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Jul 27 2006

Global warming-3: The science behind global warming

To understand the science behind global warming, it may be helpful to look at a simplified version of the science behind it.

Consider two objects, one that is luminous (i.e., an object that we can see without the aid of a light source) and another that is not luminous. Examples of luminous objects are the Sun (which generates energy due to nuclear reactions within it and sends a lot of that energy out as light) or a light bulb (that converts electrical energy into light energy). Examples of non-luminous objects are the Earth or a person in a room. The energy radiated by the luminous source spreads out in all directions and some of it will fall on the non-luminous object.

What is important to understand is that even what looks like a non-luminous object also radiates energy into space. In fact every object radiates energy. So in a sense, every object is ‘luminous’ in the sense that it sends out energy, but we usually reserve that term for objects that emit visible light. Not all radiated energy is visible. A human being radiates energy at a rate of about 500 watts, or the equivalent of five 100 watt bulbs, but the reason we do not “see” the radiation energy emitted by people is due to it being outside the visible range

The rate of energy emission of an object radiates depends to a large extent on its temperature (it actually goes as the fourth power of the temperature) and the nature of its surface (such as color, texture, material). So just as the Sun radiates energy into space, so does the Earth, except that the Sun’s radiation is much greater since it is at a much higher temperature.

The important thing about global warming is understanding what happens when the energy radiated by a luminous source (say the Sun) falls upon a non-luminous object (say the Earth). Part of it is immediately reflected back into space, and does not affect the temperature of the Earth. But the rest is absorbed by the Earth and, in the absence of anything else happening, will tend to cause the Earth’s temperature to rise. The relative amounts of the Sun’s energy that are absorbed and reflected by the Earth depends on the nature of the Earth’s surface. (As an example, a person in a room absorbs energy from the surroundings at a rate of about 400 watts, thus adding a person to a room is the net heat equivalent of turning on a 100 watt bulb.)

But as the temperature of the object rises due to it absorbing energy, the amount it radiates out again also increases, and at some point the object reaches equilibrium, which occurs when the energy absorbed by it from outside equals the energy it radiates away. Once an object reaches this state of thermal equilibrium, its temperature stays steady.

If for some reason we alter the ratio of energy absorbed by the Earth to the energy reflected, then the state of equilibrium is disturbed and the Earth’s temperature will shift to a new equilibrium temperature. If relatively more energy gets absorbed, then the equilibrium temperature will rise until the energy radiated again becomes equal to the energy absorbed. Conversely, if relatively more energy now gets reflected, then the equilibrium temperature will drop, i.e., the Earth will cool. The people warning of global warming argue that human activity is causing the former situation and they say that the reason for this is that we are changing the nature of the Earth’s surface, especially its atmosphere.

To understand what is happening at the Earth’s surface and atmosphere, we need to understand something about the energy radiated by the Sun. This comes largely in the form of “electromagnetic energy.” This is an umbrella term that encompasses X-rays, ultraviolet, light waves, infrared, microwaves, radio waves, etc. All these types of radiation are identical except for one single factor, which is called the wavelength of the radiation. The items in the list differ only in their wavelengths, with X-rays having the smallest wavelength and radio waves having the longest. (Similarly, all colors of visible light are also identical except for the wavelength, which increases as you go from blue to green to yellow to red.)

When this broad range of electromagnetic radiation from the Sun hits the Earth’s atmosphere, almost all of it, except the visible light portion, gets absorbed by the atoms and molecules in the atmosphere and does not reach us on the ground. Of the portion that does reach the ground, some of it gets directly reflected unchanged and escapes back into space. The remainder gets absorbed by the ground. It is the energy that is absorbed by the ground that is the source of concern.

Recall that the Earth, like any object, also radiates energy away. But since the temperature of the Earth is different from the temperature of the Sun, the distribution of the wavelengths in the energy radiated by the Earth is different from the distribution that we receive from the Sun (although the total energy involved is the same in both cases for an object in equilibrium). This affects how much is absorbed by the atmosphere as it passes through it. Some of the Earth’s radiation will get absorbed by the gases in the atmosphere (i.e., is trapped), while the rest passes through and goes off into space.

This is a crucial point. If the gases in the atmosphere change significantly, then you can change the relative amounts of the Earth’s radiated energy that escapes into space and the amount that is trapped by the atmosphere . The so-called ‘greenhouse gases’ (carbon dioxide, water vapor, methane, nitrous oxide, and others) are those that are very good at absorbing the energy at the wavelengths radiated by the Earth, preventing them from escaping into space.

Global warming scientists argue that human activity is increasing the concentration of greenhouse gases (especially carbon dioxide) in the atmosphere. Hence more of the energy radiated by the Earth is being absorbed and less of the energy is escaping into space. Note that the incoming visible light from the Sun is not affected much by the concentrations of greenhouse gases since they are at a different wavelength, and the greenhouse gases do not absorb them as much. As a result of this increase in the absorption levels of the outbound radiation, the equilibrium temperature of the Earth will rise.

At this point, there are various scenarios that can unfold. One is that we arrive at a new and higher but stable equilibrium temperature. If the change in equilibrium temperature is small, the consequences might not be too disastrous, although there will be some adverse effects such as some temperature-sensitive organisms (such as coral reefs) becoming destroyed or some species going extinct if they cannot evolve mechanisms to cope. If the change is large, then there could be massive floods and droughts and other catastrophes.

The worst case scenario is a kind of runaway effect, where a rise in temperature results in effects that cause an even more rapid rise in temperature and so on, in a series of cascading effects.

Some argue that we are already seeing some signs of runaway effects, and point to the melting of the polar ice caps and the general decrease in glaciers and snow coverage worldwide. Snow is white and thus reflects back unchanged into space almost all the sunlight that hits it at the Earth’s surface. When this snow melts and becomes water, not only is the amount of reflected energy decreased but water absorbs light energy. Hence the major loss of snow cover (apart from adverse environmental and ecological consequences) has a major effect on the reflection/absorption balance of the Earth, shifting it towards greater absorption. So more energy is absorbed by the Earth, resulting in even greater warming, resulting in further snow loss, and so on.

Another possible runaway factor is the amount of green cover. On balance, plants, because of photosynthesis, tend on average to be net absorbers of carbon dioxide and emitters of oxygen. Thus they reduce one of the greenhouse gases. If global warming results in less green cover of the Earth (say caused by prolonged droughts), then that would result in more greenhouse gases remaining in the atmosphere and causing yet more warming and more droughts. Human activity such as deforestation can accelerate this process.

Those are the basic elements of the science underlying global warming and the factors that go into building the models that try to predict long term climate change.

Next: The emerging scientific consensus over global warming.

POST SCRIPT: Colbert takes media apart again

As you may recall, the mainstream media did not take kindly to Stephen Colbert’s demolishing them at the White House Correspondents Association Dinner. Now he takes them apart again.

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