Last week’s column briefly described weather, climate, global climate, and the atmosphere as a global system redistributing energy received from sunlight due to the tilt of the Earth’s axis. Everything naturally moves from a higher to a lower concentration and nature seeks equilibrium or the lowest energy state. Finally, certain gases like carbon dioxide in the atmosphere allow visible light through but don’t let heat (longwave radiation) back out. Next a brief description of the what and why of global warming with apologies for the simplification.
Global Warming and the Greenhouse Effect are linked but what is the greenhouse effect. Most of us have been in a greenhouse. On a bright sunny day it’s significantly warmer than the outside air. But why? The material covering the enclosure allows in visible solar radiation which is absorbed to various degrees by what is inside the greenhouse. The absorbed solar radiation heats up what has absorbed it (water, water vapor, plants, etc.). When the matter absorbing the sunlight heats up and becomes warmer than its surroundings, it gives off heat. That heat is given off as infrared (longwave) radiation. Unlike visible wavelengths that can pass through the greenhouse covering, these wavelengths are “trapped” and the greenhouse heats up. This is basically what happens to your car interior on a sunny day. Now we need to relate that to the Earth.
First, let’s use carbon dioxide to refer to the many greenhouse gases since it is the predominate gas of a group that includes water vapor, methane, nitrous oxide, ozone and CFCs. Now consider the earth as a giant, spherical greenhouse receiving solar radiation as in the previous paragraph. Without the gases just listed, the earth would reradiate much of the solar radiation received as longwave radiation back out into space and maintain a fairly constant global temperature. A little less than eighty percent of the atmosphere is nitrogen gas with an additional twenty percent oxygen. Neither of these is a greenhouse gas. The rest consists of carbon dioxide, water vapor, and minute amounts of other gases. Water vapor is the most variable of these. Most of the atmosphere allows longwave radiation (heat) back out into space. If the concentration of carbon dioxide increases than more heat (longwave radiation) is trapped in the atmosphere and the average global temperature warms. If the concentration decreases, the reverse would be true.
Millions and billions of years ago the atmosphere had much higher concentrations of carbon dioxide and other greenhouse gases (especially water vapor). As plant life was established that could thrive in these conditions, more and more carbon dioxide was removed from the atmosphere and trapped in the form of organic molecules, both living and dead. As carbon dioxide levels dropped, the average temperature of the atmosphere dropped as did the concentration of water vapor since more longwave radiation could escape into space. This carbon dioxide trapped in organic molecules by plants eventually became peat, coal, crude oil, and natural gas. These compounds were originally formed by by plants using energy from the sun to make organic compounds so as they accumulated and transformed into fossil fuels they stored the solar energy used to form the original compounds. That’s why they release energy when the bonds are broken and can be used as an energy source. Next week’s column will attempt to tie all this into both sides of the Global Warming debate.