19.3 Human Impact on Biogeochemical Cycles

Humans have drastically altered Earth's biogeochemical cycles through activities like deforestation, fossil fuel burning, and intensive agriculture. These changes have far-reaching consequences, including global warming, ocean acidification, and disrupted nutrient cycles.

Mitigating our impact requires sustainable practices and reducing emissions. By implementing eco-friendly farming, promoting renewable energy, and protecting natural carbon sinks, we can work towards restoring balance to these vital planetary processes.

Human Impact on Carbon and Nitrogen Cycles

Deforestation and Fossil Fuel Combustion

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• Deforestation removes trees that absorb carbon dioxide during photosynthesis, reducing the amount of carbon dioxide removed from the atmosphere
• Burning trees after deforestation releases stored carbon back into the atmosphere as carbon dioxide, contributing to increased greenhouse gas levels
• Fossil fuel combustion (coal, oil, natural gas) releases carbon that was previously stored underground for millions of years back into the atmosphere as carbon dioxide
• Increased atmospheric carbon dioxide levels lead to enhanced greenhouse effect and global warming

Agricultural Practices and Nitrogen Cycle Disruption

• Agricultural practices such as the use of nitrogen-based fertilizers and the cultivation of nitrogen-fixing crops (legumes) have dramatically increased the amount of biologically available nitrogen in ecosystems
• Excess nitrogen from fertilizers can run off into aquatic ecosystems, leading to eutrophication and algal blooms that deplete oxygen levels and create dead zones
• Nitrous oxide, a potent greenhouse gas, is released from agricultural soils and during the production of nitrogen-based fertilizers, contributing to global warming
• Livestock production releases methane, another potent greenhouse gas, through enteric fermentation and manure management

Ocean Acidification and Global Warming

• Increased atmospheric carbon dioxide levels lead to more carbon dioxide dissolving into the oceans, forming carbonic acid and lowering ocean pH (ocean acidification)
• Ocean acidification negatively impacts marine organisms that build calcium carbonate shells or skeletons (corals, mollusks, some plankton) by making it more difficult for them to form and maintain these structures
• Global warming caused by increased greenhouse gas levels leads to rising sea surface temperatures, which can cause coral bleaching events and disrupt marine ecosystems
• Melting of polar ice caps and glaciers due to global warming leads to sea level rise, threatening coastal communities and ecosystems

Human Impact on Phosphorus and Sulfur Cycles

Eutrophication and Phosphorus Cycle Disruption

• Human activities such as the use of phosphate-based fertilizers, detergents, and sewage discharge have greatly increased the amount of biologically available phosphorus in aquatic ecosystems
• Excess phosphorus in aquatic ecosystems leads to eutrophication, characterized by rapid algal growth and subsequent depletion of oxygen levels when the algae die and decompose
• Eutrophication can lead to the formation of dead zones in aquatic ecosystems, where oxygen levels are too low to support most forms of life
• Phosphorus, unlike nitrogen, does not have a gaseous phase in its biogeochemical cycle, so it accumulates in sediments and can continue to contribute to eutrophication even after external inputs are reduced

Acid Rain and Sulfur Cycle Disruption

• The burning of fossil fuels releases sulfur dioxide into the atmosphere, which can react with water vapor to form sulfuric acid, a component of acid rain
• Acid rain lowers the pH of soil and aquatic ecosystems, negatively impacting plants, animals, and microorganisms that are sensitive to acidic conditions
• Acid rain can leach nutrients from soils, reducing soil fertility and damaging forest ecosystems
• Acid rain can also corrode human-made structures (buildings, statues) and pose health risks to humans through the inhalation of acidic particles

Mitigating Human Impact on Biogeochemical Cycles

Sustainable Practices and Reducing Emissions

• Implementing sustainable agricultural practices, such as precision farming, crop rotation, and the use of organic fertilizers, can help reduce the overuse of nitrogen and phosphorus fertilizers and minimize nutrient runoff
• Promoting renewable energy sources (solar, wind, hydro) and improving energy efficiency can help reduce reliance on fossil fuels and decrease carbon dioxide and sulfur dioxide emissions
• Encouraging reforestation and afforestation efforts can help restore and expand carbon sinks, removing carbon dioxide from the atmosphere through photosynthesis
• Implementing carbon capture and storage technologies can help mitigate the impact of fossil fuel combustion by capturing carbon dioxide emissions and storing them underground or using them in industrial processes

Addressing Ozone Depletion

• The release of chlorofluorocarbons (CFCs) and other ozone-depleting substances has led to the depletion of the stratospheric ozone layer, which protects Earth from harmful ultraviolet radiation
• The Montreal Protocol, an international agreement, has successfully phased out the production and consumption of many ozone-depleting substances, leading to a gradual recovery of the ozone layer
• Continued monitoring and regulation of ozone-depleting substances, as well as the development of safer alternatives, are crucial for protecting the ozone layer and mitigating the impacts of increased UV radiation on ecosystems and human health