The Greenhouse Effect
The Keeling Curve
The Greenhouse Effect
The Greenhouse Effect is commonly mentioned in the context of “Global Warming”, that is, the observation that the northern hemisphere has warmed by more than 1°F over the last 100 years, and that the decades of the 1980s and 1990s have been the warmest on record, as far as can be ascertained, for the last 1000 years. What is at issue when discussing global warming, however, is not the Greenhouse Effect by itself, but an additional Greenhouse Effect, in excess of the one that Earth has naturally. The Excess Greenhouse Effect is produced by the release of greenhouse gases to the atmosphere, from human economic activities.
To understand the natural Greenhouse Effect (which makes Earth a pleasant place to live) we have to look at the energy balance between incoming sunlight heating Earth and outgoing heat radiation cooling the planet. The two must balance, since the Earth has stayed at roughly the same temperature for millions of years (as shown by the fact that the fish and corals in the ocean and the trees and insects on land have been around for a very long time). If we were to measure the temperature of the Earth from space, the Earth's effective radiation surface (not the ground surface, but what-is-seen-from-space-in-the-infrared, actually a zone) would show a temperature of roughly –18 degrees Celsius (about 0°F). This is the temperature that is appropriate for the requirement that incoming and outgoing energy must balance. At this temperature, our planet radiates a quantity of heat into space that is the same as that amount of sunlight received from the Sun that is converted into heat on the ground and in the atmosphere.
How then can we 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 average temperature of the Earth’s effective radiation surface is only a chilly 0°F? How can we get away with wearing T-shirts?
The key to understanding this apparent contradiction is to remember that we live at the bottom of the gaseous sea called “atmosphere.” As far as the Earth’s radiation balance is concerned, the lower atmosphere and the surface of Earth form part of a “warm interior” of the planet. In the process of “global warming”, this interior – the lower atmosphere – warms up.
The relevant surface for re-radiation of heat that we see from space is located well above the real surface of the Earth where we live. The Earth’s heat radiation zone is centered 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.
How does global warming affect this picture? We must assume that, upon warming the lower atmosphere, the radiative surface (what-is-seen-from-space-in-the-infrared) moves upward in the atmosphere. The rise can be seen in the changing positions of the snow line (the elevation where snow stays on the ground) 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.
Figure demonstrating the importance of greenhouse gases in regulating the temperature of the lower atmosphere. The top diagram shows a greenhouse Earth (the real Earth) where the black-body (radiative) “surface” lies 5000m up in the atmosphere from the land surface. In the past 100 years this “surface” has been rising. The lower diagram shows what the Earth would look like without a greenhouse effect, with 0°F at the ground.
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 heat radiation surface is too low and Earth radiates too little heat to keep the balance, Earth will warm up and then radiate more heat into space. If the temperature of the heat radiation surface is too high and Earth radiates more heat than it receives, the planet will become colder and consequently 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.
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 is also near 0°F. 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 which 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, which increases 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. Clouds reflect sunlight, reducing the incoming energy that can be converted to heat. (On a hot summer day, when clouds start covering the sky, we feel relief from the shading.) Clouds also intercept heat radiation from the Earth’s surface and atmosphere, and radiate heat back down, warming the surface. (A cloudy night in the desert is much warmer than one with a starry sky.) Which of these processes – reflection of sunlight or trapping of outgoing heat – dominates in a given situation depends on circumstances and on cloud properties. The uncertainties surrounding the role of clouds prevent a precise calculation of the effects of the excess Greenhouse Effect (from release of carbon dioxide and other greenhouse gases) on the temperature of the lower atmosphere. From the fact that the natural Greenhouse Effect keeps us nice and warm, we know that the Greenhouse Effect works. Thus, we know that global warming is inevitable when greenhouse gases are added to the atmosphere. Estimates of overall warming from a doubling of carbon dioxide fall largely within the range of between 1.5 and 4.5 degrees Celsius (roughly 3 to 8°F), although values outside this range also have been suggested.