Sunday, April 8, 2012


date: Mon, 09 Feb 2009 09:39:31 +0000
from: Tim Osborn <>
subject: Re: [Fwd: climate change - CO2 effect]
to: Phil Jones <>, "ANDREW DLUGOLECKI" <>

Hi Andrew,

Ch. 2 of IPCC AR4 WG1 may cover this issue, as Phil suggests, but as
this particular topic isn't new, AR4 may not give much detail. So,
here's some more detail:

It *is* correct that, as concentrations increase, the ability to
absorb more terrestrial (note: not "solar") radiation is reduced
("law of diminishing returns"?). However, the extra absorption is
not reduced to anywhere near zero and this effect is fully taken into
account in future climate projections. This is exactly the reason
why we often talk about the warming that would occur due to a
doubling of [CO2]. If we double [CO2] from pre-industrial 280 ppm to
560 ppm (i.e. +280 ppm), we might warm by 2.5 degC. To warm by a
further 2.5 degC would require a further doubling from 560 to 1120
ppm (i.e. now +560 ppm). (Another way to express this is to say that
the radiative forcing depends on the logarithm of [CO2]).

This is all taken into account by IPCC etc. Also, for other, lower
concentration, GHGs, this law of diminishing returns has not yet been reached.

The reasons why the incremental (or enhanced) greenhouse effect by
CO2 hasn't already reduced to near zero are discussed here:

My summary of these reasons:

(1) Absorption is quite variable according to wavelength of
radiation, and there are some wavelengths where the atmosphere is not
saturated (i.e. where it does not already have the capacity to absorb
all the terrestrial radiation). As [CO2] increases, the atmosphere
can definitely absorb more of this radiation. For example, for CO2,
about half the incremental/enhanced greenhouse effect arises by
absorption of terrestrial radiation with wavelengths of 15 microns
(which happens to coincide with the peak in the Earth's radiation
spectrum -- i.e. more energy is radiation by the Earth in this
wavelength than in others, so it is particularly effective if you can
absorb this type of radiation). In this band, current CO2
concentration is not yet absorbing all the energy (so there is
capacity to absorb more as [CO2] increases further) and also water
vapour is only a partial absorber so there is still room for a CO2 effect.

(2) Even if lower layers of troposphere are fully "saturated" (i.e.
able to absorb 100% of terrestrial radiation), the weather is able to
efficiently mix the troposphere so that any additional heat trapped
within the upper troposphere also influences the lower troposphere
and the surface. So what is really important in determining the
surface climate is the energy balance at the top of the troposphere
(the "tropopause"). In the upper troposphere, water vapour
concentration is quite low, so there are plenty of gaps in the
absorption spectrum where extra CO2 can play a role.

Finally, the evidence from other planets is a powerful demonstration
of the existence of a natural greenhouse effect. However, when
making the argument, care must be made to take into account distance
from the sun, differences in albedo (reflectiveness) and in
atmospheric depth. For venus, the fact that it's closer to the sun
is insufficient to explain it much higher surface temperature (though
it contributes) and this demonstrates how strong the greenhouse
effect could actually become. Sorry, but I don't have a suitable
reference to hand.

Hope that helps,


At 09:01 09/02/2009, Phil Jones wrote:
> Andrew,
> The answer to this view is probably Ch 2 of the IPCC Report (AR4) on the
> Forcing Factors that influence the climate.
> The best person to help with one or two specific references is
> Piers Forster
> from Leeds who was one of the two CLAs on the Chapter.
> Piers is "Piers Forster" <> .
> All of these skeptic issues have been discussed and dismissed ages ago.
> Cheers
> Phil
>>Date: Mon, 9 Feb 2009 05:59:04 -0000 (GMT)
>>Subject: [Fwd: climate change - CO2 effect]
>>User-Agent: SquirrelMail/1.4.8
>>Any of you able to answer Andrew's questions?
>>---------------------------- Original Message ----------------------------
>>Subject: climate change - CO2 effect
>>Date: Fri, February 6, 2009 8:51 pm
>>To: "Clare Goodess" <>
>>Dear Clare,
>>I am sure this is old to you, but I am engaged in a debate on climate
>>chnage with a real sceptic, a (geology) Professor Plimer in
>>Australia.� One of the points he raises ( and I have heard Lord Lawson use
>>it also) is that it does not matter if we pile up masses of CO2 because
>>the capacity to absorb solar radiation dwindles rapidly, due to the narrow
>>waveband in which absorption ocurs, so that incremental concentrations
>>have little effect.
>>No doubt this has been investigated, so can you give some pointers on
>>conslusive research on this.
>>Also, does the fact that Venus has� a thick CO atmosphere and very high
>>temperatures give a real-life scale demonstration that Plimer's view is
>>wrong, or is it unwise to use planetary behaviour?
>>Sorry to bother you, but despite my fairly wide reading I have not found a
>>convincing rebuttal
>Prof. Phil Jones
>Climatic Research Unit Telephone +44 (0) 1603 592090
>School of Environmental Sciences Fax +44 (0) 1603 507784
>University of East Anglia
>Norwich Email
>NR4 7TJ

Dr Timothy J Osborn, Academic Fellow
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ, UK

phone: +44 1603 592089
fax: +44 1603 507784


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