These days, the biggest environmental issue in the news is global warming. The world is getting warmer, whether by natural cyclical events or as a result of man-made pollutants. The most accepted theory behind this phenomenon is called the greenhouse effect. Whether or not it is the sole cause of the rising temperatures is still up for debate.
The basic idea is this:
Earth's atmosphere is made up of a large number of various gases, each with different properties. Certain gases and chemicals let sunlight pass through easily, but once that light has bounced back off the planet as heat, those gases begin to trap the radiation. So instead of there being an even balance of heat coming in and going out, it begins to accumulate and the temperature goes up. The gases act exactly like the panes of glass in a greenhouse, letting light in but not letting heat out.
Certain gases have more powerful greenhouse properties than others, and many of them are naturally occuring in the atmosphere. Though when I say they are natural, that doesn't mean man's activities and pollution hasn't had an impact.
Water Vapour - This common gas is the most plentiful of the greenhouse gases in the atmosphere. It's also the most uneffected by man. The humidity levels in the air are governed by many biological and weather processes, and basically regulates itself.
Carbon Dioxide - The next most abundant greenhouse gas. Though a natural occuring gas, CO2 levels are also strongly effected by man, particularly with the burning of fossil fuels (oil, coal and natural gas) as well as wood and other solid wastes. One excellent reason to switch to alternative energy sources such as solar or wind power.
Methane - Methane is best known as a gas released by herds of livestock, and during the decomposing of organic materials in landfill dumps.
Nitrous Oxide - The last of the big greenhouse gases, released during various agricultural and industrial practices. Nitrogen-based chemical fertilizers are the biggest culprit. Another reason to look for organic produce.
Now, besides these naturally occuring gases, there are a number of strictly man-made chemicals that also contribute to the greenhouse effect, such as: hydrofluorocarbons, perfluorocarbons and other related compounds.