General Overview: The Greenhouse Effect
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 vapor, 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:
Illustration of the Earth’s radiative balance. (Adapted from: NOAA)
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 to 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.
Figure demonstrating the importance of greenhouse gases
in regulating the temperature of the lower atmosphere. The top diagram shows a greenhouse Earth where the apparent temperature “surface” lies 5000m up in the atmosphere from the land surface. In the past 100 years this apparent temperature “surface” has been rising. By contrast, without a greenhouse effect, the Earth would look like the lower diagram.
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 too little heat to keep the balance, Earth will warm up and radiate more heat into space. If the temperature of the apparent “surface” is too high and Earth radiates more heat than it receives, the planet will become colder and radiate less energy back to space. Overall, this “negative feedback” stabilizes the radiation balance despite all the variations of temperature from one place to another and within the vertical column of the atmosphere. It sets the temperature so that the incoming and outgoing energy is balanced.
Average Temperatures on the Moon
We can get another idea about what the temperature on Earth would be like without a greenhouse atmosphere by contemplating the Moon. The Earth’s satellite has no atmosphere because its gravitational force is not strong enough to retain gas for long. It has the same distance from the Sun as the Earth, but its temperature varies enormously: where the Sun is shining, the Moon’s temperature rises to 230°F and where it is dark falls to negative 290°F. The average surface temperature of the moon, about the same distance as the Earth from the Sun, is also near 0°F, but of course, the moon has no atmosphere. By contrast, the average surface temperature of the Earth is 60°F at sea level. On Earth, the contrast between maximum and minimum temperatures would not be as great as on the Moon, even without an atmosphere, because the Earth rotates once in a day, while the Moon only rotates once in a month. However, without an atmosphere the Earth’s contrast between day and night and the contrast between summer and winter would be very large indeed.
Not all the gases in the atmosphere are equally active in keeping Earth warm. In fact, the atmosphere’s most abundant gas, molecular nitrogen, does very little in this regard, and the same is true for the second most abundant gas, molecular oxygen. The most important ingredient of the air for producing the greenhouse effect is water vapor. However, its abundance depends on the air's temperature. The warmer the air, the more water vapor it can hold. (As air cools, the vapor condenses into rain or snow.) It is carbon dioxide that moves the air toward higher temperature, so that water vapor can take over and warm it some more. Carbon dioxide molecules intercept infrared radiation, warming the air and increasing water vapor through evaporation from the sea surface and from plants and soil moisture. Water vapor then increases the temperature even more. The process is checked by a rise in infrared radiation to space and by formation of clouds. Unfortunately, the role of clouds in the radiation balance is as yet poorly understood. Different types of clouds have different effects, and this makes the calculations complicated and the results uncertain.
One last point to consider when discussing the greenhouse effect is the amount of sunlight coming in to Earth. The quantity of sunlight we receive depends on the size and the brightness of the Sun and the distance between it and the Earth. As far as we know, the size of the Sun does not change much over the time spans we are considering; we can assume it is constant. The sun’s brightness varies only a little, about one-thousandth over an eleven-year sunspot cycle, but perhaps more over longer time spans. We can take that as constant too, calling the incoming energy flux "the solar constant." One of the contentious issues in the discussions about the global warming of the last 100 years that has not been fully resolved is the question of whether a brighter Sun may have contributed to the recently observed temperature rise.