By Andy May
The Earth’s dry atmosphere is 78% nitrogen, 21% oxygen and 0.9% argon. These are not greenhouse gases and they total 99.9%, leaving little space for the greenhouse gases methane, carbon dioxide and water vapor. The amount of water vapor in the atmosphere varies a lot with altitude and temperature. At low altitude and high temperatures (greater than 30°C or 86°F), over the ocean, it can reach 4.3% or more of the atmosphere and is less dense than dry air, causing it to rise. It will rise until the temperature is low enough for it to condense to a liquid or solid state and form clouds, rain or snow.
Carbon dioxide is emitted when animals and some microbes breathe, from the oceans (which contain 93% of the carbon dioxide on Earth) and when plants or fossil fuels are burned. In the 1990’s fossil fuel emissions were about 3% of the carbon dioxide entering the atmosphere according to the EPA. About half of the fossil fuel emissions were absorbed by the environment. Mostly the CO2 emissions were absorbed by the oceans, land plants, and marine algae. Additional carbon dioxide in the atmosphere is a powerful fertilizer, for a dramatic illustration of the effect, see this short youtube video. Figure 1 shows the impact of additional carbon dioxide on pine trees under controlled conditions. The four CO2 levels tested are, from left to right, 385 ppm, 535 ppm, 685 ppm, and 835 ppm.
Figure 1 (Pine trees grown at ambient CO2 and three higher CO2 concentrations under controlled conditions, source)