The primer was prepared by:
Paul L. Houston
Room B-46
Baker Laboratory of Chemistry
Cornell University
Ithaca, New York14850-1301
voice: (607) 255-4303
fax: (607) 255-8549
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http://www.msc.cornell.edu/~plh2/group/glblwarm/GLBLWPDF.HTM
For those of you who do not want to use the Acrobat Reader, visit the primer at
http://www.msc.cornell.edu/~plh2/group/glblwarm
The other "tool" we will need is the equation for the surface area of a sphere: A = 4 (PI) R**2.
Based on this analysis, the heating rate for Venus, Earth, and Mars can be calculated. This is not quite the whole story, however, since each planet has a different "albedo" or reflectivity. Venus reflects 75% of the radiation, so only 25% is available to warm the planet. Earth reflects 30%, so 70% is available. Mars reflects 15%, so 85% is avalaible. Thus, a corrected heating rate can be calculated taking these reflectivities into account. Note that the left-hand axes in these plots give the Average Solar Irradiance in Watts per square meter. The Earth receives about 240 Watts per square meter of radiation from the Sun.
In order now to calculate the temperature of a planet we need to know how this heating is balanced by cooling. The situation is much like seeing what the level of water is in a washbasin: it comes in from the tap and goes out the drain. The level is determined by the balance between the rates of inflow and outflow. In much the same way, the temperature of a planet is controlled by the balance between the inflow and outflow of heat. We have calculated the inflow above. What about the outflow?
It turns out from physical calculations that , according to something known as Stefan's Law, the cooling rate of a body depends on the fourth powed of the temperature. Thus, the hotter the planet, the more rapidly it will be cooled by giving off its own radiation. What we need to do now is to balance the heating and the radiation. The comparison between them shows that, for example, on Earth the heating of 240 Watts per square meter can be balanced by the emission of that same amount of heat if the temperature at which the Earth emits the radiation is about 255 K. For Venus and Mars, the temperature calculated from this heat balance is less. These temperatures are perhaps easier to see in an enlarged view.
Notwithstanding the validity of the above physics, when we look at the planets we find that the temperature on the surface of the planet is not given by the above calculation. In all cases, the actual temperature is higher. Let's take a quick look at the planets. The figures I'll show and much more data about the planets are available from a NASA page called Know The Planets.
Venus, when seen from Earth doesn't show many features, because it is covered by clouds. The surface of Venus looks different. Venus has a dense atmosphere, composed mostly of clouds of sulfuric atmosphere. The surface temperature is 726 K.
Earth, also has an atmosphere, and also shows clouds, continents, oceans and rivers. There is water! Its average surface temperature is 310 K, or about 30 C or 80 F.
Mars has very little atmosphere and a surface temperature of 215 K. Some comments about the planets and many of these figures come from the NASA information at Know The Planets.
So the question that remains is "Why is the surface of Earth and Venus (and to a small extent, Mars) at a higher temperature than that which we calculated from balancing the heating and cooling rate of the planets?" The answer is that the temperature at the surface is not the average temperature nor average place from which the planet radiates. Note that, since the observed surface temperature is in every case higher than the average temperature at which the planet radiates, it must be that the average place that the radiation is emitted from is cooler than the temperature of the surface. Since cooler temperatures are associated with higher altitudes, it is evident that the average place at which radiation is emitted from these planets is somewhere above the surface of the planet (more detail, if desired). The emission from a high altitude can only occur if there is something up there to absorb and emit radiation, so we are led to consider in detail the composition of planetary atmospheres.
We have come to an important conclusion. The temperature of the surface of a planet is higher than the temperature calculated from heat balance because the atmosphere absorbs and emits radiation, on average at a higher and cooler altitude than the surface. This warming of the surface is due to the atmosphere and is called the "greenhouse effect."
But what about the composition of the atmospheres? What is it that can actually absorb and emit the radiation? We will argue below that it is mostly the carbon dioxide, and to some extent the water and methane, that actually absorb the radiation.
