Dust and dirt in the air may come from natural sources such as volcanos, erosion and meteoric dust. Some of this dirt falls out within a few hours. Some is aerosol, so small that it could stay in the air for years. The aerosol particles in the atmosphere make the earth colder. The effect of dust therefore cancels out some of the effects of greenhouse gases. Even though humans also put aerosols in the air when they burn coal or oil this only cancels out the greenhouse effect of the fuel burning for less than 20 years: the carbon dioxide stays in the atmosphere much longer and keeps on warming the earth.