Unit 9: Global Change

Earth’s Energy Balance and the Greenhouse Effect

What “global change” means in AP Environmental Science

In AP Environmental Science, global change refers to large-scale shifts in Earth’s climate system and atmosphere that occur over decades to centuries and affect ecosystems, resources, and human societies worldwide. Two headline topics dominate this unit: climate change (driven largely by increasing concentrations of greenhouse gases) and stratospheric ozone depletion (caused by specific human-made chemicals that destroy ozone). A common misconception is to treat these as the same problem. They are related only in the broad sense that both involve atmospheric chemistry and human impacts, but they have different causes, mechanisms, consequences, and solutions.

Earth’s energy budget: the foundation for understanding climate

Earth’s climate is controlled by a simple idea: Earth must balance the energy it receives from the Sun with the energy it returns to space.

Incoming energy arrives mostly as shortwave solar radiation (visible and ultraviolet). Some of that incoming energy is reflected back to space by clouds, aerosols, ice, and other light-colored surfaces. The rest is absorbed by Earth’s surface and atmosphere, warming the planet. A warm Earth then emits energy back toward space as longwave (infrared) radiation.

If incoming absorbed energy increases (or outgoing energy decreases), Earth warms until a new balance is reached.

A helpful quantitative relationship is how albedo controls reflection. Albedo is the fraction of incoming solar radiation that is reflected.

\text{Reflected solar energy} = \text{albedo} \times \text{incoming solar energy}

Higher albedo (ice, snow, some clouds) means more reflection and cooling. Lower albedo (oceans, forests, asphalt) means more absorption and warming.

The greenhouse effect: what it is and why it matters

The greenhouse effect is a natural process that warms Earth’s lower atmosphere and surface. It is not “bad” by itself—without it, Earth would be much colder and far less habitable.

Mechanism, step by step: sunlight reaches Earth; the surface absorbs some energy and warms. The warm surface then emits infrared (heat) radiation upward. Greenhouse gases absorb some of that outgoing infrared radiation and then re-emit infrared radiation in all directions, including back toward the surface. This raises surface and lower-atmosphere temperature compared with a planet lacking these gases.

Greenhouse gases you should know (and what isn’t a greenhouse gas)

Greenhouse gases (GHGs) absorb and re-emit infrared radiation. Key gases and sources emphasized in APES include:

• Carbon dioxide (CO2): fossil fuel combustion, deforestation, cement production; also produced naturally (respiration, volcanic activity).
• Methane (CH4): livestock digestion, rice paddies, landfills, fossil fuel extraction/transport, wastewater.
• Nitrous oxide (N2O): agricultural soils (especially from nitrogen fertilizer), manure management, and some industrial sources.
• Water vapor (H2O): the most abundant greenhouse gas, but primarily a feedback, not the original long-term driver.
• Fluorinated gases: industrial processes, refrigeration, and consumer products; includes hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

Two clarifications prevent common confusion:

• Oxygen (O2) and nitrogen (N2) are not major greenhouse gases because they do not absorb infrared radiation effectively.
• Water vapor generally amplifies warming initiated by longer-lived gases like CO2 because warm air can hold more moisture.

Also note that black carbon (soot) is not a gas (it’s a particle/aerosol), but it contributes to warming by absorbing sunlight in the atmosphere and by darkening snow/ice surfaces (lowering albedo).

Positive feedback loops: how warming can accelerate

A feedback loop occurs when a change triggers effects that either amplify (positive feedback) or reduce (negative feedback) the original change. Key positive feedbacks emphasized in APES include:

• Ice–albedo feedback: warming melts ice/snow, lowering albedo. Darker surfaces absorb more solar energy, causing more warming and more melting.
• Water vapor feedback: warming increases evaporation and atmospheric water vapor, strengthening the greenhouse effect.
• Arctic methane feedback (important regional example): the Arctic is a large natural methane source; warming that thaws permafrost (and related frozen organic stores) can increase methane release, which then increases warming.

These feedbacks typically magnify an initial change rather than being the original cause.

Greenhouse gas sources by sector (common AP-style categorization)

APES questions often organize emissions by human activity sector. Key sources include:

• Energy supply: burning coal, natural gas, and oil for electricity and heat; often the single largest global source.
• Transportation: fossil fuels burned for road, rail, air, and marine travel.
• Industry: fossil fuels burned on-site; cement manufacturing releases significant CO2.
• Commercial and residential buildings: on-site energy generation and burning fuels for heat or cooking.
• Land use and forestry: deforestation of old-growth forests (loss of carbon sinks), land clearing for agriculture, strip-mining, fires, and decay of peat soils.
• Agriculture: especially emissions from agricultural soil management.
• Waste and wastewater: landfill and wastewater methane (CH4), and incineration.

