When sceptics say that CO2 is "saturated", what are they talking about?

CO2's properties as a 'greenhouse gas' come from its absorption of infrared radiation coming from the Earth's surface (which it then re-radiates, thus keeping the heat in the lower levels of the atmosphere rather than letting it escape to space). The contention is that *all* the IR radiation which *can* be absorbed by CO2 already is, so adding more CO2 to the atmosphere can't increase the heating because there's no more radiation to be absorbed.

Some more explanation from Spencer Weart's excellent website:

"The greenhouse effect will in fact operate even if the absorption of radiation were totally saturated in the lower atmosphere. The planet's temperature is regulated by the thin upper layers where radiation does escape easily into space. Adding more greenhouse gas there will change the balance. Moreover, even a 1% change in that delicate balance would make a serious difference in the planet’s surface temperature. The logic is rather simple once it is grasped, but it takes a new way of looking at the atmosphere — not as a single slab, like the gas in Koch's tube (or the glass over a greenhouse), but as a set of interacting layers.

The subtle difference did not occur to anyone for many decades, if only because hardly anyone thought the greenhouse effect was worth their attention. For after Ångström published his conclusions in 1900, the few scientists who had taken an interest in the matter concluded that Arrhenius's hypothesis had been proven wrong. Theoretical work on the question stagnated for decades, and so did measurement of the level of CO2 in the atmosphere.

A few scientists dissented from the view that changes of CO2 could have no effect. An American physicist, E.O. Hulburt, pointed out in 1931 that investigators had been mainly interested in pinning down the intricate structure of the absorption bands (which offered fascinating insights into the new theory of quantum mechanics) "and not in getting accurate absorption coefficients." Hulburt's own calculations supported Arrhenius's estimate that doubling or halving CO2 would bring something like a 4°C rise or fall of surface temperature, and thus "the carbon dioxide theory of the ice ages... is a possible theory." "

http://www.aip.org/history/climate/co2.htm

CO2, like all gasses, absorbs very specific frequencies of light, converting JUST THOSE FREQUENCIES into vibrational/rotational energy (=heat). These frequencies are called "absorption bands".

In the case of CO2, there is already enough CO2 in the atmosphere to absorb almost all (well over 99.99%) the light of those frequencies, so that none reaches the ground. The fact that all the light that can be converted to heat by CO2 is already being converted to heat is described as "saturation".

Thus, an increase in CO2 will not increase the greenhouse effect, as all the light of the frequencies that can be converted is already being converted. Unfortunately, a moderate reduction in CO2 will not reduce the greenhouse effect either, as the bands will remain almost satuated unless concentration CO2 declines a lot.

This means that none of the current CO2 emmission reduction targets will make any real difference (If we reduce CO2 concentration by 30%, instead of 99.99% of the light being absorbed, it will be about 97%, hence a 30% reduction in CO2 makes only a 3% difference to global warming - not worth it).

You actually need to reduce the CO2 concentration to less than 20% of current levels to get to the pre-industrial revolution levels to make any noticeable difference to the amount of light converted to heat by CO2. No one is planning to do this.

Please note that the biggest environmental damage from CO2 is not global warming, it is that it disolves in water to form carbonic acid, leading to the rise in acidity of the sea - a problem for sea creatures with shells, all of which disolve in mild acids. As 90% of the earth is sea (and probably 99% of the biomass is in the sea), this is a greater danger than warming.

Remember, the press and reporters need to make a living too. With that said, let me share with you some of the crisis I have "survived": 1. Acid Rain 2. The world was entering an ice age. 3. The world was to run out of petroleum by the year 2010 4. The Cold War The best advice is to listen and research! There is something going on with the weather. Now the question is: Is it naturally occurring, human influenced (or here is a unique one: can it be both?) Is it long term, or short term... again searc weather patterns especially those going back to 1600.... the Medevli Ice Age (are we just coming out of an ice age???), the Maunder minimum, The Year without a Summer which helped launch the French Revolution. Research Solar Acitivity and the Models we use to predict weather...why can't we predict weather more than 5 days out... so what is the accuracy of the predictions of 10, 25, 50, 100 years from now??? And have you heard anything but the world is coming to an end becuae the US did not support the Kyoto Agreement. It does not make much front page news there is a lot of research going on to control emissions. It doesn't make front page news again, like the use of our farm land to make ethanol so we can feed our cars instead of people...but... as I said l have said in other responses...learn to think critically!!!!

