From Mensagenda - February 2007

The Future: The Greenhouse Effect

by Mary Abbey

I begin this column with a hope for the future: May we all be intelligent in making our energy choices, reducing the emissions of greenhouse gases, and delaying the loss of our fossil fuels. If we don’t, the future will be a really scary place. We must all be aware of the problems associated with our lifestyle choices and work to lessen the impact that we have on this planet.

In order to understand one major consequence of human activities, global warming, we need to understand the greenhouse effect. Otherwise it’s just one (of thousands) sounding the alarm. First of all, the greenhouse effect is good; in fact, it’s essential. Without this effect our average world temperature would be around -18°C, that’s -0.4°F, too cold for life to survive. In a greenhouse, visible light passes through the glass and is absorbed by dark surfaces inside. This energy is then reradiated as infrared waves, which we detect as heat. The heat hits the glass and is mostly reflected back into the greenhouse. The air is warmed by this heat and the overall temperature goes up. You also experience this if your car sits in the sunshine, especially if you have black upholstery. Our atmosphere is warmed by a process similar to this, but also includes major convection cells, and wind and ocean currents, which work to distribute heat around the world.

Our atmosphere is composed of approximately 20.5% O2 and 78% N2. These diatomic molecules are tightly bonded together, so tight that they cannot absorb light energy. Molecules absorbing energy vibrate, flexing back and forth, until that energy is released. Therefore, most of our atmosphere is transparent to light. The nice warm air you feel on a sunny day is not warmed by light passing through the atmosphere. It is warmed by ground surfaces, the soil, trees, pavement, buildings, etc. These surfaces absorb the visible light; the darker, the more absorbent. You’ve all felt the difference between wearing a black shirt and wearing a white shirt on a sunny day. These molecules that have absorbed the light vibrate and release energy, now in infrared. This happens slowly over several hours, so that heat is being radiated throughout the night. Without gases made of larger molecules in our atmosphere, this heat would escape right off the planet. In fact, most still does.

The final 1.5 percent of our atmosphere is made up of water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxides (NOx), Sulfur oxides (SOx), CFCs, and other trace gases. There are at least 30 greenhouse gases, but these are the most important. When heat is radiated from the surface, some of it is absorbed by these gases and reradiated. This energy can be directed in any direction, so some is eventually lost to space. About 90 percent of this reradiated heat stays in the atmosphere, giving us warmer temperatures.

So what about global warming? That’s next when I’ll talk about the enhanced greenhouse effect and other variables that you should know. In the meantime, check out www.globalwarming101.com. Will Steger is leading an expedition to Baffin Island and is inviting all of you to come along. If you have students, or are yourself a student of life, check this out.