Example: why melting Arctic sea ice matters beyond polar regions

If summer sea ice declines, albedo decreases and the darker ocean absorbs more solar energy. The warmer ocean can delay refreezing into fall, which reduces ice extent the next year as well. This helps explain polar amplification (polar regions warming faster than the global average). Large-scale weather systems and ocean-atmosphere circulation also transport energy poleward, reinforcing faster warming at high latitudes.

Exam Focus

• Typical question patterns:
  • Explain how greenhouse gases warm Earth using the energy budget (incoming shortwave vs outgoing longwave).
  • Predict what happens to temperature if albedo changes (ice melt, deforestation, soot on snow).
  • Identify which gases are greenhouse gases and connect each to a human source.
  • Distinguish gases (CO2, CH4, N2O, fluorinated gases) from non-gas warmers like black carbon.
• Common mistakes:
  • Saying the greenhouse effect blocks sunlight from entering (it mainly involves trapping outgoing infrared).
  • Mixing up ozone depletion with climate change (different layers, chemicals, and effects).
  • Treating water vapor as the main long-term “cause” rather than a feedback that responds to warming.

Evidence for Climate Change and Its Primary Drivers

Climate vs. weather: the timescale matters

Weather is short-term atmospheric conditions (today’s temperature, rainfall, wind). Climate is the long-term pattern and average of weather over decades. A cold week does not contradict a long-term warming trend; climate change is detected through sustained shifts in averages and extremes.

Multiple independent lines of evidence (consilience)

AP Environmental Science emphasizes that climate change conclusions come from many independent observations that point to the same story (“consilience”), including:

• Instrumental temperature records: long-term trends in global average surface temperature.
• Ocean heat content: the ocean stores most of the excess heat added to the climate system.
• Glacier and ice sheet mass balance: widespread retreat and mass loss.
• Sea level rise: driven by thermal expansion of warming seawater and melting land ice.
• Phenology and species range shifts: earlier flowering/migration and movement toward poles or higher elevations.
• Paleoclimate proxies: ice cores, tree rings, sediment cores, corals.

A helpful supporting fact is that the world’s oceans contain more CO2 than the atmosphere, which matters because ocean uptake affects both climate (heat storage) and chemistry (acidification).

The role of carbon: why CO2 is central

CO2 is emphasized because it is emitted in large quantities by humans, persists long enough to accumulate, and is tightly linked to fossil fuel use and land-use change. Burning fossil fuels rapidly moves carbon from long-term geologic storage into the atmosphere. Deforestation removes living biomass that would otherwise store carbon and often releases additional stored carbon through burning and decomposition.

Anthropogenic (human) drivers

Major human contributors include:

1. Fossil fuel combustion (electricity, transportation, industry): releases CO2 directly.
2. Deforestation and land-use change: reduces carbon sequestration and releases carbon stored in vegetation/soils.
3. Agriculture: livestock and rice increase CH4; nitrogen fertilizers increase N2O through microbial soil processes.
4. Industrial processes: cement production releases CO2; other industries emit fluorinated gases.
5. Waste management: landfills and wastewater can emit methane.

Natural drivers and why they don’t explain recent warming well

Earth’s climate changes naturally due to factors such as:

• Solar variability: changes in solar output occur in cycles; typical global temperature influence is small (often described as about 0.1°C between solar maxima and minima).
• Volcanic eruptions: large eruptions can inject particles/aerosols into the stratosphere that reflect sunlight and usually cause temporary cooling, not sustained warming.
• Long-term orbital changes (Milankovitch cycles): operate over thousands to tens of thousands of years.
• Tectonic processes: movement of tectonic plates forms mountains and influences volcanism and long-term carbon cycling, which can contribute to climate changes over geologic time.

APES comparisons often hinge on timescale and direction: recent warming is too rapid to be explained by orbital cycles, and volcanic forcing typically cools in the short term.

Correlation vs. causation: how APES expects you to reason

You may see a graph of CO2 concentration and global temperature and be asked what you can conclude. Correlation supports the hypothesis that CO2 and temperature are linked, but stronger conclusions come from combining correlation with mechanism (greenhouse effect physics) and additional evidence (ocean heat content, ice loss, sea level rise, measured energy imbalance).

