On page 1299 of this issue, Haywood et al . Their unique integration of observations and models shows how a better understanding of the aerosol problem can be reached. Emissions from industry are a major source of aerosols in the atmosphere. A question of uncertainties. Positive forcing corresponds to warming and negative forcing to cooling of the atmosphere.
Large uncertainties exist, particularly in the aerosol forcing.
The complexity of this problem seems to grow with each new study. Uncertainties in the direct effect arise from the amount and distribution of aerosols in the atmosphere and their chemical and physical properties, which determine their effectiveness at reflecting sunlight back to space. Interactions between different types of aerosols may also affect the magnitude of direct forcing. Larger particles reflect less sunlight, and formation of sulfate on sea salt therefore reduces the overall magnitude of the sulfate forcing. Sulfate formation can also occur on mineral dust and carbon aerosols, again reducing the sulfate forcing.