“The Greenhouse Effect”
Introduction: The "greenhouse effect" is widely discussed in the media, and although its details are complicated, its principles are not difficult to understand. Without a greenhouse effect, radiation from the Sun (mostly in the form of visible light) would travel to Earth and be changed into heat, only to be lost to space. This scenario can be sketched as follows: Sun’s radiation → absorbed by Earth → Re-radiated to space as heat The greenhouse effect is a process where energy from the sun readily penetrates into the lower atmosphere and onto the surface of Earth and is converted to heat, but then cannot freely leave the planet. This can be sketched as follows: Sun’s Radiation → absorbed by Earth → some re-radiated to space as heat → some trapped by the atmosphere Due to the presence of certain “greenhouse gases” that trap heat, like carbon dioxide, methane, water vapour, and CFC’s, the atmosphere retains the sun’s radiation and warms up the planet. By increasing the abundance of these gases in the atmosphere, humankind is increasing the overall warming of the Earth’s surface and lower atmosphere, a process called "global warming." The figure below illustrates the radiation balance and the role of greenhouse effect. The Radiation Balance: Another way to think about the greenhouse effect is to consider that according to physics the radiation we receive from the Sun must be equally balanced by the heat Earth radiates out to space. If we were to give back less energy than we receive, our planet would soon be too hot for life. Likewise, if we were to give back more energy that we receive, our planet would soon be too cold for life. This can be written as a balanced equation of radiation: Solar radiation input to Earth = Earth’s output of re-radiated heat If we were to measure the temperature of the Earth from space, the Earth's "surface" would show a temperature appropriate for this requirement of energy balance: a measurement of roughly -18 degrees Celsius (about 0 °F). At this temperature, our planet radiates a quantity of heat into space that is equivalent to the amount of energy received from the Sun. At this point you may be asking how we can speak of “global warming” when we have just stated that the Earth (as seen from space) MUST stay at the same temperature. And how is it that the temperature of the Earth’s surface is only a chilly 0°F? The key to understanding this apparent contradiction is to remember that we live at the bottom of the atmosphere. As far as the radiation balance is concerned, the lower atmosphere and the surface of Earth form part of a “warm interior” of the planet. The apparent temperature "surface" that we would see from space is located well above the real surface of the Earth where we live. This apparent temperature "surface" is about 5000 meters up (17,000 feet) within the atmosphere. To get a better handle on this concept consider the following: the difference in elevation between 0 meters and 5,000 meters corresponds to a difference in temperature of about 60°F. In other words, at sea level it is 60°F warmer than it would be without the atmosphere. For the last 100 years or so this apparent temperature “surface” has been moving upward in the atmosphere as a result of global warming. As the apparent "surface" rises, the bottom of the atmosphere gets warmer, a fact that can be seen in the positions of the snow line (the elevation where snow begins to form) and tree line (the elevation where it becomes too cold for trees to grow). However, despite all these changes happening in the lower atmosphere, the overall temperature of the planet as seen from space stays the same. How is it possible that the Earth exactly balances the incoming sunlight with the outgoing heat radiation? The answer is simple: the amount of heat radiation from Earth is precisely tied to the temperature of the atmosphere. If the temperature of the apparent “surface” is too low and Earth radiates...
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