Unit 9: Global Change

Students will come to understand he global impact of local and regional human activities and evaluate and propose solutions.

Ozone Depletion

Ozone depletion refers to the thinning of the stratospheric ozone layer, which is critical for filtering harmful ultraviolet (UV) radiation from the sun. The main cause is the release of chlorofluorocarbons (CFCs) and other halogenated compounds, which break down ozone molecules in a catalytic cycle. This process is especially pronounced over Antarctica due to polar stratospheric clouds that facilitate chlorine-based reactions.
The depletion of ozone has severe ecological impacts, including increased skin cancer and cataracts in humans, reduced phytoplankton productivity, and harm to amphibian eggs that are sensitive to UV radiation. APES students should note that the ozone layer’s function is distinct from greenhouse gas regulation, as its role is primarily related to radiation filtering rather than heat trapping.
The Montreal Protocol (1987) is a landmark global agreement that successfully phased out the production of most ozone-depleting substances. This policy is often cited in APES as a prime example of effective international environmental cooperation. It also illustrates how swift, science-based action can reverse environmental damage over decades.
Understanding the chemistry of ozone depletion requires connecting to concepts from atmospheric pollution, such as photochemical reactions and free radical formation. This connection is tested on the AP Exam through questions that require differentiating between ozone in the troposphere and the stratosphere. Misunderstanding this distinction is a common cause of incorrect answers.
Recovery of the ozone layer is ongoing and is projected to take several more decades. This slow recovery highlights the persistence of CFCs in the atmosphere and the importance of preventive measures rather than reactive solutions in environmental policy.

Global Climate Change Causes and Evidence

Global climate change refers to long-term alterations in Earth's climate patterns, largely driven by increased greenhouse gas emissions from human activities. Major contributors include carbon dioxide from fossil fuel combustion, methane from agriculture and waste, and nitrous oxide from fertilizers. These gases trap infrared radiation, intensifying the natural greenhouse effect.
Evidence for climate change comes from multiple lines of scientific data, such as rising global temperatures, melting glaciers and polar ice caps, sea level rise, and shifting species ranges. In APES, students must be able to link this evidence to anthropogenic causes rather than attributing it solely to natural variability.
Paleoclimate data from ice cores, sediment layers, and tree rings reveal historical climate patterns and show that current warming is occurring at an unprecedented rate compared to natural cycles. This reinforces the concept that current climate change is primarily human-driven rather than part of a normal glacial-interglacial fluctuation.
Climate change affects every other APES unit by influencing weather patterns, water availability, agricultural productivity, and biodiversity. Understanding these connections is critical for answering synthesis-based AP questions that link multiple environmental processes together.
International agreements like the Kyoto Protocol and the Paris Agreement aim to reduce greenhouse gas emissions, but their effectiveness depends on national compliance and enforcement. Students should be prepared to discuss the strengths and weaknesses of such agreements and compare them to the Montreal Protocol's success.

Ocean Warming

Ocean warming is a direct consequence of global climate change, as oceans absorb over 90% of the excess heat trapped by greenhouse gases. Warmer waters disrupt marine ecosystems by altering species distributions, increasing metabolic rates, and causing heat stress in sensitive organisms like corals.
One of the most visible impacts of ocean warming is coral bleaching, which occurs when corals expel their symbiotic algae (zooxanthellae) under thermal stress. This weakens the coral and reduces reef biodiversity, ultimately impacting fisheries and tourism industries that depend on healthy reefs.
Ocean warming also intensifies storms by increasing the energy available for hurricanes and typhoons, leading to stronger and more destructive weather events. This connects to APES topics on ecosystem resilience and disaster preparedness.
In addition to biological effects, ocean warming alters ocean currents, which can disrupt global climate systems such as El Niño–Southern Oscillation (ENSO) patterns. This further demonstrates how interconnected ocean and atmospheric systems are in regulating Earth's climate.
Mitigation of ocean warming requires addressing its root cause — greenhouse gas emissions — while adaptation strategies focus on protecting vulnerable coastal communities and restoring damaged marine habitats. APES students should note that adaptation without mitigation only addresses symptoms, not the problem's source.

Global Change (Continued)

Ocean Acidification

Ocean acidification occurs when atmospheric CO₂ dissolves in seawater, forming carbonic acid that lowers ocean pH. This chemical shift reduces carbonate ion availability, which is essential for organisms like corals, mollusks, and some plankton to build calcium carbonate shells or skeletons. The process is directly tied to fossil fuel combustion, linking it to the same root cause as climate change.
Lower pH levels impair calcification, leading to weaker shells and skeletal structures in marine organisms. This makes them more vulnerable to predation and environmental stress, ultimately disrupting marine food webs. APES students should recognize that these effects can cascade, affecting both biodiversity and human food sources.
Acidification also impacts the sensory and reproductive behaviors of fish, potentially reducing population resilience. For example, some fish species lose the ability to detect predators or find suitable habitats, increasing mortality rates.
Ocean acidification interacts with ocean warming, compounding stress on marine ecosystems. This dual threat is particularly devastating for coral reefs, which face both bleaching and structural weakening from pH changes. These combined stressors are frequently tested in APES free-response questions.
Mitigation requires reducing CO₂ emissions and protecting “blue carbon” ecosystems like mangroves and seagrasses, which can sequester significant amounts of carbon. Restoration projects in these habitats not only combat acidification but also enhance coastal resilience to storms and erosion.