In the meantime, you might be interested in seeing how heat is balanced between the surface of the earth and various points in the atmosphere. At every elevation, the amount of heat coming in to that elevation (from both above and below) must be balanced by an equal amount of heat going out. A cartoon [7] (numbers in square brackets denote references to the graph, diagram, table, etc. that preceded the number) gives the results in percent, where 100% corresponds to 345 Watts per square meter (of which 30% is reflected).
But our next job is to understand why some atmospheric molecules absorb radiation and some don't.
What wavelengths correspond to different colors? A chart may be helpful. Most of what we see, the visible region, is actually a rather small region of wavelength from about 0.6 to 0.4 microns (A micron is a millionth of a meter). Since the radiation emitted by a body at the temperature of the Earth is mostly at longer wavelengths than the visible, we will be most interested in the infrared region of the spectrum, from about 0.7 to 100 microns in wavelength. It is interesting to look at the intensity of emission as a function of wavelength and temperature as well as some of the gases that absorb in this region [1].
Why don't we see any absorption from molecules like nitrogen and oxygen that are much more abundant in the Earth's atmosphere than those that do absorb (like methane, water, and carbon dioxide)? We need to consider that the absorption in this region corresponds to vibrations of a molecule, and that these vibrations can be excited by the oscillating field of the light only if the dipole moment of the molecule changes during the vibration. Because both oxygen and nitrogen are symmetric molecules (composed of two atoms each), there cannot be any net dipole moment on these molecules; there is thus no dipole moment to change as the molecules vibrate. On the other hand, the dipole moment of carbon dioxide changes if it vibrates in its asymmetric stretching or bending modes. The reason can be seen by considering how the oscillating field might "shake" the dipole moment.
Another way to look at the fact that the surface of the earth is hotter than we calculated by heat balance is to realize that the greenhouse gases effectively form a blanket over the earth. At every height in the atmosphere above the earth, the radiation coming into the atmospheric layer at that height (from both above and below) must be balanced by an equivalent amount of radiation going out of that layer. At the bottom of the blanket, most of the radiation is coming into the layer from the surface of the earth, which has absorbed the major portion of the sunlight. Some of the radiation going out of this bottom layer of the blanket goes up, but much goes back to the surface, keeping it warm. At the top of the blanket, radiation mostly comes in from below, but it mostly goes out the top. There is simply not much more atmosphere above to reabsorb the radiation. If we look at the radiation that actually escapes the earth, we find that most of it comes from near the top of the atmospheric blanket. The top is much cooler than the bottom, so that, on average, the radiation that goes off to space is indeed coming from a place at high altitude whose average temperature is as calculated, 255 K. The surface of the earth is considerably warmer because it has an atmosphere of gases that can absorb infrared light.
We can next ask the following questions: On a per molecule basis, 1) which are the most important gases that contribute to the warming and 2) how much to they contribute?
The answer to the first question is obtained by calculating the so-called "Global Warming Potential." The warming potential of a molecule in the atmosphere will depend on a) how strongly it absorbs light and b) how long the molecule lives in the atmosphere. Let's take carbon dioxide as a reference and define its global warming potential to be unity. CO2 absorbs with a particular strength and lives for about 120 years in the atmosphere. We now consider a particular time horizon, for example 100 years. A molecule that absorbs 1000 times more strongly than carbon dioxide but lives only one year in the atmosphere then has a relative global warming potential on this time horizon given by the ratio [(1000 absorption units)(1 year) + (0 absorption)(99 years)]/[(1 absorption unit)(100 years)] = 10. On a different time horizon, the global warming potential would be different (more for a shorter horizon and less for a longer horizon in this example). We then weigh the global warming potential by the concentration of molecules in the atmosphere to get the relative importance of the contribution of each species to the greenhouse effect. The results for naturally occurring gases are shown in a pie chart. A table [9]gives some global warming potentials for other gases. The results for all gases currently in the atmosphere are shown in the bottom of another figure [11].
