Graham Y makes some interesting points. He states that: “… temperature increase is likely to be less than 3°. Most of the warming we are going to see from CO2 has already occurred because of its logarithmic reaction to infra-red radiation”.

These are novel claims – that the correlation between increasing temperature and increasing atmospheric CO2 being logarithmic rather than linear limits the ability of CO2 to increase global temperature by more than 3°. Could you cite a peer-reviewed reputable source that provides evidence to support this, and explain what is meant by “its logarithmic reaction to infra-red radiation” and show how this limits the effect of further CO2 emissions.

Bonmot says that “at relatively low (CO2) the gradient of the logarithmic function that you think is minor is in fact major, and near linear”. The repost that anything can be made to appear linear if a short enough time span is quite valid. However, over the 100 years of the 20th century, the correlation between the rate of increase in atmospheric CO2 and temperature, particularly over the latter half of the century, appears quite good and near linear as shown at

http://zfacts.com/p/226.html
http://www.skepticalscience.com/news.php?n=61 and
http://wattsupwiththat.com/2008/01/25/warming-trend-pdo-and-solar-correlate-better-than-co2/

Asserting an international conspiracy to falsify temperature records so as to show decadal increases in global temperature over the last 50 years, when global temperature actually fell is all very well. But simply saying that is the case does not make it fact and unless supported by credible evidence, preferably science-based evidence, it certainly does not make it a proven fact. Where is the evidence?

The warming or ‘greenhouse gas’ effect of CO2, when compared to temperature, is logarithmic such that there is rapid but decreasing warming between 0ppm and approximately 50ppm of atmospheric CO2 – increasing the ‘greenhouse’ temperature effect by about 15º to 35ºK (Kelvin), with a marked decline in its warming effect between 50ppm and around 200ppm – representing about a 4ºK increase; beyond around 200ppm to well over 800ppm the warming effect declines to about 1ºK or less. Therefore, the capacity of increased atmospheric CO2 to increase temperatures, from whatever the source, is very limited above 300ppm or so.

Engineers who deal in greenhouse systems point to the classic text book “Heat Transmission” by William H. McAdams (McGraw-Hill, 1954), still used in the design of industrial furnaces, which discusses this limiting effect, which has to do with the radiation bands on which the gas absorbs heat. There is good agreement on this science, and it is well understood.

It is important to remember that it is the total amount of atmospheric CO2, derived both naturally and by the addition of anthropogenic sources that has the potential to warm the Earth’s atmosphere, with the logarithmic decay of temperature versus CO2 limiting this possible warming. Because of this logarithmic decay a doubling of CO2 to 760ppm from the current 380ppm is likely to be approximately 0.12°C.

A compilation by Sundquist [(1985) Geological perspectives on carbon dioxide and the carbon cycle, in Sundquist, E.T. & Broecker, W.S. (Eds.): The carbon cycle and atmospheric CO2: natural variations Archaean to present, American Geophysical Union, Geophysical Monograph 32, 5-59] lists the results of 36 separate studies, based on a number of different measurement methods that give for both naturally and anthropogenically-derived an atmospheric CO2 residence or turnover time (not to be confused with e-folding time) ranging between two and 25 years. (Continued)

(Continued from above) In summary, these studies show that CO2 has an average atmospheric lifetime of about five years and that approximately 135 giga-tonnes (about 18%) of the atmospheric CO2 pool is exchanged each year. This large and fast natural CO2 cycling flux is far more than the approximately 6 giga-tonnes of carbon in the anthropogenic fossil fuel CO2 now contributed annually to the atmosphere. On these figures anthropogenic CO2 accounts for just 4.45% of the annual atmospheric turnover – it therefore has only a corresponding 4.45% influence on the 0.12°C mentioned above, at any one time - in other words the effect is negligible.

Nonetheless, many climate models project a global temperature increase of several degrees; obviously then, if the models are correct, another mechanism must be involved. The assumption is that atmospheric water vapour, a known powerful greenhouse gas, acts as a positive feedback amplifer mechanism. This notion assumes that initial warming leads to increases in water vapour concentration that in turns enhances the original warming (amplifers it) which further increases water vapour concentration resulting in additional warming and so on, implying that the feedback mechanism is the dominant factor driving anthropogenic climate change.

