The Science of Global Warming

The Physics of Global Warming

The Earth is a rock orbiting the sun. 99.98% of the Earth's energy comes from the sun. The rest is from radioactive decay within the Earth. The sun emits radiant energy in the form of light and other electromagnetic radiation. Light is the specific electromagnetic radiation in the visible spectrum with a wavelength of 400-700 nanometers. Our eyes have adapted to be sensitive to the visible spectrum because our atmosphere is transparent to this range of electromagnetic radiation. This means that visible light is able to pass virtually unimpeded from the sun to the Earth. Our eyes detect the light that reflects off of objects. By interpreting the light rays that reflect off of objects our brain is able to interpret the environment around us. Our eyes and brain construct an image of the world around us by interpreting the amount of light, which is the intensity, the color of light, which is the frequency, and the distance away using parallax. Parallax is the result of having two eyes and being able to obtain depth perception.

Although our eyes are adapted for the visible spectrum, this is only a very minute portion of the entire electromagnetic spectrum. The sun produces the entire range of the EM spectrum from high energy gamma rays to x-rays to ultraviolet to visible to infra-red, which is heat, to radio waves. The infra-red is very important in global warming, but we will come to that in a minute. The key point is that our atmosphere which is made up of nitrogen, oxygen and a small amount of other gases is opaque to a majority of all waves. This means that the majority of EM waves that reach the atmosphere from the sun interact with the molecules in the atmosphere and are absorbed. This is a result of the chemical composition of the molecules. Ozone is a good commonly known example. Ozone, O ₃, is a molecule that is particularly effective at absorbing ultraviolet (UV) radiation and its depletion resulted in a "hole in the ozone" which drastically reduced our protection from UV radiation over Greenland. This is only one example but the story of dealing with the ozone problem indicates that the world is able to solve global problems when it is determined to do so. The combination of the different molecules in the atmosphere absorbs most EM radiation.

However, our atmosphere is mostly transparent to visible light. In effect, there is a window that allows almost exclusively visible light to pass through unaffected. Thus the energy from the sun in the visible spectrum reaches the Earth unimpeded, part of which is absorbed and warms the Earth. The Earth that then acts as a black body eventually emits the energy in the form of heat, known in the EM spectrum as infrared radiation. If this energy in the form of infrared radiation were able to leave as it came, the temperature of the Earth would be -25° Celsius. This is below 0 degrees, which is the freezing point for water, so the Earth would be frozen over and uninhabitable for life!

Obviously, the Earth is not frozen over, and in fact the average temperature is around 12°C which makes our existence possible. So what accounts for this increase in temperature? The answer is that the natural greenhouse effect of our atmosphere retains some of the heat that is reradiated by the Earth, raising the temperature enough to be habitable. Let's consider how this greenhouse affect works in detail.

The greenhouse effect is named because the effect of our atmosphere is the same as the process that results in a greenhouse trapping in solar energy. A greenhouse is made out of glass. Glass, like our atmosphere, is transparent to light. This means that the visible spectrum of EM radiation is able to pass through. After entering the greenhouse the light strikes an object in the greenhouse. Some of the light is reflected off of objects and the reflected light is able to pass back through the glass. But some of the light is absorbed by the objects in the greenhouse. The objects in the greenhouse, thereby absorbing the energy, heat up. Eventually the energy that was absorbed is released again, but this is the critical point. When the energy is released it is emitted based on the temperature of the object that is emitting the energy. The sun which is 6000° K radiates EM energy at a much higher frequency as we discussed before and some of that energy is in the visible spectrum. An object which is hot enough to emit light is known as a luminous object. The objects in the greenhouse will be at a much lower temperature, somewhere we suspect in the range between 0°C and 100°C. At this temperature the radiated energy will be in the form of heat, which is EM radiation in the infrared range. This heat radiates out from the objects. The key aspect of the greenhouse analogy is that like our atmosphere, the glass is opaque to infrared radiation. This means that the heat energy will not pass through but will instead be kept in by the glass. Thus the energy in the form of heat which is trapped in will cause the greenhouse to warm up. This is the essence of the greenhouse effect. The atmosphere lets light energy in but the energy that is emitted by the Earth in the form of heat is trapped raising the temperature of the Earth.

A detail of the greenhouse effect is that the atmosphere is only partially opaque to heat, so not all of the heat is trapped in. Some portion of the heat escapes. The temperature rises and eventually a new equilibrium point is established. As is always true there is a conservation of energy, so that the heat coming in has to be the same as the heat going out. What is happening, though, is equivalent to what occurs when you put on a blanket or a shirt. The extra layer traps the heat in, thereby raising the temperature close to your skin, but the outer surface is still in thermal equilibrium.