Here is a detour that explains the difference between global warming and ozone depletion
The answer to the second question, "how much warming to these greenhouse gases actually contribute," is a bit more complicated. We need to calculate from the concentration and the absorption for each species how much warming will occur. Usually this warming is expressed as the equivalent increase in radiation from the sun, which you may remember has units of Watts per square meter. The heating due to the greenhouse gases is called "radiative forcing." Some values for varioius greenhouse gases and time periods are shown in a table. The important point to note is that because the concentration of greenhouse gases has increased it is predicted that radiative forcing has and will increase. With this in mind, we might want to take a look at historical trends in the concentrations of greenhouse gases and surface temperatures.
Where does all this increase in CO2 and methane come from? A map [7]shows the countries most responsible; most of the emission comes from the burning of fossil fuels. You will talk in depth later about the fact that the biggest producers are the richest contries. Even so, the CO2 emissions caused by man are a small fraction of the total CO2 cycle [7]. Reference [5] gives more information about the global carbon dioxide budget.
One might ask what causes the large variations of temperature on a geological time scale. At least part of the answer has, again, to do with astronomy: The earth wobbles in three ways relative to the sun. First, the eccentricity of its orbit changes periodically on a 100,000 year cycle. Second, the angle of the earth's axis with respect to its orbital plane changes periodically; it traces out a cone on a time scale of about 23,000 years. And third, the angle of the cone fluctuates between about 21.5 and 24.5 degrees on a time scale of about 41,000 years. These relationships and some of the cycles they cause are shown in the accompanying figure [6].
We note, for example from one of the previous graphs, that the correlation between CO2 and temperature, while close, is not exact. This is because the warming caused by the greenhouse effect interacts strongly with the climate. Thus, if we are to predict to what extent further increases in greenhouse gases will increase our surface temperature, we need to understand the climate.
Of particular concern is the interaction of the oceans and their salt currents [6]with the atmosphere. There is some concern (see [6]) that the "Atlantic conveyer," a deep salty current, might be disrupted by further warming, as it was during glacial times.
The models have to take into account lots of problems. If the temperature increases, there are feedbacks that further affect the temperature. For example, an increase in temperature causes more water to evaporate into the atmosphere, causing more greenhouse forcing, causing a further increase in temperature. Thus there is positive feedback. One also has to consider feedback involving clouds, snow and ice, oceans, and pollutants and aerosols.
Model calculations must be verified, usually by comparing the model prediction for some past period to the actual result during that period. For example, a figure [10]gives the comparison between model and actual surface air temperature changes from 1900 to present.
One graph [11]shows the predicted CO2, CH4, and CFC11 concentrations, while another graph [11]shows the greenhouse forcing.
Here are some maps that display the predictions of the most extensive models for
Soil Moisture[10]
Precipitation[10]
Temperature[10]
These predictions, and others, are available from the Hadley Centre.
It turns out that the effect of aerosols and pollutants (both of which are produced along with CO2) actually counteract some of the effect of the greenhouse gas by causing more sunlight to be reflected. Thus, when aerosols are included in the calculation, the temperature rise that is measured and that which is predicted are in good agreement. Here is what will happen if there is a cessation of aerosol production. The lower curve is including aerosols, the upper one is the prediction without aerosols, and the dark one going between them is the prediction if we cease producing aerosols now.
Sea Levels will change. This is due to many effects, as shown in a composite[1]graph. The best estimate [11]is about a 66 cm increase in the sea level by 2100. This will have serious consequences for the worlds delta regions. There is already some indication [16] that the Antarctic Ice is breaking up more than it has in the past.
There will also be effects on:
Agriculture
Fresh Water
Ecosystems
An interesting study is that of the Edith's checkerspot butterfly by Camille Parmesan [15], who studied extinctions of different populations of this butterfly as shown on the map. Most of the extinctions occurred in the southern range of the habitat and at the lower elevations.
Human Health
But there are several reasons not to wait. Nor does there seem to be any technical solution, or so-called GeoEngineering solution. What to do, how fast to do it, who will it affect, and who will pay are the central questions remaining in this course.