Comment to mogadon

Your explanation about CO2 and the warming of the atmosphere is correct, but your assertions about the acidification of the ocean is not.

Concentration of CO2 is the critical rate limiting factor in photosynthesis, so an increase in CO2 in the oceans will increase the production of phytoplankton, which form the base of the food chain, and will fix the 'extra' CO2 in biomass. ie. negative feedback.

The ph of the ocean changed from 8.179 to 8.104 between 1751 and 1994. This is a drop of 0.075 units in 243 years. At this rate it will take about three and a half thousand years to reach neutral. So the question of dissolving shells simply does not arise in any reasonable time frame

Imagine a layer of red clingfim wrapped round a cue ball. It filters out all the other colours apart from red.

The white cue ball looks red.

If you add another layer of the same stuff around the cue ball, it has no effect. The other colours are being filtered out already. Red comes through. Cue ball still looks just as red.

That is saturation.

I don't know, I have never heard of CO2 being "saturated". Then again, we still don't know how much water the atmosphere as a whole can absorb. Yes, an increase in CO2 will increase the green house effect but not by much more then the percentage it is in the air. Also with a baseline of 280 ppm and the total 100 ppm increase being both man made and natural, it seems that man's part in global warming is not as significant as some wants us to believe.

I heard we're All Doomed and the World Government is secretly building a giant spaceship to allow all Top Contributors to populate Mars ...

Good luck, people!

(quoted from above)

...before, where it would be slightly more difficult for the heat to be re-radiated back into space. This is the principle on which most of the global warming predictions are based. "

http://brneurosci.org/co2.html

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Now, why would anyone looking for information about global-warming go to a neuroscience organization looking for answers?

I mean, if you were to suffer a brain injury in an accident, would you seek therapy from a climatologist?

Edited to add: icarus62 replies with a straightforward, layman-friendly scientific explanation of why the "saturation" argument is incorrect, and he gets (so far) 6 "thumbs down" votes. What gives? Who would give a straightforward scientific explanation a "thumbs-down"?

CO2 is compelety transparent to 92% of IR wavelegths. The more CO2 is added to the atmosphere, the closer you get to 8% of all radiation being absorbed until the energy is re-emitted at some other wavelength. We are long past that point now.

AGW proponents make all kinds of assertions to support their preconceived beliefs, but are not supported by any experimental or theoretical evidence. Science shouldn't be about supporting pre-conceived beliefs. The only models that show significant AGW use arbitrary warming factors, not scientific first principles.

Any minor increase in bands absorbed, and they are probably infinitesimally small considering the relative increase in CO2 relative to other molecules in the atmosphere, would be dramatically outweighed by Beer's law which states that the absorption of IR versus the concentration is logarithmic. That means that more CO2 means much less absorption and while the increase in bandwidth might be significant on Venus, it isn't on Earth or the real world.

"It is hardly to be expected that for CO2 doubling an increment of IR absorption at the 15 µm edges by 0.17% can cause any significant global warming or even a climate catastrophe.

The radiative forcing for doubling can be calculated by using this figure. If we allocate an absorption of 32 W/m2 [14] over 180º steradiant to the total integral (area) of the n3 band as observed from satellite measurements (Hanel et al., 1971) and applied to a standard atmosphere, and take an increment of 0.17%, the absorption is 0.054 W/m2 - and not 4.3 W/m2.

This is roughly 80 times less than IPCC's radiative forcing."

http://www.john-daly.com/artifact.htm

"Most of the infrared at these wavelengths is produced by black body radiation from objects that have been heated up by absorbing radiation at shorter wavelengths. This means that even if the carbon dioxide levels increase, it will have little effect on the total amount of infrared radiation that is absorbed from the sun. The main effect would be to trap radiation originating at the surface at lower levels in the atmosphere than before, where it would be slightly more difficult for the heat to be re-radiated back into space. This is the principle on which most of the global warming predictions are based. "

http://brneurosci.org/co2.html