It is often assumed that effective radiative forcings, regardless of
forcing agent, are additive in the temperature change. Using climate
model simulations with abruptly applied aerosol forcing we find that the
temperature response per unit forcing is larger if induced by
aerosol-cloud interactions than directly by aerosols. The spatial
patterns of forcing and temperature change show that aerosol-cloud
interactions induce cooling over remote oceans in the extratropics,
whereas the effect of increased emissions is localized around the
emission sources primarily over tropical land. The results are
consistent with ideas of how the patterns of sea surface temperature
impact radiative feedbacks, and a large forcing efficacy of
aerosol-cloud interactions could help explain previously observed
intermodel spread in the response to aerosols.