GLOBAL WARMING

At one point, many astronomers believed that there was a significant chance of finding life on the planet Venus. It was closer to the sun, and its average temperature was expected to be 10-20 degrees warmer than Earth's. It was blanketed with clouds, and it was easy to imagine that these clouds covered a moist, fertile environment, teeming with life.

That view changed throughout the twentieth century, as new observations produced a dramatically different picture of conditions on our sister planet. In the 1930's, spectrographic analysis found no oxygen and high concentrations of carbon dioxide in the Venusian atmosphere. In the 1950's, radio astronomy indicated a surface temperature over 600 F. And in December 1962, the Mariner 2 mission measured a surface temperature of 800 F. More details are in

http://www.nasa.gov/mission_pages/exploration/whyweexplore/Why_We_22.html

The discovery of these high temperatures on Venus called for an explanation. And over time an understanding emerged. Venus was so extremely hot because its atmosphere was different from Earth's. The Venusian atmosphere consists of 95% carbon dioxide, at an atmospheric pressure 75 to 100 times greater than our Earth's. We know from experiment that carbon dioxide is relatively transparent to visible and ultraviolet light, but is highly absorbent of infrared and lower energy emissions. The thick Venusian atmosphere acts as a blanket to trap solar radiation and to raise the surface temperature, making it hotter than an open flame.

This phenomenon has become known as the greenhouse effect. The term was coined in the 1930's by astronomer Rupert Wildt to describe the expected effect of high levels of carbon dioxide in the atmosphere of Venus. At the time, most Americans would have been familiar with the experience of walking into a backyard greenhouse on a cold day and feeling the warmth inside. Modern Americans are more familiar with cars than greenhouses. We know what it's like to open a car that has been parked in direct summer sunlight for a few hours with its windows rolled up tight. Maybe we should come up with a new name. Should we call this the parked car effect? The rolled up window effect? The car upholstery effect?

OK. The greenhouse effect it is.

Glass is made to be transparent to visible light and is also transparent to higher energy ultraviolet light. But glass absorbs infrared and lower energy wavelengths. The emission spectrum of the hot sun contains plenty of high energy radiation that passes through the glass and into the car. But the emissions of the lower temperature car surfaces are blocked from escaping. As solar energy is trapped within the car, the temperature rises. Eventually, the temperature difference between inside and outside starts heat flowing through the structure of the car, and its hotter inside surfaces produce higher energy emissions that can pass through the glass. An equilibrium is reached when the energy entering the car equals the energy escaping from it. Ouch, that's hot!

There are various gases that share the same property of being transparent to high energy ultraviolet radiation while being highly absorbent of low energy infrared radiation. These greenhouse gases include carbon dioxide, water vapor, methane, nitrous oxide, and others. They do not interact much with high energy light, and therefore visible and ultraviolet light from the sun passes through them unaffected. But they interact strongly with infrared radiation, and therefore act as a barrier to reflected energy from the surface of the Earth.

The physics is not in question. Adding greenhouse gases to our atmosphere will allow solar energy to reach the Earth but will form a barrier to its escape. In the absence of other factors, this added energy will cause the temperature of the Earth to rise.

But here we are talking about an entire planet, and the "other factors" and their interactions are so complex that human science can not make precise predictions of the present and future behavior of our climate. Cynical vested interests have used this uncertainty as an opportunity to sow confusion and doubt, and to make it impossible for there to be a reasoned national response to this issue.

But the truth is out there.

It can be found on the bone dry searing lifeless surface of Venus.

It can be found in the blast of heat that brings beads of sweat to our foreheads when we slide onto the seat of a car that was parked with its windows closed in the hot summer sun.

We're rolling up the windows. And we can't get out of the car.

An inconvenient truth indeed.