Introduction
Greenhouse Gases

As the book states, the greenhouse effect is not controversial. The principle by which infrared radiation is absorbed by certain gases in the atmosphere and causes heating is very well understood. Anyone who has ever spent the night both in the southwestern deserts of the U.S. and in a southern coastal region during the summer knows that the greenhouse effect works very well. For those that have not experienced this, a quick study of our nearest neighbors in the solar system shows that the greenhouse effect works quite well.

Venus is about the same size as the Earth, but it revolves around the Sun at an average distance of .72 AU (1 AU is the average distance between the Earth and the Sun). Because of its closeness to the Sun, Venus receives more energy from the Sun. However, due to the fact that Venus is much brighter than the Earth and would reflect more energy back into space, calculations predict that Venus should have an average temperature of about 230 K (-43 ^oC) if there were no greenhouse effect in its atmosphere. Measurements of the planets atmospheric temperature from spacecraft that have visited it show that the average temperature is about 740 K near the surface, which is more than three times what is expected. While Venus has no water vapor in its atmosphere, it does have almost 3,000 times the amount of carbon dioxide that is in the Earth's atmosphere. This large amount of carbon dioxide is what accounts for this difference between what is expected and what is measured.

Mars, which is much smaller than the Earth, revolves around the Sun at an average distance of 1.52 AU. Because of its distance from the Earth and its slightly darker color, it is predicted to have an average surface temperature of about 218 K if there were no greenhouse effect. Measurements from the Viking and Pathfinder spacecraft show that the average is about 223 K. Why the small difference between two? While 95% of the Martian atmosphere is carbon dioxide, the atmosphere is very thin and contains almost no water vapor. This means that there is not enough greenhouse gases in the atmosphere to produce much of an effect.

Planet	No Greenhouse Effect Temperature	Observed Average Temperature
Venus	230 K	740 K
Earth	250 K	288 K
Mars	218 K	223 K

Debate

While there is no debate over the greenhouse effect, there is debate over global warming. This debate usually breaks out into three different debates. The first is over whether the Earth's average temperature has actually increased over the last 100 years, i.e. has there actually been global warming. Checking ground-based measurement over the last 100 years shows that this is, in fact, the case, even when one accounts for heat island effects for measurements near growing cities. While the increase has not been homogeneous across the surface of the planet, the overall effect shows a net gain. However, there have been measurements of temperatures in the upper atmosphere that seem to show cooling over the last several decades. This was unexpected, as most atmospheric models predict that the temperature should be increasing at these levels, too. Proponents of global warming claims point out, though, that these models do not take into account various factors that may be present at those levels, such as a thinning stratospheric ozone layer and the presence of particles and gases from a tremendous number of large volcanic explosions that occurred in the 1980's.

The next debate is over mankind's effect on this warming. There are factors other than the increase of anthropogenic greenhouse gases that could account for temperatures to be increasing. Energy output from the Sun, while being fairly constant, does vary over time. In fact, we know that the amount varies slightly during the 11 year sunspot cycle. There could be larger cycles that we do not know about that could be causing increased heating. There could also be an increase in greenhouse gases that have nothing to do with mankind. When volcanoes erupt, they spew out tons of greenhouse gases, as well as particles. What opponents to global warming claims state is that scientist have not shown the link between mankind's increases and the measured temperature increases enough to warrant the changing of our lifestyles.

The last debate is over the predictions of what will happen. Some opponents of global warming claims will grant that the temperature has increased and that mankind has had an impact on this increase. What they state is that the dire consequences that are predicted will either not occur or will not be bad. Some will claim that the negative feedbacks to global warming will be great enough so as to cause the temperature to cool back down. Others will claim that things will not be that bad. For instance, if it gets warmer, then winters in the North will not be that brutal, allowing for a much milder climate. Along with the increase in atmospheric carbon dioxide, this will spur plant growth and allow for record crops of grains, meaning that more people can be fed.

The sites below provide information on greenhouse gases and on the controversy surrounding global warming.

Greenhouse Gas Information

Department of Energy Greenhouse Gas Report (2000)
EPA Carbon Dioxide Calculator
Time Series of Greenhouse Gases (CSIRO)

Pro Global Warming Claims

EPA Global Warming
Woods Hole Research Center
National Oceanic and Atmospheric Administration

Anti Global Warming Claims

Marshall Space Flight Center
Globalwarming.org
Science and Environmental Policy Project

After reading through these and any other sites that you might find, answer the following questions