Air pollution, in the form of aerosols, affects the climate on a large scale.[126] Aerosols scatter and absorb solar radiation. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth’s surface was observed. This phenomenon is popularly known as global dimming,[127] and is primarily attributed to sulfate aerosols produced by the combustion of fossil fuels with heavy sulfur concentrations like coal and bunker fuel.[50] Smaller contributions come from black carbon, organic carbon from combustion of fossil fuels and biofuels, and from anthropogenic dust.[128][49][129][130][131] Globally, aerosols have been declining since 1990 due to pollution controls, meaning that they no longer mask greenhouse gas warming as much.[132][50]
Aerosols also have indirect effects on the Earth’s energy budget. Sulfate aerosols act as cloud condensation nuclei and lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.[133] They also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight.[134] Indirect effects of aerosols are the largest uncertainty in radiative forcing.[135]
While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise.[136] Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.[137] The effect of decreasing sulfur content of fuel oil for ships since 2020[138] is estimated to cause an additional 0.05 °C increase in global mean temperature by 2050.[139]
