THE GOOD....The greenhouse effect, when functioning normally, keeps our planet warm. Natural  gases in the atmosphere form a blanket which allows sunlight to reach the earth's surface, but prevents heat from escaping
(like the glass in a greenhouse). This gas blanket traps heat close to the surface, and warms the atmosphere.

THE BAD....But now human activities are increasing the amount of 'greenhouse gases' in the atmosphere, altering the Earth's climate...Unless we reduce emissions of these gases, the Earth may keep getting hotter, and the stable climate on which civilization is based could disappear.

THE UGLY....The Greenhouse Gases:

Carbon Dioxide. Responsible for about 71% of the greenhouse effect.
Main sources of carbon dioxide: Burning fossil fuels like coal, oil, and natural gas,
and destruction of forests...which releases carbon dioxide when they're burned or cut down.

Chlorofluorocarbons. Responsible for 10% of global warming;
also destroys t he earth's ozone layer.

Methane. 9% of the greenhouse effect. Produced by cattle, rice fields, landfills.

Nitroous Oxide. Responsible for 3%. Formed by microbes, breaking down chemical fertilizers, and by burning wood and fossil fuels.

Carbon Monoxide, Ozone, etc. 7% is from ground-based pollution caused by cars, power plants, oil refineries.