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The impact of modern black carbon aerosols on climate via their changes in radiative balance is studied using a coupled model where sea surface temperatures (SSTs) are allowed to vary and an atmosphere-only version of the same model where SSTs are held fixed. Allowing the ocean to respond is shown to have a profound impact on the pattern of temperature change. Particularly, large impacts are found in the North Pacific (which cools by up to 1K in the coupled model) and in north central Asia (which warms in the coupled simulation and cools in the fixed SST simulation). Neither set of experiments shows large changes in surface temperatures in the Southeast Asian region where the atmospheric burden of black carbon is highest. These results are related to the stabilization of the atmosphere and changes in oceanic heat transport. Over the North Pacific, atmospheric stabilization results in an increase in stratiform clouds. The resulting shading reduces evaporation, freshening the surface layer of the ocean and reducing the inflow of warm subtropical waters. Over the land, a delicate balance between greater atmospheric absorption, shading of the surface and changes in latent cooling of the surface helps to determine whether warming or cooling is seen. Our results emphasize the importance of coupling in determining the response of the climate system to black carbon and suggest that black carbon may play an important role in modulating climate change over the North Pacific.

Plain Language Summary Particles of soot produced by combustion have greatly increased around the world over the last century and a half. As these particles absorb sunlight that might otherwise have been reflected back to space, they are believed to warm the planet as a whole. We tried to isolate the fingerprint of temperature change associated with soot (also known as black carbon) by taking a climate model and instantaneously increasing atmospheric concentrations from their 1860 values to modern values. The resulting pattern of temperature change looks very different than the pattern of atmospheric heating, with the largest warming found away from the most polluted regions and a region of cooling over the North Pacific. Changes in clouds, mixing, and ocean circulation all play a role in explaining these changes. In different regions these three processes have differing responses to the stabilization of the atmosphere produced by the additional heating at high altitudes. Over the North Pacific, the additional heating aloft reduces mixing of dry air into the near-surface boundary layer and shades the surface, increasing relative humidity and allowing for more stratus clouds. Reduced evaporation causes the surface waters to freshen, slowing the overturning circulation that supplies warm water to this region.