Example: why oceans keep warming even if air temperatures vary year to year

Year-to-year surface air temperature fluctuates due to natural variability (for example, ocean-atmosphere cycles). But if greenhouse gases continue increasing, the overall energy imbalance persists, and oceans continue absorbing excess heat over time. This is why ocean heat content is a powerful indicator of long-term warming.

Exam Focus

• Typical question patterns:
  • Interpret graphs showing temperature, CO2, sea level, or ice extent and describe the trend.
  • Identify anthropogenic vs natural drivers of climate change and explain mechanisms.
  • Explain how deforestation affects atmospheric CO2 using the carbon cycle.
  • Compare solar-cycle/volcanic effects with modern warming using timescale and direction.
• Common mistakes:
  • Using a single cold event to argue against warming (weather vs climate confusion).
  • Claiming volcanoes are the main cause of recent warming (large eruptions usually cool short-term).
  • Treating correlation as proof without mentioning a mechanism (connect to greenhouse physics).

Impacts of Climate Change on Earth Systems and People

Why impacts are uneven (and why environmental justice shows up here)

Climate change is global, but its effects are not evenly distributed. Impacts vary with latitude, ocean proximity, elevation, ecosystem type, and socioeconomic conditions. APES often emphasizes environmental justice: communities with fewer resources frequently face higher risks and have less capacity to adapt.

Physical impacts: heat, precipitation, and extreme events

Climate change shifts averages and can change the probability of extremes.

• Heat waves become more frequent and intense as average temperatures rise.
• Precipitation patterns shift; some regions see more intense rainfall and flooding, while others face more severe drought.
• Storm impacts can increase when ocean surface temperatures rise, raising the potential energy for stronger tropical cyclones, though storm behavior depends on multiple factors.

A key misconception is that climate change means “everywhere gets hotter and wetter.” Regional patterns can diverge: some places become hotter and drier, others hotter and wetter, and some may even see localized cooling due to ocean circulation changes.

Ocean circulation and heat storage

Ocean currents carry heat around Earth. As oceans absorb more heat from the atmosphere, sea surface temperatures rise and ocean circulation patterns can change. Because oceans store a very large amount of heat, even small changes in currents can have large and lasting effects on global climate.

Sea level rise: two main mechanisms (and a common trap)

APES expects you to know two primary contributors:

1. Thermal expansion: water expands as it warms.
2. Melting land ice: glaciers and ice sheets add water to the ocean.

Melting floating sea ice does not raise sea level much directly (it already displaces water), though it strongly affects albedo and ecosystems.

Sea-level rise threatens to inundate coastal wetlands and estuaries, and it can trap wetland species that cannot move inland due to coastal development. Some estimates (including widely cited UN-related projections) suggest very large numbers of people—on the order of hundreds of millions, sometimes stated as about 150 million by 2050—may need relocation due to coastal flooding, shoreline erosion, and agricultural disruption.

Cryosphere changes: glaciers, ice sheets, and permafrost

Glacier retreat affects freshwater availability in regions that rely on seasonal meltwater. Globally, glaciers have lost substantial area since the late 1800s.

Antarctica is the main ice-covered landmass, holding about 90% of the world’s ice and roughly 70% of its freshwater. If essentially all Antarctic ice melted, global sea level would rise by about 60 m (roughly 200 feet).

Permafrost thaw can damage infrastructure and can release greenhouse gases (especially methane) as previously frozen organic matter decomposes, creating a positive feedback.

Ocean impacts: warming and acidification

Oceans absorb both heat and CO2.

• Ocean warming affects currents, nutrient mixing, and habitats.
• Ocean acidification occurs when atmospheric CO2 reacts with seawater to form carbonic acid, lowering pH and reducing carbonate availability needed by many organisms (corals, shellfish) to build shells and skeletons.

A common confusion is to blame acid rain; ocean acidification is primarily driven by atmospheric CO2 uptake.

Ecosystem impacts: range shifts, phenology, and biodiversity

As climate zones shift:

• Species may move poleward or upslope.
• Timing of life events (phenology) shifts (earlier flowering, breeding, migration).
• Species that cannot move or adapt fast enough face higher extinction risk.

Climate change interacts with other stressors such as habitat fragmentation, invasive species, and pollution. Fragmented habitats can block migration corridors that would otherwise allow range shifts.

Examples emphasized in high-latitude ecosystems include Arctic food webs: polar bears and other fauna that depend on sea ice (ice floes), birds, and marine mammals can be drastically affected as ice conditions change.

As rivers and streams warm, warm-water fish can expand into areas previously inhabited by cold-water species.

