Thursday, May 27, 2010

Six Minutes to Midnight - Part 2

Natural Greenhouse Effect

For mankind to inhabit this planet a stable temperature is required. The Earth’s atmosphere provides a mechanism which helps regulate this temperature, called the greenhouse effect. In essence there must be a balance between the heat that we get from the sun and the heat we radiate back into space.

The sun emits electromagnetic radiation across a range of wavelengths, the most obvious to us here on Earth is the visible spectrum that makes up all that we see. Of the EM radiation which arrives to Earth, we are bathed in the visible spectrum and the ultra violet spectrum. Of all the energy that reaches Earth, one third is reflected back into space. This reflection is due to the sun rays reflecting off of clouds and aerosols in the atmosphere and the ice sheets which cover the polar and higher elevation regions around the Earth. The remainder of this incident radiation is absorbed by gases in the atmosphere or by the surface.

The gases which absorb this radiation play a key role in this greenhouse effect; hence we refer to them as greenhouse gases. The most common of these gases are water vapour, carbon dioxide, methane, ozone, sulphur dioxide and nitrous oxide. A point I wish to clear up here before I continue is the common misnomer in both the media and many scientific publications: the term "carbon emissions". This term is widely banded about in discussions on climate mechanics, it of course refers to all the greenhouse gases not just the carbon based ones. The greenhouse gases absorb some of this solar radiation and as a result heat up, they then re-emit this heat as a longer wavelength em radiation which we know as infra-red radiation. These gases act as a blanket and keep the temperature of the Earth about 33 K higher than it would otherwise be.

Figure 1


The remainder of the solar energy on the Earth is used to evaporate water which in turn drives the cloud system. The heat released provides thermal differences which support the winds which aids the transportation of heat around the globe. This system is summarized in figure 1. This natural cycle is very finely balanced. Too little heat absorbed and we get a global cooling and too little radiated back into space we get a global warming.

This cycle which we call the natural greenhouse effect was first theorized by a French mathematician Jean-Baptiste Fourier. He compared how a greenhouse acted similarly to how the Earth maintains its temperature. In a greenhouse, the glass allows the light from the sun to pass unimpeded. This light is in turn absorbed by the plants which re-emit this light as heat. The glass acts as a barrier to this heat and it becomes trapped, heating up the greenhouse. In essence the glass plays the same role as the gases in our atmosphere.

You may now be asking the question what is so important about these gases that make them so important to this cycle. The atmosphere is made up of several different gases in various concentrations, the most abundant of these is nitrogen followed by oxygen. These two gases alone account for almost 99% of the atmosphere. In the remaining 1% of the atmospheric gases, the greenhouse gases can be found. What differentiates these greenhouse gases from the main atmospheric gases? The simple answer is oxygen and nitrogen are monatomic gases and as such their bonding structure does not vibrate with incident infrared radiation so they don’t readily absorb heat. However the greenhouse gases are a combination of two elements and the bonding structure in these configurations absorb infrared radiation easily. This phenomenon was known since the mid 1800’s due to the work of Irish physicist John Tyndall. From Tyndall’s experiment he was able to deduce the mechanism for why the Earth is able to maintain its temperature. The absorption characteristics of these gases are key to understanding the mechanism of the greenhouse effect.


  
Figure 2     

In the above diagram it shows the absorption of em radiation at certain wavelengths for the following gases, oxygen + ozone, carbon dioxide and water vapour. As can be seen from figure 2 oxygen/ozone is a very strong absorber in the ultra violet and a very narrow band in the infrared range. The key feature to pick up from this figure is the absorption of infrared radiation in water vapour and carbon dioxide. 

2 comments:

Terence said...

have you heard of the gaia hypothesis? I watched an interesting documentary about the guy who came up with it, James Lovelock. He is a brilliant scientist but his theory of a planet-wide biosphere that is self-regulating has been heavily criticised by the scientific community. This documentary suggested that it is becoming more widely accepted now. Its an attractive idea... if living things, and ecosystems, can regulate themselves to maintain homeostasis (and therefore life) why would the whole biosphere not do so? People like Dawkins dont believe that evolution could possibly extend so far though.

Daily Blaa said...

I read his books on the Gaia hypothesis. He was very far ahead of his time when he first published them. I think the acceptance of this hypothesis arises more from hope than scientific evidence. Even Lovelock himself in a recent interview conceded that we have reached a tipping point and have passed the point of no return. It does appear to be all doom and gloom unfortunately.

I will put up the next part of this post on the weekend.

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