I hear and read that CO2 (weight:12+16+16=44) causes global warming. The EPA has even designated it as a pollutant. Yet it weighs three times the average molecular weight of air (78% N=14, 21% O=16 and .9% Ar=40). If CO2 is heavier than air, and there is only .038% of our atmosphere, is there enough to cause warming? Also, why do all discussions quote the amount of carbon (meaning CO2) EMITTED, and never the amount of CO2 ABSORBED, via photosynthesis?

Air (nominally 80% nitrogen, 20% oxygen) and Carbon Dioxide are completely miscible in all proportions. Although it's heavier than air (air has an average molecular weight of 28.8 vs carbon dioxide's molecular weight of 44 -- so it's not three times heavier than air), it still dissolves in air and can be dispersed throughout the atmosphere of the world.

The theories behind global warming say that the CO2 molecule, wherever it is (high or low in the atmosphere) absorbs light from the sun and converts it into heat.

There is discussion of the amount of CO2 that is absorbed by the world's forests. That's one of the points of contention -- some people say it's more than others say.

(This is a question that should have been posted under Chemistry.)

Also, if you consider the equations involved, photosynthesis converts carbon dioxide to oxygen in the ration 1:1.



At the same time, in respiration in living things, the ratio of oxygen to carbon dioxide is the same.



Looking at those only, adding a surplus of carbon dioxide will tend to remain in the atmosphere and not used in photosynthesis.

Now adding other factors like photosynthesis occurs only in the presence of light, things are being burnt in uncontrolled amount at times, the effect of marine organisms which also try to decrease the amount of carbon dioxide, the plants which have a higher rate of photosynthesis due to the increase in carbon dioxide... all those if considered may make anyone's head hurt. That is why, I prefer referring to those two equations first.

The overall result though, is that the amount of carbon dioxide in the atmosphere is gradually increasing.

CO2 is only 0.039% of our atmosphere, but the relative abundance is largely meaningless. Consider, instead of the mass of the atmosphere, the ratio of atmospheric CO2 to the entire earth itself. Vanishingly small, and equally meaningless.

The reason the ratio of CO2 to N2 and O2 is meaningless is because N2 and O2 are not heat trapping gases. Solar radiation passes through them without being absorbed--for the most part. The gasses that matter are H2O, CO2, CH4, NO2, and others. The biggest heat trapping gas is H2O but it tends to be self regulating. If you have ever stood outside during a thunderstorm you have witnessed H2O literally raining out of the atmosphere. CO2 and the others do not behave that way. CO2 concentrations ranged from 250 to 280 ppm over much of the past 20 million years, never rising above 300 ppm. Natural fluctuations would change concentrations of the gas perhaps 10 ppm over the course of a thousand years.

Then from 1700 to 1900 CO2 rose 10 ppm within just two brief centuries. Incredibly fast. Though at that rate, we could have continued burning fossil fuels for more than a thousand years before exceeding 350 ppm. Trouble is, we kept doubling consumption, such that we passed 350 ppm by the 1980s. Today (2012) we are at nearly 400 ppm, and we will exceed 500 ppm before 2050 at the current rate. These numbers are serious cause for alarm.