 Greenhouse gases aren't the only thing that we humans are pouring into the atmosphere. We're also pouring particulate matter or we often call them aerosols into the atmosphere. Now these aren't gases. These are small particles and they stay up anywhere from a couple of days to a few months depending on how big and how heavy the particles are. A good example is smoke. Smoke from a smokestack from burning coal for example or smoke from a bush fire which is a natural aerosol which goes into the atmosphere. Aerosols mainly work to cool the atmosphere so they act in opposition to greenhouse gases and they do that because they scatter incoming sunlight. So some of that sunlight doesn't make it to the earth's surface. So that has the effect of actually cooling the earth's surface. So in fact ironically even though it's not good for us because it's local air pollution and it affects human health negatively directly it does provide us a benefit by slowing the cooling a bit. So right now the earth is at about 1.1 degree Celsius above pre-industrial but if we stripped out the aerosols that would go to 1.4 or 1.5 degrees even as we speak today. So they are cooling by 2 or 3 or 4 tenths of a degree. Now there is one exception to that and there are some carbon particles put out as soot. They actually absorb sunlight and they actually add to the heating. So we have to look at this balance between the aerosols that scatter and cool the aerosols that absorb and heat. On balance it's a net cooling effect so we understand that pretty well. But ironically as we clean up air pollution and there's a good reason for doing that for health reasons we're actually going to add to the warming of the atmosphere.