Stronger Response to the Aerosol Indirect Effect Due To Cooling in Remote Regions

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. Plain Language Summary Aerosols, small particles suspended in the atmosphere, emitted by humans tend to cool the climate. They do this directly by reflecting incoming sunlight, and indirectly by affecting cloud properties foremost such that clouds reflect more sunlight. Here, we investigate how the global surface air temperature responds to changes in the two types of aerosol interaction with solar radiation. We find that the cloud effect causes a relatively larger global mean temperature change than the direct effect of the aerosol particles. Interactions between aerosols and clouds are difficult to represent in climate models and are sometimes excluded entirely. Our results highlight the importance of including the cloud effect to get an accurate representation of the Earth's climate. Key Points The forcing efficacy from an enhanced aerosol indirect effect is larger than unity The aerosol indirect effect induces remote cooling at mid‐ to high‐latitudes, in contrast to the local cooling from the direct effect The different spatial patterns of temperature change from the aerosol direct and indirect effects excite different feedbacks