Even outside the research community, almost everybody has heard of the greenhouse effect: more CO2 in the atmosphere warms up the Earth’s surface. But interestingly, this only applies to the air near the ground. Above a height of a few kilometres above ground, CO2 has a cooling effect, not a warming one. In the stratosphere (at heights of about 15-50 km), this cooling is even much stronger than the warming at ground level.

Why is this? Basically, there are two reasons for this. The first reason is related to the presence of the ozone layer. Ozone absorbs solar irradiation. This energy warms the air, and is radiated back into outer space by means of greenhouse gases such as CO2. An increase in CO2 makes this radiation more efficient, whereas there is no change in the absorption by ozone – thus the air becomes colder. This effect should not be confused with the problem of the hole in the ozone layer. The latter involves the decomposition (not the mere presence) of ozone by chlorofluorocarbons (CFCs), which also leads to cooling in the stratosphere.

The second reason why CO2 cools the atmosphere at medium heights is what is called the “blocking effect”: more CO2 makes the atmosphere more opaque for radiation of certain wavelengths. So the radiation which a CO2 molecule in the stratosphere receives from lower down has covered a shorter distance. Therefore it originated in higher air layers, which tend to be colder. As a reaction to the lack of energy input from below, the atmosphere at medium altitudes becomes colder. This does not apply to other greenhouse gases than CO2, such as methane (CH4) and nitrogen monoxide (N2O). The radiation which these gases absorb comes mainly from the ground which is warming up, so that they do not contribute to cooling of the atmosphere at medium heights.

By the way, the cooling of the atmosphere at medium heights is one of many pieces of evidence showing that the cause of the change in climate is CO2 emissions of human origin, and not a natural process.

Publications

  • Goessling, H. F. & S. Bathiany, 2016: Why CO2 cools the middle atmosphere – a consolidating model perspective. Earth System Dynamics 7, 697–715.

http://www.earth-syst-dynam.net/7/697/2016/