What governs the spread in shortwave forcings in the transient IPCC AR4 models?

• Published in: Geophysical research letters Vol. 36; no. 1; pp. L01806 - n/a
• Main Authors: Storelvmo, T., Lohmann, U., Bennartz, R.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Geophysical Union 01.01.2009; Blackwell Publishing Ltd; John Wiley & Sons, Inc
• Subjects: aerosol indirect effect; Aerosols and particles; Atmospheric aerosols; Atmospheric Composition and Structure; Atmospheric sciences; Chemistry; Climate change; Cloud physics and chemistry; Cloud/radiation interaction; Clouds; Earth; Earth sciences; Earth system modeling; Earth, ocean, space; Exact sciences and technology; Global Change; Impacts of global change; Physics; Radiation; radiative forcings; transient simulations; transient simulations; radiative forcings; aerosol indirect effect; short period waves; aerosols; droplets; simulation; concentration; global; Forcing; clouds; transient phenomena; uncertainties; Ocean model
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2008GL036069

The coupled global atmospheric‐ocean models used for transient simulations in the IPCC AR4 report differences in the present‐day shortwave forcing of more than 2 W/m2. We show here that about 1.3 W/m2 of this spread could be explained by the different methods used to calculate cloud droplet number concentration (CDNC) from aerosol mass concentrations. Although we cannot rule out that other forcing agents could yield comparable uncertainties, this strongly points to the aerosol indirect effect as the main contributor to the wide spread in the shortwave forcing reported in IPCC AR4.