Let’s examine the actual data collected at the Mauna Loa Observatory and figure out what it means.
To make the analysis of data more interesting and realistic, imagine that you are working your way through college, and answered an ad in the paper for a job as a student assistant. You interviewed with the director and you were hired.
It appears there is a natural yearly change in the concentration of carbon dioxide. The data from Mauna Loa shows the concentration of carbon dioxide in the Northern Hemisphere increases slowly most of the year, but decreases rapidly in the summer. Data from the South Pole shows a similar pattern but with two important differences:
1) The peak in carbon dioxide concentration at the South Pole corresponds to the minimum of carbon dioxide concentration at Mauna Loa.
2) The difference between the peaks and valleys is much less at the South Pole. In fact, there is barely any decrease at all.
One explanation for the seasonal change in carbon dioxide concentration is the cycle of plant growth. First, consider late spring and summer in the Northern Hemisphere, when plants are growing rapidly. Rapid growth means that they must make leaves, stems, roots, and other plant tissues through the process of photosynthesis. In this process, the plants take in carbon dioxide and water vapor through tiny openings, or stomata, in their leaves. Inside the plant cells, the carbon is extracted from carbon dioxide and combined with hydrogen from water to provide food for the plant and to form new plant tissue. Light energy drives this chemical reaction; thus the name photosynthesis (photo meaning light and synthesis meaning the combination of materials to form new substances).
When some plants die or become dormant in the fall and winter, they stop absorbing carbon dioxide. As the dead parts decompose, they return carbon dioxide to the atmosphere. Carbon again combines with oxygen and is released in the form of carbon dioxide. Carbon dioxide continues building up in the air until late spring, when the rate of new plant growth exceeds the rate of old plant decay. Carbon dioxide is removed from the atmosphere until fall, when the rate of decay again equals the rate of growth. This exchange between foliage and air accounts for the annual change in the concentration of carbon dioxide that is recorded at Mauna Loa each year.
The process of fall die-off and spring revival is similar in the Southern Hemisphere. However, when people who live in the Northern Hemisphere experience summer, those in the Southern Hemisphere are feeling the cold of winter. That’s why the maximum concentration of carbon dioxide in the air of Earth’s Southern Hemisphere is shifted by about six months from that of the Northern Hemisphere.
Another difference is in the amount of carbon dioxide extracted by plants in the Southern Hemisphere. Since there is less land area in the Southern Hemisphere, there are fewer plants. Compare the land and sea areas in the portions of the globe shown here.
To determine how much industrial activity has changed the composition of Earth’s atmosphere, we must look back further, to the beginning of the industrial age.
We can learn about past climates by analyzing cores of ancient layers of ice drilled from the ice sheets near the poles. It is also possible to use the same ice cores to measure the amount of CO2 that was present in the atmosphere. This is done by placing a sample of ice from a given layer, representing a particular year, into a chamber. The air is pumped out of the chamber. The ice is crushed, so bubbles trapped in the ice are broken, releasing ancient air trapped inside for thousands of years. Instruments are used to determine the precise concentration of carbon dioxide in the atmosphere when that layer of ice was formed.
The bubbles of air trapped in the ice cores from Greenland, Antarctica, and other sites around the world have been analyzed. As can be seen in the top graph in this section, the concentration of carbon dioxide in the year 1854 was about 288 ppm. During the period known as the Industrial Revolution, the concentration of carbon dioxide climbed to almost 370 ppm. The present rate of increase of carbon dioxide is 50 times faster than at any time in the past.
The bottom graph shows the average temperature of Earth since 1866. Scientists like James Hansen (see Chapter 1), who believe the increase in the average global temperature is caused in large part by the increase in carbon dioxide from the burning of fossil fuels, point out that both graphs are heading upward. They say that confirms the prediction of the greenhouse effect. Scientists who disagree say the increase in world temperatures may be due to other factors. They also note the cooling trend, between 1939 and 1976, which is not consistent with the predictions of the global warming theory.
QUESTION 6.18. Compare the top and bottom graphs. In your opinion, are these results consistent with the theory of global warming and the greenhouse effect, or do they refute the theory? Can any definite conclusions be drawn from these graphs? What would it take to convince you that Hansen is definitely right, or definitely wrong?
Scientists are rarely satisfied with evidence from only one source. Ice cores preserved bubbles of old air. Where else might bubbles of old air be found? One answer was in the hollow buttons of Civil War uniforms. Some of them were sealed so well that they preserved the air inside for 130 years. Scientists removed the air from the buttons and measured the carbon dioxide concentration. In other cases old glass bottles that had been sealed for 100 years or more were carefully opened and the air inside was analyzed. Information gathered from these different sources all indicated that the concentration of carbon dioxide had increased from about 280 parts per million before the industrial revolution to more than 360 parts per million today.
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