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GUEST,KP BS: Where's the Global Warming (1541* d) RE: BS: Where's the Global Warming 07 Dec 09

You asked:
"Please explain the difference between "CO2 absorbs heat" and "CO2 reflects heat back to Earth".

Hope this is neither too pedantic or simplistic, but here goes.

A multi-atom molecule like carbon dioxide absorbs electromagnetic radiation. The radiation hitting each molecule imparts energy to the molecule (at uv and high frequencies the molecules vibrate, at lower frequencies the molecules just spin and twist). Some of the energy is therefore used up in making the molecule spin/twist/vibrate (yup, sounds like 1960's pop music...) and less is available for other purposes. This is energy absorption. Only certain frequencies excite these various rotation/vibration modes (like the way a guitar body vibrates best at certain frequencies/notes), so if you measure the spectrum of em radiation going through CO2, you'll see reductions in radiation intensity at these particular frequencies. One big point of argument is how broad these absorption bands are, and whether the large band for water vapour overlaps and swamps the smaller one for CO2.

The 'greenhouse' process can be summarised as:
1. high energy radiation from the sun impacts the earth, warming it
2. As the earth warms up, it then radiates heat in the form of lower energy infra-red rays
3. CO2, water, and methane absorb much of this radiation coming from the earth which would otherwise be 'lost in space'
4. This absorption of radiation speeds up/twists/turns the various absorbing molecules so they have more energy. This then causes (but its not the only cause) the variety of secondary effects we call 'climate change'.

I find it more conceptually useful to say that CO2 puts more energy into the global climate system than to talk about average temperature. With more energy in the system, you'd expect to get more extremes (hurricanes being more intense, hotter places being hotter, wetter places being wetter, but also anomalous cold spots). One consequence of climate change might be to disrupt the Gulf Stream warm current which would make northern Europe a lot colder.

Reflection on the other hand is where the radiation just 'bounces off' the molecule without being absorbed. This happens when solar radiation hits for example a symmetrical molecule like nitrogen (N2) or oxygen (O2). It also happens, (as BeardedBruce's article points out) when you have aerosols - tiny droplets in the atmosphere. These aerosols reflect the sun's radiation back out to space and mitigate the warming effects. Producing more aerosols in the atmosphere is a potential approach to controlling/reversing climate change. Just burning more dirty coal probably isn't the answer though - for one the cooling impact discussed in the final paragraphs of that article are quite small, partly because the aerosols are largely in the lower atmosphere. Also, the aerosols you get from coal burning are essentially dilute sulphuric acid aka 'acid rain'. What you'd need would be less toxic aerosols, higher in the atmosphere where smaller quantities would have a bigger impact.

So it is not really accurate to say that CO2 reflects heat back to earth - it is really the absorption process that is causing the 'greenhouse effect'.

Hope this helps. I'll see if I can find some references, and address your other point about CO2 in plants, a bit later.


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