In part 2 and 3, we discussed how different substances reflect, emit and absorb different wavelengths of radiant energy, and how they impact the energy balance. Many pollutants also selectively absorb and emit radiation. Greenhouse gases are an example, but not all greenhouse gases are pollutants. Other substances in the atmosphere selectively absorb and emit radiation at other wavelengths, which impact many chemical processes. Many of these processes can have very adverse effects on the environment. For example, if concentrations of greenhouse gases increase significantly, the average temperature of the atmosphere could rise, possibly melting the polar ice caps. If the polar ice caps were to melt completely, coastlines around the world would shift inland around 30 miles. This would flood most coastal cities and river cities, where over 50% of the human populations live. Of course this change would probably happen gradually, but where would we put so many people? However, an increase in average air temperature would also mean that more water could be evaporated into the air. It would then make sense that there would be more clouds present, especially in moist areas (in tropics and near coasts). The increase in cloud cover would block incoming solar radiation, producing a cooling effect. Would this cooling effect counteract the increase in temperatures due to the greenhouse effect? We don't really know, as scientists are still arguing that question. The interaction between pollution and heat balance is a major focus of many air quality researchers.
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