Interactions between air quality and climate change - Air Pollution Modeling and Its Application - page 441

6.4 Assessing Impacts of Aerosol Processes
on Equilibrium Climate Sensitivity

Trond Iversen 1,2 , Alf Kirkevåg 1 , Øyvind Seland 1 , Jens Debernard 1 ,
Jon Egill Kristjansson 2 , and Corinna Hoose 2

1

Norwegian Meteorological Institute, Oslo, Norway

2

Department of Geosciences, University of Oslo, Oslo, Norway

Abstract We study possible climate impacts of anthropogenic aerosols by modeling
their atmospheric life-cycles, and parameterizing their extinction of solar radiation
and interactions with clouds. We have developed and implemented such schemes
in the global atmospheric climate model CAM3 of the National Centre for Atmo-
spheric Research (NCAR). We present results from CAM-Oslo coupled to a slab
ocean. Equilibrium climate response, assuming different aerosol properties, are
estimated as the change in equilibrium values of surface air temperature and
precipitation due to changed atmospheric content of CO 2 or aerosol production.
Feed-back processes with aerosols are discussed, along with effects of an added
background cloud droplet number concentration (cdnc), and how CO 2 -induced
climate change may influence aerosols.

Keywords Aerosols, climate change, equilibrium climate sensitivity

1. Introduction and Model Tool

Indirect effects of aerosols and feedback effects of clouds contribute large uncertainty
to calculated climate projections (Stainforth et al., 2005; Solomon et al., 2007).
Modeling the cooling effects of aerosols is a major source of uncertainty in
estimates of climate sensitivity to forcing (Andréa et al., 2005; Figure 2.20 in
Forster et al., 2007), implying a risk of a stronger 21st century warming than
anticipated from greenhouse gases alone. The uncertainty in climate sensitivity is
therefore important when assessing possible anthropogenic climate change.
We have calculated atmospheric life-cycling and physico-chemical properties
of aerosols on-line in the global community atmospheric model 3 (CAM3) of the
National Center for Atmospheric Research (Collins et al., 2006). This model version
is named CAM-Oslo. Size-distributed composition and numbers are estimated
based on primary particle production of sea-salt, mineral dust, and nucleated
sulphate (SO 4 ), and condensation, coagulation, and aqueous production involving