Recall from our previous discussion of ecosystems that human activity has significant impact on each of the three main processes that affect the levels of carbon dioxide in the atmosphere: photosynthesis, respiration, and burning. These impacts decrease the flow of carbon out of the atmosphere while increasing the flow into the atmosphere. The net effect of these changes is to increase carbon dioxide levels, with unpredictable but potentially catastrophic consequences. The increase in human population combined with a strong tendency to increase resource utilization with increased living standards means that rather than slowing the loading of the atmosphere with carbon dioxide is likely to accelerate.

Section 37.7 in your text is a good discussion of the potential results of human interference with the carbon cycle. Note that it is impossible to carry out a real controlled experiment that will accurately predict the results of global warming. In fact, the only way to know what will happen is to find out. It is certain that the level of carbon dioxide and other greenhouse gasses is increasing (Figure 37.15). It is also certain that these changes are leading to warming, especially in polar regions. The ultimate consequences, as we mentioned, are unpredictable, but your text discusses some of the more unpleasant ones.

Will disruptions in weather patterns and agriculture be one of the factors that impose limits on the size of the human population? Possibly. Other potential limiting factors include the availability of fresh water for personal, agricultural, or industrial use (Section 39.7), and the conversion of marginal land (arid lands and tropical forest) to agriculture (Sections 39.4 - 39.6). It is important to remember that human impacts are inevitable. As long as there are humans, we will use land for agriculture and energy for all kinds of purposes. The question facing people today is one of the magnitude of the impact. Just as it is cheaper in the short run to dump toxic waste than to treat and dispose of it properly, it is cheap to be dirty and destructive, and more expensive to be clean and constructive. In the long term, though, the dumped toxic waste will cause huge problems, and destructive lifestyles will lead to irreparable damage. Adding fifty cents to the price of a gallon of gasoline, for example, would decrease driving, and therefore decrease air pollution, carbon dioxide levels, particulate emissions, and other undesirable consequences. Measures like this are often politically unpopular. Does this represent short term or long term thinking?