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Everybody understands how a backyard greenhouse works. Sunlight shines through the glass and gets trapped inside as heat. Even on a cold winter day, the inside of a greenhouse will be warm and cozy. In fact, if you do not block some of the incoming light, it can actually get too hot. This is why one often sees whitewash on the window panes of a greenhouse. Some of the incoming light is reflected from the painted windows, and so less energy enters the greenhouse and it doesn't get quite as hot.

The Earth is a greenhouse. Our atmosphere is quite transparent to sunlight, except for ultraviolet rays, which are absorbed in the upper atmosphere to form the ozone layer. Light reaching the surface generally is reflected or reradiated as infrared energy what you feel radiating from a warm pavement. The atmosphere is less transparent to infrared than to visible light, so it retains much of this reflected energy. Just like in the backyard greenhouse, incoming energy is trapped in the planetary greenhouse.

Two atmospheric gases are especially opaque to infrared: carbon dioxide and water vapor. Their presence in significant quantities can dramatically affect global temperature. Other gases can have an effect - methane is a good example (if you raise cows you know what I mean) - but the main ones are carbon dioxide and water vapor.

Carbon dioxide is the result of combustion. It is emitted whether you burn leaves, run an engine, or simply live and breathe. Wildfires are the largest natural source of carbon dioxide, although volcanoes and forests contribute measurably. You probably learned in school that trees absorb carbon dioxide and give off oxygen. During the day this is true, but at night many trees give off carbon dioxide. Generating electricity by burning coal, oil, or gas is the largest manmade source of this gas, with automobiles a distant second. Other sources of carbon dioxide such as fireplaces and barbecues don't count.

Water vapor in the atmosphere is always present. The amount depends upon air temperature - the higher the temperature, the more water vapor. We are not talking about clouds here. Clouds consist of specks of dust surrounded by water droplets. Water vapor is a colorless, odorless gas that makes up a measurable percentage of the atmosphere. We experience this gas as humidity.

In the backyard greenhouse, the only way to increase the internal temperature is to find a way to retain more of the sun's energy that comes through the glass in the first place.

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