The current atmospheric CO2 level is about 380 ppm. This is a ~30% increase in CO2 levels since the Industrial revolution. This has SOME effect on the climate. SOME. But how much?
First question: what is the approximate level of temperature change we can expect from Anthropogenic Global Warming (AGW)?
The relationship between CO2 and temperature in the lower atmosphere is broadly logarithmic. As CO2 increases in the atmosphere, it absorbs less and less additional ground-emitted energy to produce correspondingly less and less additional warming. At some point adding more CO2 to the atmosphere doesn’t significantly change atmospheric temperature.
This logarithmic response is just a first approximation: calculating the actual warming effect of CO2 is non-trivial as many of the atmospheric processes involved are still being quantified. A common starting ground amongst modellers are estimates for the amount of cooling Earth would experience for a hypothetical zero-CO2 cloud-free atmosphere.
The results of these 'clear-sky' models can be seen here:
These three models show the simple logarithmic behaviour. They suggest that a doubling of atmospheric CO2 would increase temperature by between 0.64 to 1.46 deg C.
However these models are first steps that explicitly leave out many factors - e.g. the effects of water vapour and cloud cover. For instance Professor Lindzen (MIT, author of one of the clear sky models) states that a doubling of CO2 would lead to an increase of only ~ 0.22 deg C with 40% cloud cover, about a 1/3 of the clear sky model.
Lord Monckton (Science Adviser to Margaret Thatcher) gives a technical gloss of the frankly complex modelling of CO2 doubling: this can be found here
His summary is that the best-attested effect of CO2 doubling is ~0.5 deg C per doubling of CO2.
In distinction to these luminaries, the IPCC's assessments of climate change predict that the temperature goes up ~ 3 degrees C per doubling of CO2.
Readers can see right there the dynamo that drives the current AGW furore - the UN's assessment of climate-forcing from CO2 is about SIX times that of MIT meteorologists and other modellers.
Which is correct? After all it's not an easy task for relative laymen to distinguish between one set of apparent experts and another.
Fortunately there is an experimental test which distinguishes the models. The IPCC models all predict a mid-troposphere “hot spot” about 10km above the Earth’s surface. The dissenting models predict that this spot doesn't appear. This hot spot is not, as it happens, observed by radiosondes - therefore the lower estimating models (Lindzen et al) should be given preference.
As stated earlier the current atmospheric CO2 level is about 380 ppm. This is a ~30% increase in CO2 levels since the Industrial revolution. Fitting a 30% increase against Monckton's median result from non-IPCC models of 0.5 degrees per doubling gives us a temperature increase of ~ 0.15 degrees Centigrade.
So our best estimate of the effect of the 30% post-revolution change in CO2 is a temperature increase of about 0.15 degrees C.
Second question: is 0.15 degrees significant?
To get an idea of how significant a 0.15 degree shift would be, let us examine temperature variations over the last thousand years.
The most exhaustive temperature reconstructions covering the last ~ thousand years that I have been able to locate were commissioned by the US Senate in 2005. A copy of the collated graphs is shown below:
Looking at these graphs one can trace a (broadly speaking) sinusoidal variance in temperature over the last 1000+ years. It was warm in 1100, cold in 1600/1700 and warm again in 2000. The variation is + or minus 2 degrees C.
An extra 0.15 degrees from AGW on this scale is just noise - it's more than an order of magnitude down from the sinusoidal variation of effective solar heating.
Digression: Mediaeval Warm Period
Before we leave this graph we should note the Mediaeval Warm Period (MWP) centred at about 1100 AD. It lasted about 200 years.
During the MWP it was at least as warm as it is now. Greenland was colonised by the Vikings, the great cathedrals were built in Europe, diets were good enough that people grew to approximately the same height as they reach today (this last intriguing tidbit is from here)
The importance of the MWP to our present discussion is that it points up the cyclical nature of our climate due to variation in effective insolation. No-one would maintain that the MWP was caused by CO2 emissions during the Dark Ages: it is evidently a perfectly natural maxima in the insolation graph.
However AGW proponents such as the Royal Society and the IPCC maintain that our current warm period is starkly different from the rest of human history. According to these sources we have recently had "the eleven warmest years since records began in 1850". Also: they frequently opine that no factors apart from anthropogenic CO2 can possibly explain the current warm period.
But the Senate reconstructions make it evident that in 1850 Europe was only just beginning to climb out of a prolonged cold period (the so-called 'Little Ice Age') that started way back in ~ 1300. Also: our modern warm period looks just like the MWP - we are currently in just another local maxima in effective insolation. The MWP didn't come about from AGW: why would the author assume that the modern warming period has?
To reach the conclusions they do, IPCC and Royal Society AGW advocates would have to believe that CO2 causes a large amount of temperature change per doubling (in line with the discredited IPCC models) AND that the entire MWP is somehow not significant in regard to the status of our current warm period.
Third question: 0.15 degrees C may not be significant, but what if or when CO2 doubles? What would we be looking at?
CO2 is currently growing at ~ 30 ppm per 10 years: this is currently thought to be due to a prolonged growth spurt by the Chinese and Indian economies. If they or others sustain the growth in CO2 emitting industries for another 130 years the concentration of CO2 will double, assuming no buffer mechanisms kick in. The predicted warming effect of this doubled CO2 is (pace Monckton) ~ 0.5 degree C at current levels of insolation.
Half a degree is hovering on the edge of being a significant amount of warming. But to put this in context: in 130 years we will have certainly come out of the current Modern Warm Period and be moving into the next five hundred year 'Little Ice Age'. A 0.5 degree increase in global temperature from Anthropogenic CO2 would buffer the two degree drop one would expect to be associated with the 'Post-Modern Little Ice Age'.
In Summary:
The sum total of AGW is a minor effect that is (for instance) completely swamped by the eleven year variation in solar radiance. Our climate is driven by effective insolation: the effect of AGW is a small perturbation on this.
To project a slightly more significant role for AGW in the next century it is necessary to make a rather broad assumption about coal and oil-based industrialisation for the next 130 years. By that date the role of any AGW would still merely act to buffer the descent into a long cold period.
Further Reading:
I highly recommend http://www.junkscience.com/Greenhouse/ as an excellent overview of common questions about the (somewhat misnamed) 'Greenhouse' effect.
Saint Al will not be mocked.
