What environmental issue is primarily linked to the combustion of fossil fuels?

The combustion of fossil fuels is primarily linked to air pollution due to the release of various harmful pollutants into the atmosphere. When fossil fuels such as coal, oil, and natural gas are burned for energy, they produce carbon dioxide (CO2), nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter. These emissions contribute to the formation of smog, respiratory problems, and other health issues, as well as affecting the environment through the degradation of air quality.

Air pollution from fossil fuel combustion can also lead to acid rain, which further impacts ecosystems, water quality, and soil health. The extensive use of fossil fuels in transportation, industry, and electricity generation enhances the concentration of these pollutants, making air pollution one of the most significant environmental concerns associated with fossil fuel usage. Other options listed, such as desertification, ocean acidification, and overfishing, stem from different causes associated with land use, carbon dioxide absorption in oceans, and fishing practices, respectively, but are not directly linked to the direct emissions from fossil fuel combustion.