The biosphere encompasses all life-sustaining regions of the Earth, its atmosphere, and its oceans. The most significant changes occurring in the biosphere today include:
• The deforestation of wilderness areas through the clear-cutting of trees to make wood products
• The pollution of rivers and oceans due to fertilizer and sewage runoff
• The depletion of the protective ozone layer of the Earth’s atmosphere due to the overuse of certain air pollutants
• The desertification of grasslands due to livestock overgrazing, which reduces agricultural output and available habitats for native plant and animal species
Which is most clearly true of all four of the described changes in the biosphere?

They are the result of human activity.


All four changes described in the passage are the result of human activity: clear-cutting forests, channeling fertilizer and sewage into rivers and oceans, overusing air pollutants, and allowing livestock to overgraze grasslands.

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