Agriculture and food security

Climate change affects crop yields through heat stress, shifting rainfall/irrigation needs, increased pests and diseases, and higher risk of crop failures from extreme events. Some regions may get longer growing seasons, but benefits are often limited by water availability, soil constraints, and heat extremes.

Human health and infrastructure

Key pathways include:

• Heat-related illness and mortality during heat waves (increased risk of heat exhaustion, heat stroke, hyperthermia, and cardiovascular stress; heat also worsens risks for people with conditions such as diabetes).
• Air quality impacts, including higher ground-level ozone formation in hotter conditions and wildfire smoke exposure.
• Vector-borne diseases shifting as mosquitoes and ticks expand ranges; warmer conditions can increase risks of malaria, dengue fever, Zika virus, and yellow fever.
• Waterborne disease risks can rise as warmer water increases bacterial activity, potentially affecting illnesses such as cholera, giardia, and amoebic dysentery.
• Coastal flooding and storm surge damaging homes, roads, and water systems.

Sea level change across time (context)

Since the peak of the last ice age about 18,000 years ago, global sea level has risen by roughly 120 m (about 400 feet) due to melting land ice. In the modern era, measured sea level rise has accelerated to about a few millimeters per year (often cited around 3 mm per year since the early 1900s), largely from thermal expansion and land-ice melt.

Example: connecting multiple impacts in one coastal scenario

Consider a coastal city experiencing sea level rise. Higher baseline sea level makes storm surge flooding more damaging. Saltwater intrusion can contaminate freshwater aquifers. If the city builds seawalls, that can protect infrastructure but may also increase coastal erosion elsewhere or harm coastal wetlands that would have provided natural protection.

Exam Focus

• Typical question patterns:
  • Given a region (coastal, arid, polar), predict likely climate impacts and justify them.
  • Explain sea level rise using both thermal expansion and land-ice melt.
  • Describe how climate change can affect biodiversity, agriculture, or human health with a clear mechanism.
  • Connect ocean heat storage/currents to climate patterns.
• Common mistakes:
  • Claiming melting sea ice is a major direct cause of sea level rise (land ice is the key for added water).
  • Discussing impacts without connecting them to a climate variable (temperature, precipitation, sea level, extremes).
  • Forgetting that impacts are region-specific and influenced by social vulnerability.

Adapting to Climate Change: Reducing Harm and Building Resilience

What adaptation is (and what it is not)

Adaptation means adjusting human systems and, where possible, supporting ecosystems to reduce harm from climate change impacts. It does not replace mitigation (reducing emissions); it addresses impacts that are already occurring or unavoidable due to past emissions.

A frequent APES pitfall is mixing up:

• Mitigation: reduce the size of climate change (cut greenhouse gas emissions, increase carbon sinks).
• Adaptation: reduce the damage caused by climate change (prepare for flooding, heat, drought).

Types of adaptation strategies

Adaptation is often described in categories.

Infrastructure and engineering adaptations

Elevating buildings, redesigning stormwater systems, and improving drainage can reduce flood risk. Seawalls, levees, and surge barriers can protect coastal development, but they are often expensive and can create “lock-in” dependence on hard defenses that may fail under extreme conditions.

Nature-based solutions

Restoring wetlands and mangroves can reduce storm surge and erosion while also providing habitat. Increasing urban tree canopy reduces heat islands and can improve air quality.

Water and agriculture adaptations

Switching to drought-tolerant crops, improving irrigation efficiency (for example, drip irrigation), and adjusting planting dates can reduce crop losses under hotter or drier conditions.

Public health and emergency preparedness

Heat warning systems, cooling centers, and disaster planning for floods, hurricanes, and wildfires reduce mortality and economic disruption.

Managed retreat and land-use planning

In some high-risk coastal areas, managed retreat—moving infrastructure and development away from vulnerable zones—may be the most sustainable long-term option. APES often frames this as a tradeoff: retreat reduces future risk but can be socially and politically difficult when culture, livelihoods, and property are tied to place.

Climate resilience and equity

Resilience is the ability of a system to withstand shocks and recover. Climate resilience is stronger when communities have reliable infrastructure, strong social networks, healthcare access, financial resources/insurance, and effective governance. Environmental justice shows up when you ask who caused emissions, who experiences risks, and who has resources to adapt. On exams, you may be asked to propose adaptations that are realistic for low-income communities, not only expensive engineering projects.

Example: choosing an adaptation strategy for heat waves

If a city is experiencing more frequent extreme heat, options include expanding tree canopy (cooling plus air-quality benefits), using cool roofs/reflective pavements (reduce heat absorption), and providing cooling centers with targeted outreach for vulnerable populations. Strong answers explain the mechanism (reducing absorbed heat, preventing heat illness) and recognize constraints (cost, maintenance, access).

