Greenhouse gases help trap the earth's radiant heat, keeping our lower atmosphere warm. The majority of scientists believe that increased production of these gases is contributing to changes in the earth's climate. The possible effects of this trend include hotter temperatures, increased disease and unstable climates causing extreme weather events.
Air quality is also heavily affected by global warming. Hotter days and increased sunlight produce optimal conditions for the creation of ground level ozone, the chief component of smog.
Many of the steps we take to reduce air pollution will also reduce emissions of greenhouse gases. By reducing our consumption of fossil fuels we will be simultaneously improving air quality and helping to "cool" the planet.