By giving dominance to the water vapour feedback mechanism this signifies an amplifer input signal that is much greater than the initial anthropogenic CO2 forcing. Is this really the case? If it were true then at the end of the last glaciation, when atmospheric CO2 was increasing, we would expect to see runaway climate change – this clearly did not occur. Rather the climate has ranged or oscillated between interglacial warmth heat and glacial cold as the world has swung in and out glacial periods – this would suggest (in part) a negative feedback mechanism.

There are many other problems associated with the positive feedback amplifier mechanism, not least of all it is impossible to obtain empirical confirmation of the value of any feedback by direct measurement. On this, and it would take a few pages worth of explanations to discuss, the science on the positive feedback amplifier mechanism is definitely not settled.

Very well articulated Raredog.

Raredog

In response to my call for credible evidence of the claims made by Graham Y, you assert that anthropogenic emissions are 4.45% of natural emissions. It follows that anthropogenic emissions are only responsible for 4.45% of the increase in CO2 since pre-industrial times. Ergo, anthropogenic emissions can only have a 4.45% effect on temperature.

This is wrong because natural CO2 sources are balanced by natural sinks and have no long term effects on concentration. ALL of the increase of CO2 concentration in the atmosphere is anthropogenic.

I am indebted to Dr Lambert (UNSW) for his advice on the behaviour of Logarithms in which he notes that: A logarithmic response means that the warming for increase in CO2 from 50 to 200 (a quadrupling) will be the same as from 200 to 800 (also a quadrupling). It does not mean that the first will be four times the second. Logarithms do not behave like that.

That invalidates the material you put forward arguing the effects of increasing CO2 concentration on temperature. It certainly does not mean, as claimed by Graham Y, that increasing CO2 concentration will not result in temperatures exceeding 3°C. They most certainly can and unless anthropogenic emissions are curbed, they will.

The penultimate paragraph of your comment in effect argues that climate sensitivity can not be as high as the IPCC range because otherwise we would have had runaway climate change at the end of the last ice age. This is exactly backwards. It was 5K colder at the last Glacial Maximum with CO2 concentrations half what they are to-day.

If sensitivity was as low as you suggest, there is no way that there could have been an ice age. The fact that we have ice ages is one of the strongest pieces of evidence that climate sensitivity is around the IPCC value.

Agnostic of Mittagong - sorry, about the late reply. I have abbreviated your quotes to fit in with the word limit.

"You assert that anthropogenic emissions are 4.45% of natural emissions."

No, anthropogenic CO2 accounts for just 4.45% of the ANNUAL atmospheric CO2 turnover.

"It follows that anthropogenic emissions are only responsible for 4.45% of the increase in CO2 since pre-industrial times."

No, most of the increase is possibly anthropogenic with a contribution from the natural warming post the Little Ice Age, for instance.

“Ergo, anthropogenic emissions can only have a 4.45% effect on temperature.”

Most likely, in terms of the warming effect of atmospheric CO2, at any one point in time.

“This is wrong because natural CO2 sources are balanced by natural sinks and have no long term effects on concentration.”

There is a range in concentration as exemplified by changes in atmospheric CO2 levels between the glacial/interglacials periods as well as in response to natural warming/cooling periods (eg Little Ice Age, etc) - as such atmospheric CO2 concentrations oscillate over the longer term. The assumed STATIC balance of natural sources and sinks is another argument based on a number of assumptions.

“ALL of the increase of CO2 concentration in the atmosphere is anthropogenic.”

Mostly but with the addition of possible natural sources.

“A logarithmic response means that the warming for increase in CO2 from 50 to 200 (a quadrupling) will be the same as from 200 to 800 (also a quadrupling).”

I do not understand this example in terms of CO2 concentration verus temperature.

“That invalidates the material you put forward arguing the effects of increasing CO2 concentration on temperature. It certainly does not mean that increasing CO2 concentration will not result in temperatures exceeding 3°C.”

Not so. The effect of increasing atmospheric CO2 concentration on temperature increase, due to its inverse logarithmic relationship, declines as the concentration increases – in other words, there is a diminishing return of temperature increase for each additional increase in CO2 concentration. (Continued)