Exam Focus

• Typical question patterns:
  • Propose an adaptation plan for a given impact (sea level rise, drought, heat) and justify it.
  • Compare engineered vs nature-based adaptations and discuss tradeoffs.
  • Identify barriers to adaptation and connect them to equity.
• Common mistakes:
  • Listing adaptations without explaining how they reduce risk.
  • Confusing adaptation with mitigation (for example, calling solar panels an “adaptation” instead of mitigation).
  • Ignoring unintended consequences (seawalls harming wetlands, air conditioning increasing energy demand).

Mitigating Climate Change: Cutting Greenhouse Gases and Increasing Carbon Sinks

What mitigation is trying to accomplish

Mitigation aims to limit the magnitude of future climate change by reducing greenhouse gas emissions and enhancing carbon sequestration (capturing and storing carbon in biomass, soils, or geologic formations). Mitigation is fundamentally about changing energy systems, land use, and industrial processes.

Major mitigation approaches (and the logic behind them)

Mitigation strategies are strongest when you connect each strategy to a specific emission source.

1) Decarbonizing electricity

Because electricity generation is a major CO2 source, switching to low-carbon electricity reduces emissions across multiple sectors.

• Renewable energy (solar, wind, hydroelectric, geothermal): low direct CO2 emissions.
• Nuclear power: low direct CO2 emissions with tradeoffs (radioactive waste, cost, accident-risk perception).
• Energy efficiency: doing the same work with less electricity (LEDs, insulation, efficient motors).

A misconception to avoid: “renewable” does not automatically mean “no environmental impact.” Hydroelectric dams alter ecosystems; wind and solar require materials and land. APES rewards recognizing tradeoffs while comparing carbon benefits.

2) Electrifying end uses

If electricity is cleaner, shifting cars, heating, and some industrial processes from direct fuel combustion to electricity can cut emissions. Electric vehicles reduce tailpipe emissions; the total benefit depends on the electricity mix.

3) Reducing transportation emissions

Public transit, biking, walkable city design (reduce vehicle miles traveled), higher fuel efficiency, and alternative fuels in certain applications can lower emissions.

4) Cutting methane and nitrous oxide

Because CH4 and N2O have strong warming effects per molecule, reducing them can yield relatively quick benefits.

• Capture methane from landfills or manure lagoons.
• Reduce leaks from natural gas systems.
• Improve fertilizer management (right source, right rate, right time, right place) to reduce N2O.

5) Protecting and expanding carbon sinks

A carbon sink absorbs more carbon than it releases. Reforestation and afforestation increase carbon storage, and soil conservation/regenerative practices can increase soil organic carbon. A key nuance: forests store carbon, but they can become sources if burned or degraded; storage is not guaranteed without long-term protection.

Policy tools: how governments and markets reduce emissions

APES tests the structure and logic of policy tools.

Carbon taxes

A carbon tax charges a fee proportional to fossil fuels’ carbon content (or directly on CO2 emissions). The goal is to internalize climate externalities, making high-carbon choices more expensive and low-carbon alternatives more competitive. Strength: broad, simple price signal. Challenge: political resistance and equity concerns unless revenue is recycled or used to offset burdens.

Cap-and-trade systems

A cap-and-trade program sets a total emissions limit (cap) and issues allowances that can be traded. The cap ensures total emissions stay within the limit; trading helps reductions occur where cheapest. Students often confuse the parts: the cap is the environmental limit, the trade is the market mechanism.

Regulations and standards

Fuel economy standards, emissions performance standards, renewable portfolio standards, building codes, and appliance efficiency standards all reduce emissions through rules rather than prices.

International cooperation

Climate change is a global commons problem, so international agreements matter. Key agreements commonly referenced include:

• Kyoto Protocol (entered into force 2005): an international agreement under the UN framework aimed at reducing greenhouse gas emissions through binding targets for certain countries.
• Paris Agreement (2016): focuses on greenhouse gas emissions and mitigation, with the goal of keeping global temperature rise well below 2°C above pre-industrial levels, while each country determines its own plans.

Example: selecting mitigation strategies for a coal-heavy region

If a region relies heavily on coal, high-impact mitigation includes retiring coal plants and replacing them with renewables, nuclear, or lower-carbon fuels while reducing methane leakage; investing in grid upgrades and storage for variable renewables; and improving efficiency to reduce demand. Strong responses explain why coal is high-carbon and why efficiency is often the fastest “new supply.”

