of storm tracks;

    • narrowing of the Intertropical Convergence Zone;

    • new particle formation (nucleation) and condensation in aerosol formation;

    • aerosol dry deposition.

  • Using Effective Radiative Forcing (ERF) and distinguishing it from Feedbacks;

    • Net global mean cloud radiative effect (CRE) is -20 W m^-2 (p580);

    • Net feedback due to all cloud types likely to be +0.6 W m^-2 °C^-1 (p592);

    • Radiative forcing due to aerosol-radiation interaction (RFari) is -0.35 W m^-2 (p615, 682);

    • Radiative forcing due to black carbon (BC) on snow and ice is +0.04 W m^-2 (p618);

    • Effective radiative forcing due to aerosol-radiation and aerosol-cloud interactions excluding BC on snow and ice (ERFari+aci) is -0.9 W m^-2 (p620, 683), and is unnecessarily equal to the sum of ERFari and ERFaci calculated separately;

  • Over oceans at the large scale, there is a trend of "wet-get-wetter" and "dry-get-drier". Aerosol-cloud interactions might at most affect individual storms. The evidence for a systematic aerosol effect on storm or precipitation intensity is lacking;

  • Solar Radiation Management (SRM) needs continuous implementation. Any abrupt temination will cause harmful side effects on ecosystem due to rapid bouncing back to 'non-SRM' temperature and precipitation rate.

Chapter 7 (Clouds & Aerosols) is very closely related to Chapter 8 (Anthropogenic and Natural Radiative Forcing). The assessments made by Chapter 7 also affect the assessments in Chapter 11 (Near Term Climate Change) and Chapter 12 (Long Term Climate Change) due to the influence of clouds/aerosol radiative forcing estimates on projected future climates. The models used to assess the clould/aerosol radiative effects are evaluated in Chapter 9 (Evaluation on Climate Models).

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