As this is written there are a few inches of snow of the ground at this author's residence, the results of a winter's storm which breezed through a couple of nights earlier. In the meantime, much of the midwestern region of the U.S. is digging out from a major winter storm which brought significant amounts of snow, freezing rain, and bone-chilling temperatures. Under such conditions a term like "global warming" may not seem to have much meaning, but on the other hand, why not take a look at it?

Any object in the presence of a heat source, be it sunlight, an electric heater, or a campfire, will warm up, and will start to radiate that heat and cool off once it is removed from the presence of that source. If there was nothing else involved this process would operate quite straightforwardly. But as is usually the case in nature, reality is seldom so simple.

The Earth, of course, receives the vast majority of its heat from the sun, and the side of the Earth that is facing the sun becomes warm as it is exposed to sunlight. Under simple circumstances, as soon as a section of the Earth's surface rotates away from the sun and experiences nighttime, it would very rapidly cool off and become extremely cold. This in fact is precisely what happens to the regions of the moon's surface that are not facing the sun.

This does not happen on the Earth due to the presence of the Earth's atmosphere. The various gases making up the atmosphere "keep heat in," so to speak, and prevent the surface heat from rapidly leaking off into space.

The atmospheric gases most responsible for this are water vapor and carbon dioxide. The molecules of these gases are rather transparent to what we call the "visible" portions of light, and this is where the majority of the sun's energy output lies. As a result, the sun's energy arrives at Earth's surface relatively uninhibited, and the surface heats up. Meanwhile, these same gases are significantly opaque to infrared light, and since the radiation that a warm body gives off is primarily infrared, these gases prevent the heat from leaving. Glass has some of these same optical properties (i.e., transparent to visible light, opaque to infrared) and since this process is used in glass greenhouses to keep the internal environment warmer than it otherwise would be, this phenomenon has come to be known as the "greenhouse effect."

Water and carbon dioxide are natural constituents of the Earth's atmosphere, and thus the Earth experiences a natural greenhouse effect; in fact, the Earth's surface temperature is already about 50 degrees warmer than it would otherwise be without this. The process is pretty much self-regulating, as there are processes that place these gases into the atmosphere (e.g., evaporation for water, animal respiration and volcanic eruptions for carbon dioxide) and other processes that remove it (e.g., rainfall for water, plant respiration for carbon dioxide).

Within the relatively recent past (geologically speaking), a new player has entered the scene: humanity, or perhaps more accurately, humanity's civilization. Many of humanity's industrial processes are injecting significantly greater amounts of carbon dioxide into the atmosphere than would otherwise be the case. At the same time, in order to meet humanity's increasing needs brought on in part by its expanding population, significant amounts of plant life, which remove carbon dioxide from the atmosphere via the process of photosynthesis, are being removed from the surface. This is especially acute in the Earth's tropical regions, where large sections of rain forest -- responsible for removing significant portions of the atmosphere's carbon dioxide content -- are being cut down at drastic rates.

The results of these activities are starting to show. Careful measurements reveal that, over the past century, the Earth's average surface temperature has increased by about one degree. While this may not sound like much, global weather patterns and processes such as melting of polar cap ice are very sensitively attuned to even small changes in temperature. A recently-released comprehensive international study suggests that, based upon observed changes during the past few years, the Earth's average temperature will increase anywhere from three to eleven degrees during the next hundred years. This could have devastating consequences to all of humanity, and the sweltering late-summer heat waves experienced by some parts of the U.S. and the violent hurricanes that we have seen during the past few years may only be a precursor to what is to come.

It can be argued, of course, that not everything that is happening may be due to human activities, and it is in fact quite possible that there are natural processes operating on timescales of thousands to millions of years that we as yet only dimly understand. Nevertheless, there can be no question that we are affecting the Earth's atmosphere by our activities, and our ever-expanding population (which has more than doubled during this author's lifetime) can only add to this. Even if our understanding is incomplete, it would nevertheless seem prudent to take steps now to alleviate these consequences as much as possible.

Perhaps our best lesson is shining brightly right now in our southwestern evening sky. The planet Venus was named over two thousand years ago for the Roman goddess of love and beauty, but spacecraft measurements indicate that the surface conditions are anything but beautiful. A massive carbon dioxide atmosphere has produced an enormous greenhouse effect which has raised Venus' surface temperature to a hellish 900 degrees. Could Earth become another Venus? Perhaps, perhaps not -- there are of course other variables and processes involved, not all of which we understand -- but the sight of our "sister" planet shining in our nighttime sky should convince us to try to protect the planet that we do have.

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