Exam Focus

• Typical question patterns:
  • Propose mitigation strategies targeting specific sectors (electricity, transport, agriculture) and justify them.
  • Compare carbon tax vs cap-and-trade conceptually (how each reduces emissions).
  • Evaluate tradeoffs of energy sources using emissions and environmental impacts.
  • Identify major climate agreements (Kyoto, Paris) and their general purpose.
• Common mistakes:
  • Describing mitigation as “recycling more” without linking to greenhouse gas reductions.
  • Confusing carbon sequestration (long-term storage) with short-term capture that may be re-released quickly.
  • Explaining cap-and-trade as if it guarantees each company reduces emissions the same amount (trading means reductions can differ by firm).

The Stratospheric Ozone Layer and How Ozone Depletion Happens

Ozone: same molecule, different role depending on altitude

Ozone (O3) is a highly reactive molecule made of three oxygen atoms. In APES, it’s essential to distinguish:

• Stratospheric ozone (high altitude): beneficial because it absorbs much of the Sun’s harmful UV radiation.
• Tropospheric ozone (near the ground): a pollutant and a major component of photochemical smog that harms lungs and plants.

“Ozone is good” is incomplete—altitude matters.

Where the ozone layer is and what it does

The stratosphere contains about 97% of the ozone in the atmosphere, with much of it roughly 15–40 km above Earth’s surface. The ozone layer is often described as a belt of naturally occurring ozone between about 15–30 km altitude that serves as a shield—especially by absorbing much of the harmful UV-B radiation.

UV radiation types (UVA, UVB, UVC)

Ultraviolet radiation is commonly subdivided into:

• UVA: closest to blue visible light; commonly associated with skin tanning.
• UVB: causes blistering sunburns and is associated with skin cancer.
• UVC: largely absorbed in the upper atmosphere/stratosphere and is important in the reactions that form ozone.

How ozone forms and breaks down naturally

Stratospheric ozone is created and destroyed naturally in a dynamic balance. UV light can split an oxygen molecule (O2), creating atomic oxygen (single oxygen atoms). Atomic oxygen can combine with O2 to form ozone (O3). Ozone can also be broken down back into O2. Ozone concentration depends on the balance of formation and destruction.

Human-caused ozone depletion: CFCs, halons, and catalytic destruction

Ozone depletion is a decrease in stratospheric ozone, mainly caused by chlorofluorocarbons (CFCs) and related halogen-containing chemicals such as halocarbons/halons. There are no natural reservoirs of CFCs or halons in the atmosphere, but their chemical stability allows them to persist in the lower atmosphere long enough to reach the stratosphere.

• Chlorofluorocarbons (CFCs) are nonflammable chemicals containing carbon, chlorine, and fluorine.
• Halocarbons (halons) are organic molecules with at least one carbon and one or more halogen atoms (commonly fluorine, chlorine, bromine, iodine).

In the stratosphere, UV radiation breaks these compounds apart, releasing chlorine (and sometimes bromine). These atoms destroy ozone through catalytic cycles, meaning the halogen atom is not consumed and can destroy many ozone molecules.

Why the “ozone hole” is strongest over Antarctica

Ozone depletion is most severe over Antarctica because extremely cold stratospheric temperatures lead to polar stratospheric clouds. Reactions on these cloud surfaces convert chlorine into forms that rapidly destroy ozone when sunlight returns in Antarctic spring. The polar vortex helps isolate Antarctic air, intensifying the effect.

Example: consumer products historically linked to ozone depletion

Historically, CFCs were used in aerosol propellants, refrigeration, and foam-blowing agents. Their stability made them useful, but that same stability allowed them to reach the stratosphere and contribute to ozone depletion—an important example of unintended technological consequences.

Exam Focus

• Typical question patterns:
  • Distinguish stratospheric ozone (protective) from tropospheric ozone (pollutant).
  • Explain why CFCs cause ozone depletion (stable enough to reach stratosphere; UV releases chlorine; catalytic destruction).
  • Identify UVA vs UVB vs UVC and connect UVB to biological harm.
  • Describe why ozone depletion is severe over Antarctica.
• Common mistakes:
  • Saying “the ozone hole causes global warming” (different problems).
  • Confusing smog ozone with the ozone layer.
  • Claiming the ozone hole is a literal hole letting all sunlight in (it is thinning, not an open gap).

Impacts of Ozone Depletion and How the World Responded

What increases when ozone decreases: UV-B exposure

When stratospheric ozone decreases, more UV-B radiation reaches Earth’s surface. UV-B can damage tissues and DNA.