Invasive Species

Invasive species are non-native organisms introduced to an ecosystem, often through human activity, that outcompete native species and disrupt ecological balance. They typically have no natural predators in the new environment, allowing them to spread rapidly and dominate resources. Examples include zebra mussels in North American lakes and cane toads in Australia.
These species can alter nutrient cycling, reduce biodiversity, and cause economic damage to agriculture, fisheries, and infrastructure. For APES, students should understand that invasive species are a leading cause of species extinction worldwide, second only to habitat loss.
Introduction pathways include ballast water discharge, ornamental plant trade, pet releases, and intentional introduction for pest control. Preventing these introductions requires strict biosecurity measures, international cooperation, and public education.
Eradicating invasive species once they are established is extremely difficult and costly. Strategies like mechanical removal, chemical control, and biological control have mixed success and can sometimes cause unintended ecological harm if not carefully managed.
APES connections include the relationship between invasive species and disturbed ecosystems. Human-altered environments, such as urban areas or degraded wetlands, often provide ideal conditions for invasive species to thrive, linking this topic to land use, pollution, and climate change discussions.

Human Impacts on Biodiversity

Human activities have accelerated biodiversity loss through habitat destruction, pollution, overexploitation, climate change, and the introduction of invasive species. This loss reduces ecosystem resilience and the availability of ecosystem services that humans depend on, such as pollination, water purification, and climate regulation.
Deforestation, urban expansion, and agriculture are major drivers of habitat loss, often fragmenting landscapes and isolating wildlife populations. Fragmentation increases vulnerability to genetic bottlenecks and local extinctions.
Pollution, including plastic waste, heavy metals, and pesticide runoff, disrupts species health and reproductive success. These pollutants can bioaccumulate in food webs, causing long-term harm to both wildlife and humans.
Overharvesting of species for food, medicine, and trade depletes populations faster than they can recover. Examples include overfishing, poaching, and unsustainable logging practices, which can collapse entire ecosystems when keystone species are lost.
APES students should link biodiversity loss to the HIPPCO framework: Habitat destruction, Invasive species, Population growth, Pollution, Climate change, and Overexploitation. This framework integrates human impacts into a clear, testable model for understanding biodiversity decline.

Global Change (Conclusion)

Mitigation and Adaptation Strategies

Mitigation strategies aim to reduce the causes of global change, primarily by lowering greenhouse gas emissions and enhancing carbon sinks. This includes transitioning to renewable energy sources, improving energy efficiency, and implementing carbon capture and storage technologies. In APES, mitigation is often linked to proactive policy measures like the Kyoto Protocol and Paris Agreement.
Adaptation strategies focus on adjusting human and ecological systems to minimize the impacts of global change. Examples include building sea walls to protect against rising sea levels, shifting agricultural practices to accommodate changing precipitation patterns, and designing climate-resilient infrastructure.
Nature-based solutions, such as reforestation, wetland restoration, and regenerative agriculture, provide both mitigation and adaptation benefits. They sequester carbon while also buffering communities from climate impacts like flooding and drought.
International cooperation is critical for effective global change responses. Climate change and biodiversity loss are transboundary issues that require coordinated actions, technology sharing, and equitable financial support for developing nations.
In APES, students should connect these strategies to the concept of sustainability, recognizing that addressing global change requires balancing environmental, economic, and social considerations. Test questions often ask students to evaluate trade-offs between mitigation and adaptation approaches.

Common Misconceptions

Misconception: Ozone depletion and climate change are caused by the same pollutants.
Clarification: Ozone depletion is primarily caused by chlorofluorocarbons (CFCs) and related compounds, while climate change is driven by greenhouse gases like CO₂ and CH₄. Although both involve atmospheric chemistry, they are separate environmental issues with different causes, effects, and policy responses.

Misconception: Ocean acidification is caused by acid rain entering the ocean.
Clarification: The primary cause is atmospheric CO₂ dissolving in seawater, forming carbonic acid. While acid rain can locally affect coastal waters, its contribution to global ocean pH changes is negligible compared to CO₂ absorption.

Misconception: All invasive species are harmful in every situation.
Clarification: While many invasive species cause ecological or economic damage, some have neutral or even beneficial effects in certain contexts. However, APES focuses on their negative impacts because they often outcompete native species and disrupt ecosystems.

Misconception: Biodiversity loss only affects plants and animals in remote areas.
Clarification: Loss of biodiversity has direct consequences for human societies, including reduced food security, increased disease risk, and diminished ecosystem services like water filtration and pollination. These impacts are global and affect both urban and rural areas.

Misconception: Mitigation strategies alone can fully stop climate change.
Clarification: Even with aggressive mitigation, some climate impacts are inevitable due to the persistence of greenhouse gases in the atmosphere. Adaptation is equally necessary to manage the changes already underway.