Human-caused emissions of aerosols – tiny, light‑scattering particles produced mainly by burning fossil fuels – have long acted as an invisible brake on global warming. 

This is largely because they absorb or reflect incoming sunlight and influence the formation and brightness of clouds. 

These combined effects act to lower regional and global temperatures.

Aerosols also have a substantial impact on human health, with poor outdoor air quality from particulate matter contributing to millions of premature deaths per year.  

Source: Carbon Brief