Chapter 24 Summary

CONCEPT 24.1 Elements move among geological, atmospheric, and biological pools at a global scale.

• The global carbon cycle includes large exchanges of CO₂ between the atmosphere and Earth’s surface associated with photosynthesis, respiration, and diffusion into the oceans, and within the last 150 years, anthropogenic emissions of CO₂ and CH₄.
• Atmospheric concentrations of CO₂ and CH₄ are increasing due to burning of fossil fuels, deforestation, and agricultural development.
• Elevated atmospheric CO₂ may increase terrestrial plant growth and the acidity of the oceans, causing significant ecological changes.
• Global fluxes of nitrogen are associated with biological uptake and transformations, but human fixation and emissions now dominate the global nitrogen cycle.
• The global cycles of phosphorus and sulfur include both geochemical and biological fluxes.
• Anthropogenic fluxes of phosphorus associated with mining and industrial emissions of sulfur far exceed natural fluxes associated with weathering.

CONCEPT 24.2 Earth is warming at an unprecedented rate due to anthropogenic emissions of greenhouse gases.

• Elevated levels of CO₂, CH₄, N₂O, and other greenhouse gases in the atmosphere have warmed Earth, particularly since the 1950s, and this warming trend is expected to continue throughout the twenty-first century.
• Large changes in species distributions, community composition, and ecosystem function are expected as a result of global climate change.
• Recent changes in species’ geographic ranges and in ecosystem carbon exchange have been attributed to climate change.

CONCEPT 24.3 Anthropogenic emissions of sulfur and nitrogen cause acid deposition, alter soil chemistry, and affect the health of ecosystems.

• Sulfuric and nitric acids form in the atmosphere from compounds emitted mainly by the burning of fossil fuels, and are subsequently deposited on Earth’s surface as acid precipitation.
• Acid precipitation causes nutrient imbalances and aluminum toxicity in soils, altering community composition and ecosystem function.
• Atmospheric deposition of reactive nitrogen compounds emitted from anthropogenic sources can increase productivity in some ecosystems, but may also lead to soil acidification, dead zones in nearshore aquatic ecosystems, losses of species diversity, and increases in invasive species.

CONCEPT 24.4 Losses of ozone in the stratosphere and increases in ozone in the troposphere each pose risks to organisms.

• Anthropogenic emissions of chlorinated compounds have led to a loss of stratospheric ozone since the 1980s, particularly at high latitudes, and thus to an increase in the levels of harmful ultraviolet-B radiation reaching Earth’s surface.
• Reactions involving volatile organic compounds, many of which are anthropogenic air pollutants, generate ozone in the troposphere, where it can harm organisms.