4. Feedback and Climate Models

In the last two articles I have written at great length (sorry) about the way carbon dioxide affects the transmission of infrared light vertically through the atmosphere.

Changes in this transmission are – we think – causing Global Warming.

The physics of the effect on infrared transmission is beyond argument. However this is just one component in the energy flows that constitute Earth’s climate system.

What else do we need to consider before we can conclude that carbon dioxide is causing global warming?

What else do we need to consider?

atmosphericmodelschematic

There is so much! The calculations in the previous articles only considered the transmission of infrared radiation and light up and down a vertical column of air with a variable temperature and pressure.

However in reality:

  • Light transmission does not just take place in one dimension (up and down) but in three dimensions.
  • Illumination from the Sun strikes each part of the Earth at different angles.
  • The infrared radiation from the Earth also takes place at many angles, and from many different heights in the atmosphere.
  • There are clouds which dramatically change atmospheric transmission.
  • The air moves in complicated ways “up and down and round and round”.

Additionally, the energy balance is dynamic – all the above factors change from minute to minute – around the surface of the Earth. And the Earth is not a sphere, and is not uniform and does not move in a circle around the Sun. And the Sun’s output varies from year to year.

In order to calculate the long-term averages of temperature and rainfall that determine the climate, we need to take into account all – or as many as possible – of the above effects.

There will be a cascade effects caused by increased  atmospheric carbon dioxide – such as changes in the location or timing of cloud formation. Additionally changes in the Earth’s surface temperature will affect the temperature of the atmosphere.

These changes may either ameliorate or exacerbate the initial effects of increased carbon dioxide concentrations.

In the end one ends up with a complex General Circulation Model of the entire Climate of the Earth – such as that illustrated above. The MODTRAN code – or something similar – is incorporated as one element of all the extant general circulation models.

If it’s all so complicated…why are scientists so sure of themselves?

There are, I think, two or possibly three reasons.

The first concerns calculations of the future effect of increasing carbon dioxide concentrations.

Simple calculations made more than 100 years ago agree pretty well with the results of most recent complicated calculations.

This indicates that the simple calculation has captured the essence of the problem.

Secondly, there is broad agreement with experimental observations – the Earth’s surface really is warming (Data Analysis 1 and Data Analysis 2). You can download the raw data from land stations here.

The third reason – which is really just a different way of thinking about the previous two reasons – is that given its effect on infrared transmission, it would be truly astonishing if adding carbon dioxide to the atmosphere did not affect the climate at all!

Once one admits to this point it becomes a question of asking what the effect will be? And every calculation I have ever seen predicts warming. If anyone has found something different, I would love to hear about it.

What about the saturation of the carbon dioxide bands?

A friend of a friend wrote an analysis in which he argued that increased concentrations of carbon dioxide could not cause global warming because:

The bottom line is that once Carbon Dioxide reaches a concentration that makes the atmosphere completely opaque in the band where it resonates,  further increases in the concentration cannot result in any additional blocking.

He was imagining that the ‘band’ where carbon dioxide molecules resonate is fixed. He was wrong.

For the individual molecules, the frequencies at which they vibrate are fixed. And the width of their ‘natural’ absorption line is fixed by the local temperature and pressure.

But transmission through the atmosphere is complicated, and the width of the band that absorbs radiation just keeps growing in width as the concentration increases.

Additionally the height in the atmosphere at which the absorption takes place gets lower – and hence warmer – and re-radiates more radiation back down to Earth.

That’s all for this article:

Here we looked at how the MODTRAN calculations fit into more complex models of global climate.

The next (and final) article is about the conclusions we can draw from these calculations.

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One Response to “4. Feedback and Climate Models”

  1. Ed Davies Says:

    Additionally additionally, the layer in the atmosphere at which radiation to space happens (on average – it’s actually smeared over quite a large height range, of course) gets higher but its temperature stays the same (in order to radiate the same amount of energy) so the adiabatic lapse rate has more height to work over resulting in higher surface temperatures.

    In my (amateur) understanding that’s the main reason for the warming. It also has the advantage from an explanatory point of view that even if the CO₂ and water vapour absorptions did saturate in the lower atmosphere it would still indicate warming as the height band in which saturation occurred would increase with increased average concentrations.

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