So the greenhouse affect is a natural consequence of our atmosphere and is a very good thing because otherwise the Earth would not be habitable. For a planet to be habitable, from what we know of the requirements of life, we assume that there must be liquid water so the temperature must be between 0°C and 100°C. There is a very real example of the greenhouse effect making a planet uninhabitable. Let us look at the planet Venus. Increases in greenhouse gases, such as carbon dioxide (CO ₂) cause the equilibrium temperature to rise. Without an atmosphere, and assuming an albedo equal to Earth, Venus would be approximately 20°C, but it turns out that the actual surface temperature of Venus is 400°C which is enough to melt lead. So what is different about Venus, which otherwise is very similar to the Earth, that caused it to be uninhabitable? It is a very interesting story and one that we on Earth must take very seriously. It turns out that the early atmosphere of Venus was very similar to the Earth but Venus being closer to the sun was a little bit hotter. The CO ₂ in the atmosphere, as on Earth, was partially absorbed by the rocks, which formed a carbonate. But on Venus the temperature was slightly hotter and the process of rocks forming carbonates was temperature dependent so as the temperature on Venus rose the process became less efficient, consequently less CO ₂ was absorbed and the temperature rose more. This made the process even less efficient and the levels of carbon dioxide continued to rise until the atmosphere was almost 90-95% CO ₂. Therefore, the temperature continued to rise until it reached an equilibrium point around 400°C. However, on the Earth the rocks literally saved us! The rocks were able to absorb enough carbon dioxide to keep the levels of CO ₂ in check. The levels of carbon dioxide in the Earth's atmosphere stabilized at 0.03% and resulted in a comfortable average temperature of 12°C. Thus Venus is a huge warning to us of the dangers of increased CO ₂ levels and other greenhouse gases.

At the Earth's formation the atmosphere was mainly made up of the light elements of hydrogen and helium with traces of argon and other gases. At the extreme temperatures of the young Earth these light elements were not bound to the Earth strongly enough by gravity and they had enough kinetic energy to achieve escape velocity. Thus these gases escaped. It took billions of years for the Earth to build up its current atmosphere and there is a very delicate balance of greenhouse gases that enables the Earth to be habitable, as we have seen from the example of Venus. The primary greenhouse gases are water vapor, carbon dioxide, methane and ozone. Water vapor is the predominant greenhouse gas and constitutes about 1% of the Earth's atmosphere. The levels of these greenhouse gases have fluctuated naturally, but the current concern is the drastic increase in the level of greenhouse gases from human causes. Primarily the greenhouse gases are the result of the burning of fossil fuels. The level of CO ₂ is the primary concern although increases in methane and other greenhouse gases also contribute to global warming. Since the industrial revolution the concentration of CO ₂ has increased by 35% and is projected to continuing increasing precipitously unless the emissions of CO ₂ is drastically curbed. It is projected from the current climate models that the levels of CO ₂ will double what they were before industrialization, 150 years ago, in the next 40 to 200 years!

Although the science of weather prediction is very complicated, and we will go into the reasons later, scientists are in agreement that a drastic increase in CO ₂ will result in increased temperatures. Already, in the past 50 years the Earth's average temperature has risen more than a half a degree Celsius and the rate is accelerating. In addition, this is only half of the predicted amount as a result of the time delay for the oceans to heat. Although there is a lot of uncertainty about the exact amount, the best predictions indicate that the average global temperature will rise between 1.5°C and 4.5°C according to the IPCC (Intergovernmental Panel on Climate Change), when the pre-industrial level doubles. Although this may not sound like a lot it has been shown that the ice ages were likely the result of a change of less than 4.5°C! It is possible that it was even less because the amount of warming is not consistent across the globe. The poles experience as much as ten times the amount of warming at the equator. So we are facing a very grave situation and something must be done right away. The consequence of such an increase in the average global temperature is potentially catastrophic. This might seem hard to believe since the daily temperature can vary by as much as 20°C. However, the difference is that we are talking about the average temperature of the entire globe including the poles. Even though the daily and seasonal temperatures vary greatly, the average global temperature is remarkably constant! Virtually all of the energy that heats the Earth comes from the sun and the energy that reaches us from the sun is relatively constant; therefore, the overall average is relatively constant although the local temperatures can vary dramatically. Next let us consider the implications of the increase of global temperatures as a result of global warming.