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Climate Change - A quick rebuttal to
Augie
Auer's opinion in the NZ Herald. In the first week of May 2006, former NZ MetService chief meteorologist Augie
Auer said global warming was a myth. He blamed journalists and bad science.
In a nutshell, his main argument against
anthropogenic (human made) climate change, and an argument made by many other
climate sceptics is, that water vapour in the atmosphere is a much more potent
'greenhouse' gas than CO2 and that due to the vast abundance of water vapour
the relatively small (380 parts per billion) CO2 content in the atmosphere
would not contribute much to global warming, let alone the small contribution
that humans were making to that CO2 content. And as most water vapour is
naturally generated due to evaporation of the oceans, there was nothing that
humanity would be able to do about it. Let us look at the situation a bit more
carefully: It is true that by comparison of quantity
there is a hell of a lot more water vapour in the atmosphere than CO2. Water vapour
is a perfect absorber of infrared radiation and thus the main contributor to
the so called 'greenhouse' effect. So far Augie is
right. And he is also right, that the Earth would
be covered in ice, if it was not for the warming blanket of that water vapour. However Augie
omits to say this: Water vapour is not absorbing (shielding against radiation
heat loss) in the entire infrared spectrum. In fact there are, thankfully, a
few relatively clear 'open windows' in the absorption spectrum of water vapour because
of the physical properties of the water molecule. If it was not for these
'windows' in the absorption spectrum of water vapour, our earth would find it
more difficult to radiate heat away to space. And radiating in the
infrared spectrum, a bit like the glow from your hot potbelly stove in winter,
is the only way that Earth can loose thermal energy to space! If these remaining open windows in the
infrared spectrum between the various bands in which water vapour absorbs were
not there, then Earth would a much warmer place, and probably not very conducive to
the current forms of life. Water is so abundant in the atmosphere that
in those areas where it absorbs heat radiation, the atmosphere is practically
impermeable or 'black'. In other words in these areas the absorption spectrum
is completely saturated. And no matter how much more water vapour there would
be, it can't get 'blacker than black'. It’s a bit like putting a black tarp
over your window at home. After the first one it dose not matter how many more
you add, it won't get any darker - at least through that window! But here is the catch: CO2 and also Methane
have absorption areas of infrared radiation in some of the few remaining 'windows'
in which the earth can 'shine out' so to speak and loose heat, because these
windows in the spectrum are not already closed by water vapour's thick blanket.
And it does not take much of a CO2 concentration at all, to lead to a strong
absorption in these areas. In fact CO2 is so good at absorbing heat radiation
in its bands that even at the natural background concentration of 280 parts per
billion of CO2, the atmosphere is practically already almost 'black' in the
centre of the CO2 absorption bands! The anthropogenic additions of CO2 - in fact
we will be practically doubling it by the mid of the century - will have a very
measurable effect to the ability of Earth to radiate out of this 'window'
precisely because the natural CO2 concentration is so low (compared to water)
and the absorption is not yet saturated in these frequency bands so that any
additional CO2 we bring to release is directly contributing to the darkening
around the CO2 window in the absorption spectrum. Thus Augies
argument of the low concentration of CO2 relative to Water is actually coming
home to roost!
Absorption spectra of Water (H2O) - blue, Carbon
Dioxide (CO2) - green, Oxygen and Ozone (O2 and O3) - red and the total (added)
absorption spectrum - top. The black line marked 5780K is the incoming solar
spectrum, the 255K is the spectrum of the earth radiating out. It is clear that the green CO2 absorption lines are
blocking an important part of the 'water window' (blue line) in which Earth
can radiate in the infrared out of the atmosphere. To come back to the example of the house:
Imagine that you have 6 windows through which you can see out. 4 are covered by
a stack of black blankets (water vapour). Now somebody darkens
one of the last two open windows with a thin sheet of dark fabric. How would
that affect your house? Augie says: How come you argue about one little thin sheet of fabric when
I can see that stack of thick blankets…. Well, it all matters where you put it
as it seems…. And so it goes that these few hundred parts
per billion of CO2 will have a very dramatic effect on life on Earth. All this is not to say, that there is
nothing left to discover about the mechanisms of climate change and the
regulatory system of our Earth atmosphere, far from it. And water plays a vital
role in transporting heat around the Earth, especially in though processes of
evaporation and condensation and the fact that these two processes happen often
in different places. But the basic science behind the effects of
CO2 and Methane on the ability of Earth to radiate heat away to space is pretty
sound. And once you calculate this, then it is obvious that Earth must warm
measurably in response to our human made doubling of CO2. Everything else is almost
an afterthought, i.e. what happens to the trapped heat, where will it go and
how will it affect us. Thomas Everth back to the green blog page |
