GO GREEN: How does C02 fit into our planet's atmosphere? sponsored by Environment Agency.

THIS week Brian Walters has a scientific poser for our Dr Green:

"We are told that huge quantities of man-made carbon dioxide (CO2) are rising through the atmosphere to collect at an undisclosed altitude as a greenhouse gas.

"As an engineering student in the 1950s I recall learning that CO2 is about one and a half times heavier than the surrounding air when air and gas temperatures were in equilibrium.

"When carbon is completely burnt in air, gases are produced at a temperature greater than that of the surrounding air, the principle gas being the colourless and therefore invisible carbon dioxide. As carbon dioxide does not support combustion, the heat from combustion expands the volume of the gas sufficiently to reduce the density, such that it is able to lift clear of the heat source. On entering the surrounding cooler air of the atmosphere, the hot gas begins to cool as it rises, which progressively reduces the gas volume, with a proportional increase in the gas density.

"As the gas continues to rise through the atmosphere at a progressively slower rate it reaches an altitude where the gas has cooled sufficiently to a state where the volume can no longer support the density, therefore the gas ceases to rise any further. With continued cooling, the gas now becomes increasingly heavier than air, with the result that the CO2 falls back to the surface of the Earth under its own gravity.

"My question is: How can man-made CO2, having cooled to a state where it becomes heavier than air, continue to rise through the atmosphere and remain suspended as a greenhouse gas at an altitude where the surrounding air temperature is sub-zero when the gas requires considerable heat for it to remain in suspension?"

Prof Paul Younger said: "You are right that carbon dioxide is heavier than the two main components of air. If weight was the only thing controlling the position of molecules in the atmosphere, then we'd find a thin layer of carbon dioxide at ground level, overlain by a layer of oxygen, then finally a really thick layer of nitrogen.

"Thankfully, air doesn't separate out neatly into layers of its component gases like this; if it did, we'd all be dead, as we'd be trying to live in the pure carbon dioxide layer! Two things happen to prevent the air from separating out into separate layers of pure gas: firstly, individual molecules of all gases in the air have a tendency to 'bounce' off each other, rather like the balls in a pinball machine. This happens because of the fields of static electricity which surround each molecule.

"The result is a random movement of molecules leading to an even spread of molecules of all types through a given volume of air.

"Also, the winds which sweep across the face of the earth cause dramatic mixing of different bodies of air, making sure that isolated pockets of individual gases (such as those emitted by power stations) get thoroughly diluted away into the atmosphere as a whole. The result is, at the scales of importance to us as oxygen-breathing mammals, we can think of air as pretty much unlayered.

"This is not to say that molecular weight counts for nothing of course - although its effects are blurred, it's still the reason that we find "thin" air (i.e. air short of oxygen) at the top of mountains. But the key point is carbon dioxide emissions have affected the atmosphere as a whole. This is the only credible explanation we have for most of the atmospheric warming in recent decades."

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HEAVY QUESTION: Prof Paul Younger