Human health impacts

Increased UV exposure is linked to:

• Higher risk of skin cancer.
• Increased cataracts and other eye damage.
• Reduced effectiveness of the immune system (immune suppression).
• More sunburns and skin damage.

A common mistake is to attribute these effects to heat from climate change rather than UV exposure; ozone depletion is about UV filtering, not trapping infrared heat.

Ecosystem, agriculture, and animals

Increased UV-B can:

• Reduce phytoplankton growth in surface waters, with cumulative effects on aquatic food webs.
• Damage plant tissues and reduce crop yields (a reduction in crop production).
• Cause deleterious effects on animals.
• Increase mutations because UV radiation can change DNA structure.

Atmospheric and climate-related effects in the stratosphere

Ozone depletion can contribute to cooling of the stratosphere and other atmospheric changes. (In APES, keep the causal chain straight: ozone depletion changes UV absorption in the stratosphere; greenhouse warming is driven by infrared-trapping gases.)

Materials impacts

More UV radiation can accelerate degradation of some materials (for example, certain plastics and paints), shortening product lifetimes.

Solutions: phasing out ozone-depleting substances

The major policy success story is the Montreal Protocol (1987), an international treaty designed to phase out production and use of CFCs and other ozone-depleting substances. It worked relatively well because the causes were concentrated in a narrower set of chemicals/industries, alternatives were developed for many uses, and costs/benefits were clearer and more immediate than for climate change.

Replacing CFCs reduced ozone depletion, but some replacement chemicals have their own environmental tradeoffs, reinforcing the APES theme that solutions can introduce new impacts.

Additional ways to reduce ozone depletion (policy and consumer-system approaches)

Ozone recovery required coordinated manufacturing and policy changes, not only individual choices. Examples of strategies include:

• Support legislation that reduces ozone-destroying chemicals in products such as medical inhalers, fire extinguishers, aerosol hairsprays, wasp and hornet sprays, refrigerator and air conditioner foam insulation, and pipe insulation.
• Introduce tariffs on products produced in countries that allow the use of CFCs.
• Offer tax credits or rebates for turning in old refrigerators and air conditioners.
• Use helium, ammonia, propane, or butane as coolant alternatives to HCFCs and CFCs where appropriate.

Example: why policy matters even when individuals “do their part”

Even if consumers avoid certain products, ozone recovery required coordinated changes in manufacturing and global chemical use. This illustrates a broader APES idea: large-scale environmental problems often require systemic solutions (policy, technology shifts, and international cooperation) in addition to individual action.

Exam Focus

• Typical question patterns:
  • Predict impacts of increased UV-B on humans and ecosystems.
  • Identify the major policy solution (Montreal Protocol) and explain what it did (phase-out of ozone-depleting substances).
  • Propose additional policy/economic tools (tariffs, rebates, product regulation) to reduce ozone-depleting emissions.
  • Compare ozone depletion solutions (chemical phase-out) with climate change solutions (broad energy and land-use transformation).
• Common mistakes:
  • Attributing ozone depletion impacts to temperature increase rather than UV exposure.
  • Saying the problem is fully solved everywhere (recovery takes time).
  • Confusing the Montreal Protocol (ozone) with climate agreements (climate change).

Biodiversity, Endangered Species, and Invasive Species in a Changing World

Why climate change connects to biodiversity

Climate change affects biodiversity by shifting habitats, altering timing of life-cycle events, and increasing stress from extremes (heat, drought, storms). These impacts are often compounded by habitat loss and fragmentation, which can block migration corridors.

Why plants can be especially vulnerable to habitat loss

Plants are often initially more susceptible to habitat loss than animals for several reasons. Plants generally cannot migrate as individuals, cannot seek nutrients or water, seedlings must survive in degraded conditions, and seed dispersal can be slow.

Animals may cope with habitat destruction through migration, adaptation, and/or acclimatization, although each has limits.

Migration, adaptation, and acclimatization (key definitions)

Migration success depends on access routes/corridors, the magnitude and rate of degradation, the organism’s ability to migrate, and the proximity and availability of suitable new habitats.

Adaptation is the ability of a population/species to survive across generations in changing environmental conditions. Adaptation depends on birth rate, gene flow between populations, genetic variability, population size, generation length, and the magnitude/rate of degradation.

Acclimatization is the process by which an individual organism adjusts to gradual environmental change to maintain performance across a range of conditions. It depends on physiological and behavioral limitations of the species and the magnitude/rate of environmental change.

Invasive species

An invasive species is an animal or plant transported to an area where it does not naturally live and then spreads, often harming ecosystems, economies, or human health.

