Climate Change 2001:
Working Group I: The Scientific Basis
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5. Aerosols, their Direct and Indirect Effects


Executive Summary

5.1 Introduction

5.1.1 Advances since the Second Assessment Report
5.1.2 Aerosol Properties Relevant to Radiative Forcing
5.2 Sources and Production Mechanisms of Atmospheric Aerosols
5.2.1 Introduction
5.2.2 Primary and Secondary Sources of Aerosols Soil dust Sea salt Industrial dust, primary anthropogenic aerosols Carbonaceous aerosols (organic and black carbon) Primary biogenic aerosols Sulphates Nitrates Volcanoes
5.2.3 Summary of Main Uncertainties Associated with Aerosol Sources and Properties
5.2.4 Global Observations and Field Campaigns
5.2.5 Trends in Aerosols
5.3 Indirect Forcing Associated with Aerosols
5.3.1 Introduction
5.3.2 Observational Support for Indirect Forcing
5.3.3 Factors Controlling Cloud Condensation Nuclei
5.3.4 Determination of Cloud Droplet Number Concentration
5.3.5 Aerosol Impact on Liquid-Water Content and Cloud Amount
5.3.6 Ice Formation and Indirect Forcing
5.4 Global Models and Calculation of Direct and Indirect Climate Forcing
5.4.1 Summary of Current Model Capabilities Comparison of large-scale sulphate models (COSAM) The IPCC model comparison workshop: sulphate, organic carbon, black carbon, dust, and sea salt Comparison of modelled and observed aerosol concentrations Comparison of modelled and satellite- derived aerosol optical depth
5.4.2 Overall Uncertainty in Direct Forcing Estimates
5.4.3 Modelling the Indirect Effect of Aerosols on Global Climate Forcing
5.4.4 Model Validation of Indirect Effects
5.4.5 Assessment of the Uncertainty in Indirect Forcing of the First Kind
5.5 Aerosol Effects in Future Scenarios

5.5.1 Introduction
5.5.2 Climate Change and Natural Aerosol Emissions Projection of DMS emissions in 2100 Projection of VOC emissions in 2100 Projection of dust emissions in 2100 Projection of sea salt emissions in 2100
5.5.3 Simulation of Future Aerosol Concentrations
5.5.4 Linkage to Other Issues and Summary

5.6 Investigations Needed to Improve Confidence in Estimates of Aerosol Forcing and the Role of Aerosols in Climate Processes


Co-ordinating Lead Author
J.E. Penner

Lead Authors
M. Andreae, H. Annegarn, L. Barrie, J. Feichter, D. Hegg, A. Jayaraman, R. Leaitch, D. Murphy, J. Nganga, G. Pitari

Contributing Authors
A. Ackerman, P. Adams, P. Austin, R. Boers, O. Boucher, M. Chin, C. Chuang, B. Collins, W. Cooke, P. DeMott, Y. Feng, H. Fischer, I. Fung, S. Ghan, P. Ginoux, S.-L. Gong, A. Guenther, M. Herzog, A. Higurashi, Y. Kaufman, A. Kettle, J. Kiehl, D. Koch, G. Lammel, C. Land, U. Lohmann, S. Madronich, E. Mancini, M. Mishchenko, T. Nakajima, P. Quinn, P. Rasch, D.L. Roberts, D. Savoie, S. Schwartz, J. Seinfeld, B. Soden, D. Tanré, K. Taylor, I. Tegen, X. Tie, G. Vali, R. Van Dingenen, M. van Weele, Y. Zhang

Review Editors
B. Nyenzi, J. Prospero

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