AIR POLLUTION

As residents of Earth, most of us are more immediately aware of the state of our air than of our land or water. We live in the lower atmosphere and interact with the air for our daily survival. On a hot, hazy day in summer, pollution may hang low over our cities: We can feel it, breathe it in, and depending where one lives, perhaps actually smell it. In recent decades our awareness of the toxins and particulates that have entered the atmosphere has grown. We also sense that the air from day to day is warmer, and we have learned that the 1990s were possibly the warmest decade of the last millennium. The greenhouse gases in Earth's atmosphere are actually good for the planet. They have been there since the atmosphere first formed, and they keep our planet from becoming an icy mass. Without the natural greenhouse effect, the Earth would be more than 50°F colder than it is today, with an average global temperature of only 5°F much less hospitable to life. But in the case of greenhouse gases, too much of a good thing is not good. The planet has been experiencing an increase in carbon dioxide in the atmosphere, generated in large part from fossil fuel emissions and the proliferation of other greenhouse gases such as nitrous oxide and methane. What has caused this increase? By and large, human activities are to blame. At one level, greenhouse gases protect us, but when they reach a higher level, they become a threat to com fort, safety, and even life. Global temperatures are predicted to increase by 4 percent to 20 per cent on average through the end of the 21st century. Scientists who study the air and the atmosphere are not all in agreement about either the causes or the ramifications of the current trend toward global warming. Most do agree, however, that the trend began as far back as 1750, the beginning of the industrial revolution. In the absence of definitive knowledge of the future, say many, we must plan for a worst-case scenario. The efforts begin by understanding the current science behind the changes in Earth's ozone layer and the resulting greenhouse effect.