1Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
2Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
3Department of Physical Geography and Ecosystems Analysis, Lund University, 22362 Lund, Sweden
4Institute of Meteorology and Climate Research, Karlsruhe Institute for Technology, Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany
5Department of Forest Science, University of Helsinki, P.O. Box 27, 00014 University of Helsinki, Finland
Abstract. The number concentration of cloud droplets determines several climatically relevant cloud properties. A major cause for the high uncertainty in the indirect aerosol forcing is the availability of cloud condensation nuclei (CCN), which in turn is highly sensitive to atmospheric new particle formation. Here we present the effect of new particle formation on anthropogenic aerosol forcing in present-day (year 2000) and future (year 2100) conditions. The present-day total aerosol forcing is increased from −1.0 W m−2 to −1.6 W m−2 when nucleation is introduced into the model. Nucleation doubles the change in aerosol forcing between years 2000 and 2100, from +0.6 W m−2 to +1.4 W m−2. Two climate feedbacks are studied, resulting in additional negative forcings of −0.1 W m−2 (+10% DMS emissions in year 2100) and −0.5 W m−2 (+50% BVOC emissions in year 2100). With the total aerosol forcing diminishing in response to air pollution control measures taking effect, warming from increased greenhouse gas concentrations can potentially increase at a very rapid rate.