SUMMARY
CONCEPT 25.1 Elements move among geologic, atmospheric, oceanic, and biological pools at a global scale.
25.1.1 Summarize the major pools and fluxes associated with global-scale cycles of carbon, nitrogen, phosphorus, and sulfur.
The global carbon cycle includes large fluxes of CO2 between the atmosphere and Earth's land surface associated with photosynthesis and respiration and, within the last 160 years, anthropogenic emissions of CO2 and CH4.
Global fluxes of nitrogen are associated with biological uptake and chemical transformations. Anthropogenic nitrogen 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.
25.1.2 Describe why anthropogenic perturbations to the global carbon cycle are important mediators of environmental change, even though the fluxes are relatively small compared to net primary production and respiration.
Atmospheric concentrations of CO2 and CH4 are increasing because of burning of fossil fuels, deforestation, and agricultural development.
Elevated atmospheric CO2 concentrations may increase terrestrial plant growth and the acidity of the oceans, causing ecological changes.
25.1.3 Evaluate why changes to individual element cycles at the global scale have implications for the cycling of other elements.
Individual changes in global fluxes of C, N, P, or S can affect the fluxes of the other elements through impacts on primary production and environmental degradation.
CONCEPT 25.2 Earth is warming because of anthropogenic emissions of greenhouse gases.
25.2.1 Relate the observed increase in global temperature over the past century to the potential causes for the change in climate.
Elevated levels of CO2, CH4, N2O, and other greenhouse gases in the atmosphere have warmed Earth, particularly since the 1950s. This warming trend is expected to continue throughout the twenty-first century.
25.2.2 Describe the expected and observed responses of organisms’ geographic distributions resulting from climate change.
Large changes in species distributions, community composition, and ecosystem processes are expected as a result of global
climate change.
Recent changes in the geographic ranges of species and in carbon source-sink relationships have been attributed to climate change.
25.2.3 Evaluate why current climate-community distribution relationships may not adequately predict the future composition of communities under climate change.
Individual species will vary in their response to climate change due to variation in dispersal rates, abilities to acclimatize and adapt, and changes in biotic interactions, resulting in a rearrangement of community composition.
25.2.4 Determine the factors that may increase the susceptibility of species to extinction due to climate change.
Climate change will increase extinction rates of species with low dispersal rates or facing dispersal barriers due to habitat fragmentation, and species that cannot adapt or acclimatize to the new climate.
CONCEPT 25.3 Anthropogenic emissions of sulfur and nitrogen cause acid deposition, alter soil chemistry, and affect the health of ecosystems.
25.3.1 Describe the causes of acid deposition and the mechanisms by which it affects ecosystems.
Sulfuric and nitric acids form in the atmosphere from compounds emitted by human activities. These compounds are subsequently deposited on Earth's surface as acid precipitation.
Acid precipitation causes nutrient imbalances and aluminum toxicity in soils.
25.3.2 Assess how atmospheric nitrogen deposition can be both beneficial and detrimental to organismal and community function.
Atmospheric deposition of reactive nitrogen compounds can increase productivity in some ecosystems, but it may also lead to soil acidification, eutrophication and dead zones in nearshore aquatic ecosystems, losses of species diversity, and increases in invasive species.
CONCEPT 25.4 Losses of ozone in the stratosphere and increases in ozone in the troposphere both pose risks to organisms.
25.4.1 Describe how the release of chlorofluorocarbons poses a serious threat to organisms, including humans.
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.
25.4.2 Explain how ozone in the stratosphere can benefit life on Earth but threaten diversity and ecosystem functioning when it occurs near the ground surface.
Reactions involving volatile organic compounds, many of which are of anthropogenic origin, generate ozone in the troposphere, where it can harm organisms.
REVIEW QUESTIONS
1. What are the major biological influences on the global carbon cycle? How have human influences during the past two centuries affected the fluxes of CO2 associated with these biological influences (i.e., other than by fossil fuel burning) and, subsequently, atmospheric CO2 concentrations?
2. Terrestrial animals are capable of migrating to regions where the climate is optimal for their function. Despite animals' mobility, ecologists are still predicting that as the climate changes, many animal species will experience local extinctions. Explain why animals' responses to climate change will depend on factors other than physiological tolerances and dispersal rates.
3. How can ozone in the atmosphere be both good and bad for organisms?
HONE YOUR PROBLEM-SOLVING SKILLS
Forests are important to the global carbon cycle, taking up a substantial amount of CO2 from the atmosphere via photosynthesis.
As we discussed in Concept 20.2, the production of forests is often limited by the supply of N, and thus greater C uptake may occur with elevated N deposition.1. How much additional C has been taken up and stored as wood if N deposition has added an average of 15 kg N per hectare per year to temperate deciduous forests for 20 years? Assume that 10% of the N deposition has been taken up and used for increased plant growth, that the C:N ratio in wood is 500:1, and that the temperate deciduous biome makes up 13 million km2 (1 hectare [ha] = 0.01 km2).
2. Make the same calculation for the boreal forest biome, with a N deposition rate of 5 kg N per hectare per year for 20 years, the same N uptake amount and C:N ratio of the wood, and an areal coverage of 19 million km2 for this biome.
3. Calculate the annual sum of the C taken up and stored as wood from your answers to Questions 1 and 2, and compare that to the amount of anthropogenic C emitted in Figure 25.3.
LIST OF KEY TERMS
acid neutralizing capacity anthropogenic
Arctic ozone dent
Climate change greenhouse effect greenhouse gases ozone hole