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Global warming: where the heat comes from

17 July 2014, 07:43
In my previous article, I explained what global warming was, what climate change was, and how global warming contributed to climate change. I highlighted how global warming drove climate change, but wasn’t itself climate change. In this article, I’m going to look at what drives global warming.

An increase in temperature implies an increase in energy. Sure, fine, if you want to get technical, you can also get an increase in temperature by increasing the pressure, but we can discount that one because, while we’ve observed a change in temperature, we haven’t observed an increase in atmospheric pressure. Therefore, the change in temperature must be due to an increase in energy. That’s basic physical chemistry and thermodynamics.
So how can you get a global increase in surface temperature? Well, it turns out there are several ways – in theory.

a) An increase of energy output by the sun
b) A change n the earth’s orbit bringing it closer to the sun
c) An increase in heat radiance from the earth’s core
d) A change in the earth’s atmosphere that’s trapping more heat

There is no single cause for global warming. Some of these components will interact to create the effect. In order to understand how this works, you need to understand a bit about how the sun interacts with the earth’s atmosphere.
Solar radiation has various components: x-rays, gamma rays, visible light, ultraviolet, infrared and all the other forms of radiation that the sun outputs. This radiation has to pass through the atmosphere to reach the surface. Various atmospheric gasses affect the various wavelengths of solar radiation in different ways. For example, the sky is blue because the composition of the atmosphere is such that the blue light component of visible light is scattered while other wavelengths of light pass through. Plants appear green because chlorophyll absorbs most other “colours” and reflects green light. Gas, like chlorophyll, can also reflect back certain wavelengths and absorb others. These various wavelengths hold different amounts of energy. Ultraviolet contains much more energy than infrared for example. We also know that different gasses and other components of the atmosphere absorbs or reflects different wavelengths of radiation, and therefore the composition of the atmosphere will determine how much solar radiation is reflected back and how much is absorbed. We call this radiation balance. An example of just how critical the atmospheric composition can be would be concerns over the hole in the ozone layer. Ozone absorbs ultraviolet light, preventing it from reaching the surface. Remove the ozone via emissions of CFC’s, and more ultraviolet radiation will reach the surface.

The earth’s atmosphere is composed of various gasses: oxygen, nitrogen, carbon dioxide, ozone, argon and many, many other gasses. These other gasses may make out a very small component of the atmosphere, but it still adds to a lot when you take into account the volume of the atmosphere.

Some of these gasses absorb heat, thus warming the surface of the earth. We call those “greenhouse gasses”, and greenhouse gasses are a good thing to have. If we didn’t have them, the earth would look like Mars. On the other hand, having too much of it would turn the earth into Venus. It takes surprisingly little to drive it one way or the other.

Now let’s look at option a: global warming as a result of an increase in solar radiation.

We’ve been measuring the sun’s output with great accuracy since 1978. To make a long story short, there is no evidence for an increase in solar output since we started measuring solar output directly. There’s also no evidence to indirectly indicate that solar radiation increased since 1900. One way to know this is to look at plant growth. An increase in solar radiation would mean more energy for plants to absorb. Plants, being photo-synthesisers, would show an increase in growth with an increase in absorbable energy, and yet there is no indication that plants grow better. What variability there is in the output of various radiation components from the sun is simply not sufficient to have caused the increase in temperature we observe. While variance in solar radiation cannot be completely discounted as a contributor, it isn’t the main contributor.
We can also safely say that the earth didn’t move closer to the sun. The only way that could happen is if the earth slowed down in its orbit, (in which case the length of the year would be different than it is now) the earth lost mass, or the sun gained mass. We can discount all of these. We can also discount an increase in heat coming from the earth’s core. The earth’s core contains the same things now that it did a hundred years ago. There is no reason, and indeed, no observed effect that would suddenly cause the earth’s core to get hotter.

So, if the sun isn’t getting hotter and the earth didn’t move, what are we left with? Only one thing: changes in the atmosphere that is trapping more heat. I’ve already explained how the earth’s atmosphere works in absorbing and reflecting energy, and how changes in the composition of the atmosphere can change how much energy is absorbed and reflected. What I haven’t said yet is what this change in absorption and reflection is called: we call it solar forcing. Solar forcing takes into account radiation balance, along with the earth’s albedo (albedo being how reflective the earth is), reflection from clouds and from particles in the atmosphere to determine the change in absorption and reflection. There’s a good (and easily understood) data table showing solar forcing available on Wikipedia. What this data shows us is that the carbon dioxide component of the atmosphere is increasing, and that it correlates very well with the increase in temperature we’re observing. Carbon dioxide being a greenhouse gas, it absorbs solar radiation, and in doing so increases the temperature. There are other components contributing to this of course. Methane is one of them. Carbon dioxide is, however, the main contributor.

In a follow-up article, I’ll look more closely at the sources of carbon dioxide, and then will come the really fun bits, where I start digging into climate change denial. Be patient for those. I’ll be doing a LOT more research when it comes to that. Oh, and if you’re worried about my sources, I’ll direct you to the site I found my sources at: www.sciencedirect.com All the articles I used I got from there. Feel free to run a search yourself. Far be it from me to rob you of the joy of discovery.

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