Little Grey Rabbit's Historical Skepticism Blog

Temporary Post: Scenarios of CO2 effects on global warming

Posted in Miscellanous by littlegreyrabbit on June 23, 2011

With apologies on the interruption of service, but I have been asked to lend this blog for a few days to my good friend, Mr Lamb, for a temporary post:

In discussion with a Professor Enting recently, he said the following:

But, simpliying the numbers a bit, think of it as each time you multiply the concentration by 1.5, you add about 2 degrees to the medium term warming – the instantaneous warming lags and there may be more long term warming from ice sheet feedbacks.
so 180 to 270 adds 2 degrees, 270 to 405 adds another 2, 405 to 607 adds another 2.
If you have acess to a spreadshet like excel (or the open office equivalent) you can plot up a more detailed version yourself.

So I thought what-ho, I will do that.  The problem with Professor’s Enting’s model is it fitted no actual real life points.  He admits he can’t get it to go through zero or 1 ppm CO2, he uses a figure of 2 degrees warming for industrial age increase which he admits is 2.5 times that observed, then uses 2 degrees warming for the ice-age to pre-industrial age variation; which is also too high.  In fact that the only thing it has going for it is that it fits a 2 degree warming to around 600 ppm CO2 – which seems to represent a “6 million” figure for climate scientists; you can add up your numbers anyway you like, so longs as you reach an additional 2 degrees by 600 ppm.

So lets take Prof. Enting’s logarithmic relationship and put it on a proper scientific footing.  It is commonly stated that without greenhouse insulation the earth would be 33 degrees colder or 28 degrees colder than the temperature during minima of recent ice ages.  However, it is also understood that CO2 only represents a fraction of the greenhouse gases – the bulk of the warming being provided by water vapour. So I have prepared 3 different scenarios, a conservative scenario of CO providing 15% of the GH effect, a mid range where it provides 20 % of the GH effect and a “bend over backwards” scenario where it provides 25-30% of the GH effect

TABLE: Temperature rises for each scenario of CO2 levels in ppm – starting at 180 ppm of the Ice Age minimas.

Degrees above Ice Age minimums CO2 contributing 4 degrees at 180 ppm CO2 contributing 6 degrees at 180 ppm CO2 contributing 8 degrees at 180 ppm


180 180 180



























The number bolded is that closest to what is the pre-industrial point: Conservative scenario has CO2 contributing to around 0.3 of the observed warming from the Ice Ages, the mid range scenario around 0.55 degrees contribution and the “bend over backwards scenario” contributing 0.75 degrees.

Considering that orbital variation, axial wobble and albedo feed forward almost certainly contributes the bulk of warming since the ice ages, the conservative or mid range estimate should be about right.

To get to the canonical 650 ppm CO2, under the conservative model we should expect a further 0.7 degrees rise over pre-industrial, the mid-range model a 1.0 degree rise and the “bend-over-backwards” model a whooping 1.25 degree rise.

Note, this does not mean the temperature will rise by this, simply that will be the proportion of temperature rise that can be directly attributed to CO2, without including any negative feedbacks, positive feed-forwards, variations in cloud cover, solar variation or variations in solar magnetic flux.

In the final analysis since the nature of climate science is that it can-not be a experimental science, it is impossible verify their modelling.  This makes the field susceptible to group-think and manipulating assumptions to achieve political desirable outcomes.