Several factors could adversely affect the biological diversity of the Earth and serve to amplify the current rate of extinction. If we accept that the current levels of extinction are related to the activities of humankind—most importantly, the conversion of previously undisturbed habitat such as rainforest or native grassland into agricultural areas—then anything causing an increase in such conversion should adversely affect the biodiversity baseline. Any reduction in the land currently available for agriculture would be likely to spur more conversion. This could happen in several ways:
Several types of climate change could reduce the current area of farmland and hence create pressure for further habitat conversion. Global warming due to the greenhouse effect could cause the tropical regions to increase in size. This in turn would cause an expansion of the desert areas at higher latitudes, producing an adverse effect on the grain belts located there. If grain regions migrate to higher latitudes in turn, they will suffer shorter and harsher growing seasons, and thus reduced yields.
A second and opposite effect would be a return to a new glacial interval. The current warm period is but an interglacial interval in a long pattern of glacial cycles that has been operating for more than 2 million years. If past patterns are any guide, some thousands of years from now ice sheets will again begin to grow and cover vast regions of the Earth in some of the most productive agricultural regions.
Disruption of agriculture could also come from a rise in sea level. Even small rises in global sea level will result in significant land reductions in agricultural regions, and such small-scale rises will come about if current global warming patterns continue. River deltas, for example, are among the richest of all agricultural regions, and the first to be inundated by any rise in sea level. New evidence gathered from a study of Antarctic glaciers in 2001 indicates that the rate of sea level rise may be three or four times faster than the worst-case scenario of the late 1980s and early 1990s. There may be a 20-foot sea level rise in the next two centuries.
As we will see in greater detail in a subsequent chapter, the number of humans on Earth greatly affects the rest of its biota, and surely extinction rates as well. If the human population reaches some of the more extreme estimates over the next few centuries—over 50 billion people, for instance—there will certainly be greatly elevated extinction rates.
The Earth currently has more species than at any time during previous geologic epochs. This general pattern of increase in diversity over time may not continue, however. How it might change is described in the next chapter.
Even a completely degraded environment can be successfully exploited by certain species- but others are sure to perish.
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