There’s a tricky chemical trade-off at work in our skies. As greenhouse gases provide their famous warming effect to Earth’s surface, aerosol pollution in the atmosphere actually partly counteracts it. Aerosols are tiny particles suspended in the air, both natural and industrial, including sea salt, mineral dust, ash, soot, sulphates, nitrates, and black carbon. They hang around in the air for around 10 days, scattering and absorbing radiation from the sun. Aerosols also provide nuclei for water droplets, boosting cloud formation, thus decreasing the amount of energy reaching the ground and providing a net cooling force. In short, greenhouse gases warm the surface; aerosols cool the surface.