Attribution of Regional and Global Climate Change: Relative Effects of Fossil-Fuel Soot, Methane, Other Greenhouse Gases and Particles, and Urbanization

Black carbon, the main component of fossil-fuel soot (FFS), warms the air first by absorbing sunlight. Its absorption is enhanced by optical focusing when it becomes coated during vapor condensation or aerosol- aerosol coagulation, when it enters cloud drops or ice crystals during nucleation scavenging or aerosol- hydrometeor coagulation, and when it is surrounded by sea ice or snow following its precipitation or dry deposition. Its absorption over snow, sea ice, desert, fog, and cloud surfaces is further enhanced by the high reflectivity of these surfaces, which increases the number of photons hitting a soot particle. Although soot has a short lifetime, the air that it warms persists to larger scales. Soot's effective lifetime is also extended when it deposits to snow and sea ice. Since the organic material emitted with FFS is mostly hydrophobic, soot's effects on cloud activation are delayed thus weaker than sulfate's effects. Here new results for the climate response of fossil-fuel soot (black carbon, organic matter, sulfate), accounting for the factors listed above and for size resolution of aerosol particles and clouds and the aging of soot through the treatment of two size distributions, are presented. The results are compared with the climate responses of all anthropogenic aerosol particles, anthropogenic methane, all anthropogenic greenhouse gases, all anthropogenic greenhouse gases and aerosol particles, and urbanization. Fossil-fuel sources of black carbon treated include land-based, shipping, and aircraft. The study finds that fossil-fuel soot appears to have a stronger effect on global near- surface temperatures than either methane or urbanization, thus it may be the second-leading cause of historic near-surface global warming after carbon dioxide. Methane is found to have a stronger effect on near-surface temperatures than urbanization. FFS exacerbates warming due to greenhouse gases in Russia and over the Arctic sea ice. FFS causes little regional cooling in contrast to all aerosol particles, which, on their own, cause strong cooling in the southeast U.S., Europe, and China. The combination of all anthropogenic aerosol particles and greenhouse gases explains much of the difference between current and historic regional temperatures on a global scale. Whereas methane and other greenhouse gases cool the stratosphere, neither FFS nor urbanization do so significantly. The results here apply only to fossil-fuel soot. Biomass-burning particles, which contain black carbon, have a different composition from FFS and a different climate effect.