Hot and Cold
Lubos Motl has another global warming critique posted. He imagines that he has found a contradiction:
Motl thinks he has scored a debating point here, but in fact he has failed to understand some elementary physics. The warming of the surface (and lower troposphere) and the cooling of the stratosphere both naturally follow from the increased infrared absorption coefficient of the atmosphere. As every physics student should know, increasing the absorption coefficient leads to an exactly proportional increase in the emissivity.
To understand the different effect on stratosphere and troposphere one needs to look at the details of how each is warmed. The stratospheric case is simpler. It is heated by UV radiation from the sun, absorbed by oxygen. The stratosphere is a poor thermal radiator, since it lacks water vapor, and the combination of absorbed energy and inefficient radiation makes it warmer than the underlying troposphere. Adding CO2 has little effect on energy absorption, but makes a significant effect on energy emission. Ergo, adding CO2 cools the stratosphere.
Little of that UV reaches the troposphere. The troposphere is heated almost entirely by contact with the ground, which in turn is heated mainly by the visible light of the sun, which passes through all the atmospheric gases almost unhindered. The ground emits in the infrared. In the absence of absorbers, that infrared goes out into space, significantly cooling the ground, and hence the atmosphere in contact with it. Infrared absorbers (mainly water vapor and CO2) in the atmosphere absorb radiation, and emit it, some of which goes back to the ground to further warm it. Thus CO2 in the troposphere warms it warms the earth below by resending some outgoing CO2 back to it.
To summarize: CO2 in the stratosphere cools it by increasing its emissive capability while negligibly affecting the net absorption (since most of that is in the UV). CO2 in the troposphere warms the Earth below (and hence itself) by sending some of the outgoing radiation back to the Earth.
“Also, just like the carbon dioxide increases the infrared absorption in the troposphere, its increased concentration in the higher layer, the stratosphere, is - on the contrary - expected to increase the ability of this layer to emit energy and to cool down. Maybe.
I hope that it is not difficult for the reader to understand that the global warming theory actually predicts cooling for most of the volume of the atmosphere. There's really no serious catch here. ;-. Maybe.
Motl thinks he has scored a debating point here, but in fact he has failed to understand some elementary physics. The warming of the surface (and lower troposphere) and the cooling of the stratosphere both naturally follow from the increased infrared absorption coefficient of the atmosphere. As every physics student should know, increasing the absorption coefficient leads to an exactly proportional increase in the emissivity.
To understand the different effect on stratosphere and troposphere one needs to look at the details of how each is warmed. The stratospheric case is simpler. It is heated by UV radiation from the sun, absorbed by oxygen. The stratosphere is a poor thermal radiator, since it lacks water vapor, and the combination of absorbed energy and inefficient radiation makes it warmer than the underlying troposphere. Adding CO2 has little effect on energy absorption, but makes a significant effect on energy emission. Ergo, adding CO2 cools the stratosphere.
Little of that UV reaches the troposphere. The troposphere is heated almost entirely by contact with the ground, which in turn is heated mainly by the visible light of the sun, which passes through all the atmospheric gases almost unhindered. The ground emits in the infrared. In the absence of absorbers, that infrared goes out into space, significantly cooling the ground, and hence the atmosphere in contact with it. Infrared absorbers (mainly water vapor and CO2) in the atmosphere absorb radiation, and emit it, some of which goes back to the ground to further warm it. Thus CO2 in the troposphere warms it warms the earth below by resending some outgoing CO2 back to it.
To summarize: CO2 in the stratosphere cools it by increasing its emissive capability while negligibly affecting the net absorption (since most of that is in the UV). CO2 in the troposphere warms the Earth below (and hence itself) by sending some of the outgoing radiation back to the Earth.
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