Soot and smoke aerosol may not warm climate

[1] Soot and smoke aerosol contain black carbon, which absorbs solar radiation. These aerosols may reduce the overall negative climate forcing of anthropogenic aerosols by absorbing radiation that might otherwise be scattered back to space. They may also reduce overall cloudiness, an effect termed the "semidirect'' effect, which is thought to enhance climate warming. Here, we evaluate the climate forcing associated with black carbon and other aerosols using the concept of "relaxed forcing,'' which is the forcing associated with two simulations using fixed sea surface temperatures. The consideration of longwave perturbations associated with the relaxed forcing leads to a diminished or even negative semidirect effect associated with absorbing aerosols rather than an enhanced warming. The overall forcing depends significantly on the altitude of injection of the aerosols because higher-altitude injections tend to enhance the negative longwave forcing. In addition, high-altitude injection of absorbing aerosols can increase cloudiness at lower altitudes where temperatures, in general, may decrease.