Future warming exacerbated by aged-soot effect on cloud formation

Aged soot particles in the atmosphere enhance future warming through their influence on cloud formation and thus Earth's radiation balance, according to global climate simulations. Clouds play a critical role in modulating the Earth's radiation balance and climate. Anthropogenic aerosol particles that undergo aging processes, such as soot, aid cloud droplet and ice crystal formation and thus influence the microphysical structure of clouds. However, the associated changes in cloud radiative properties and climate effects remain uncertain and are largely omitted in climate models. Here we present global climate simulations of past and future effects of both ozone-aged soot particles acting as cloud condensation nuclei and sulfuric acid-aged soot particles acting as ice-nucleating particles on the structure and radiative effects of clouds. Under pre-industrial conditions, soot aging led to an increase in thick, low-level clouds that reduced negative shortwave effective radiative forcing by 0.2 to 0.3 W m(-2). In the simulations of a future, warmer climate under double pre-industrial atmospheric carbon dioxide concentrations, soot aging and compensating cloud adjustments led to a reduction in low-level clouds and enhanced high-altitude cirrus cloud thickness, which influenced the longwave radiative balance and exacerbated the global mean surface warming by 0.4 to 0.5 K. Our findings suggest that reducing emissions of soot particles is beneficial for future climate, in addition to air quality and human health.