Atmospheric concentrations of many CFCs are now falling. In a curious turn of events, the warming effect of CFCs is offset by the fact that they destroy ozone, also a greenhouse gas, in the lower stratosphere. Ozone is a component of photochemical smog. There is strong evidence that ozone concentrations in the lower atmosphere are higher than in pre-industrial times, especially in the northern hemisphere. Atmospheric aerosol Aerosol are fine particles and droplets suspended in air.
Some human activities increase the levels of aerosol in the atmosphere.
The greatest source of aerosol is sulfur dioxide, a gas released by coal-burning power stations and during the processing of mineral ores. Once the sulfur dioxide is in the atmosphere, chemical reactions convert it into sulfate aerosol. Likely changes to global climate Scientists are using sophisticated computer models of the world's atmosphere, surface and oceans to examine likely future changes to climate due to the enhanced greenhouse effect. The models confirm that increasing levels of greenhouse gases will produce a warming at the earth's surface. They also show that the enhanced greenhouse effect is likely to lead to world-wide changes in weather and climate.