All objects emit electromagnetic radiation continuously
All objects emit electromagnetic radiation continuously. The radiation emitted by a solid object (or a liquid) has certain fundamental characteristics which are described using the term ``blackbody radiation''. This term has nothing to do with an object's color, but instead it refers to how it absorbs and emits radiation. A blackbody is considered to be perfect absorber and emitter of energy (everything it absorbs it will emit back, with no energy lost). For this course we will assume the earth's surface and liquid water in the atmosphere emit blackbody radiation with the following characteristics:
Unlike solid objects or liquids, gases in our atmosphere do not behave as blackbody emitters/absorbers of radiation. Instead, they are referred to as selective absorbers and emitters. Gases only absorb and emit radiation at specific wavelengths.
The specific wavelengths at which a gas aborbs radiation is determined by its molecular structure, and so different types of gases absorb at different wavelengths.
Clouds, remember, are not selective absorbers; instead they absorb infrared (IR)
radiation over a wide range of wavelengths. As a result, the presence of clouds
in the atmosphere will in effect "close" the atmospheric window. At night, the
infrared radiation emitted by the earth's surface would normally pass through
the 8 - 11 mocrometer window, but in the presence of clouds this radiation is
absorbed and re-emitted back to the surface. The net effect is that, at night,
clouds can keep the surface warmer than it would be otherwise.
Because gases such as water vapor and carbon dioxide are selective absorbers of infrared radiation, they keep the sirface of the earth warmer than it would be otherwise through the so-called greenhouse effect, which operates as follows:
- Solar radiation at visible wavelengths passes through the atmosphere to the surface without being absorbed
- The earth's surface absorbs this radiation, heats up, and emits infrared radiation upward
- Greenhouse gases (water vapor, carbon dixoide, methane, ozone, CFC's) absorb some of this upward radiation; some of this energy is radiated back down to the surface by the gases (note: energy is radiated, not reflected!).
- Net effect: surface is warmer than it would be if no greenhouse gases are present (about 60 degrees F warmer)
Positive and negative feedback
What happens if we keep putting more carbon dioxide into the atmosphere? If we keep going at the present rate, we will double the amount of CO2 in our atmosphere (500 ppm by 2100 AD) compared to preindustrial times (250 ppm). The effect of increasing CO2 alone is believed to warm the surface by about 1 degree Celsius on average. Whether the surface continues to warm depends on the presence of feedback in the earth/atmosphere system, a process by which an initial change can either be reinforced (positive) or weakened (negative).
Positive feedback: This is based on the fact that water vapor is the most important greenhouse gase, mainly because there is a lot of it in our atmosphere. This is also based on the fact that "warmer air holds more water vapor"; the exact meaning of this will be discussed in Chapter 4, so for now we will just take it on faith. Positive feedback scenarios generally work as follows:
Under positive feedback conditions, we might expect the earth's average surface temperature to increase 4 - 6 degrees Celsius.
- More CO2 causes surface to warm (about 1 degree C under double-CO2 conditions)
- Warmer surface means warmer air near surface; this warmer air will drive additional evaporation from the ocean, and so there will be more water vapor in the air
- More water vapor means more greenhouse gases in the atmosphere, so surface continues to warm
- Return to step 2.
Negative feedback: This is based on the role that clouds play in the radiative balance of the earth's surface. As stated above, clouds at night can warm the surface. During the day, clouds will of course cool the surface by shading it from the sun's rays. Overall, the net effect of clouds is believed to be a cooling effect, meaning on average, the earth's surface would be warmer if there were no clouds in our atmosphere. Negative feedback involving clouds works as follows:
- More CO2 causes surface to warm
- Warmer surface means warmer air near surface, and thus more water vapor in the atmosphere
- More water vapor leads to more clouds; more clouds means, on average, a cooler surface
- Initial warming (step 1) is weakened.
Clearly, whether positive or negative feedback takes place depends on the role of clouds. Clouds are still not very well understood, in terms of their radiative effects on climate, which is why scientists currently can't predict with absolute certainty what will happen in the future. What is clear is that we are putting more CO2 into the atmosphere, and it will change the radiative balance of the earth, one way or another.