which should provide the effective temperature of the planet. We get the right number for Mars.
However, if we try the same calculation for Earth, it does not work. We would calculate that Earth’s surface is a subfreezing 18°C below zero on average and should be frozen into a solid block of ice. As far as we know, that has never been true throughout geologic history, let alone being correct now. The reason it is wrong is that we have neglected the ability of the gases—notably water vapor, carbon dioxide, ozone, and a few others—to assist in heating the surface of the planet. There is a level in our atmosphere at which the temperature is exactly −18°C, but not at the surface. That is because of the greenhouse effect.
If we do the same calculation for Venus, we get a really incorrect answer. We underestimate the temperature of Venus by several hundred degrees. That is because the Venus atmosphere, as we have learned from the space program, is very thick, very heavy, has a much higher pressure than Earth’s, and is composed largely of carbon dioxide. Some people attribute the Venus situation as being due to a runaway greenhouse effect. Venus is not that different in distance from the Sun than Earth, but its temperature is grossly different, around 540°C.
This is powerful evidence that the greenhouse effect is natural, real, and was operating before humans were even here.
Beginning in the fall of 1957, David Keeling started measuring carbon dioxide in the air on the flanks of the Mauna Loa volcano on the big island of Hawaii roughly once an hour, and he averaged the data every month. Keeling died about 3 years ago, but this recording (Figure 2.2) is being carried on by hundreds of other people around the world. Each one of the black dots in Figure 2.2 is the average of a month’s data.
The average carbon dioxide amount started out around 312 parts per million (ppm), and by 2005 it was about 380 ppm. The most important thing that the graph shows is that carbon dioxide in the atmosphere continues to increase rather smoothly. Superimposed on the long-term trend are the annual cycles, almost like a sine wave.
In either hemisphere, in the spring or summer the carbon dioxide amounts are lower. In the fall and winter, they go up. The next spring and summer they come down a little bit, and again in the fall and winter, they go up. In the spring and summer photosynthesis is drawing carbon dioxide out of the air, and in the fall and winter the decay of annual plants, the decay of organic matter in soils, and root respiration exude carbon dioxide back into the atmosphere. In fact, the peak-to-peak amplitude of the depth of this oscillation tells us something about the total amount of photosynthetic activity on the planet. This is where our oxygen comes from. There is a lot of geochemical and