Common characteristics associated with invasive species include being abundant in their native range, broad diet, high dispersal, high genetic variability, high reproduction, close association with humans, long-lived or opportunistic life histories (including pioneer traits), short generation times, tolerance of wide environmental conditions, and vegetative or clonal reproduction.

Examples:

• Dutch elm disease transmitted to elm trees by elm bark beetles, killing over half of elm trees in the northern United States.
• European green crabs introduced to the San Francisco Bay area in 1989, threatening commercial fisheries.
• Water hyacinth introduced to the United States from South America; forms dense mats that reduce sunlight for submerged plants and aquatic organisms, crowd out native aquatic plants, and clog waterways and intake pipes.
• Zebra mussels attach to hard surfaces, clogging water intake/discharge pipes, attaching to boat hulls and docks, and even attaching to native mussels and crayfish.

Endangered species

An endangered species is a species considered to face a very high risk of extinction in the wild. Factors considered when labeling a species endangered include breeding success rate, known threats, the net increase/decrease in population over time, and the number of individuals remaining.

Arguments for protecting endangered species include maintaining genetic diversity; preserving keystone species and indicator species; and protecting aesthetic, ecological, educational, historical, recreational, scientific, and even yet-to-be-discovered value.

Characteristics that contribute to endangerment (with examples)

Many traits increase extinction risk, especially when combined with habitat loss and climate stress:

• Compete for food with humans (African penguins)
• High infant mortality (leatherback turtles)
• Highly sensitive to environmental change (cotton-top tamarins)
• Hunting for sport (passenger pigeons, blue whales, Bengal tigers)
• Introduction of nonnative invasive species (bandicoots threatened by cats introduced by Europeans)
• Limited environmental tolerance ranges (frogs; eggs sensitive to pollution, temperature changes, and wetland destruction)
• Limited geographic range (pandas)
• Long or fixed migration routes (Pacific Northwest salmon impacted by dams, logging, and water diversion)
• Loss of habitat (red wolves; whooping cranes)
• Low reproductive rates (whales, elephants, orangutans)
• Move slowly (desert tortoises)
• No natural predators (dodo birds, Steller’s sea cows, sea otters)
• Not able to adapt quickly (polar bears)
• Commercially valuable characteristics (sharks, elephants, rhinoceros horns, gorillas)
• Require large territory (tigers)
• Small numbers limiting genetic diversity (tigers)
• Specialized diet (pandas eating bamboo)
• Disease spread by humans or livestock (African wild dogs)
• Superstitions (aye-ayes killed in Madagascar due to bad-luck beliefs)

Maintaining biodiversity (solutions)

Common strategies include creating/expanding wildlife sanctuaries, establishing breeding programs for endangered or threatened species, managing habitats and monitoring land use, designing and updating protective laws, protecting habitats through private/governmental land trusts, reintroducing species into suitable habitats, restoring compromised ecosystems, and reducing nonnative/invasive species.

Exam Focus

• Typical question patterns:
  • Distinguish adaptation (population-level, genetic, over generations) from acclimatization (individual-level) and apply each to a scenario.
  • Explain why habitat fragmentation can prevent climate-driven range shifts (loss of corridors).
  • Identify traits that make species vulnerable to endangerment or likely to become invasive.
  • Use an invasive species example to describe ecological and economic impacts.
• Common mistakes:
  • Treating adaptation and acclimatization as the same process.
  • Listing biodiversity strategies without explaining the mechanism (how sanctuaries, corridors, or laws reduce extinction risk).
  • Forgetting that invasive impacts often include infrastructure/economic damage (for example, zebra mussels clogging pipes).

Connecting the Big Ideas: Systems Thinking and Tradeoffs in Global Change

Why APES treats global change as a systems problem

Global change is ideal for systems thinking because changes in one part of the Earth system propagate through many others.

Emissions (human system) alter atmospheric composition (Earth system). Atmospheric change alters climate patterns (physical system). Climate patterns affect ecosystems and agriculture (biological system). Impacts feed back into human decisions (economic and political systems).

Distinguishing “cause,” “effect,” and “response”

A reliable way to organize free-response answers is:

• Cause: greenhouse gas emissions (CO2, CH4, N2O, fluorinated gases; plus warming influences from black carbon) from energy, land use, agriculture, industry, waste.
• Effect: warming, sea level rise, precipitation changes, ecosystem shifts, ocean acidification, changing extremes.
• Response:
  • Mitigation reduces causes.
  • Adaptation reduces harm from effects.

Ozone depletion follows a parallel structure:

• Cause: CFCs and halons.
• Effect: increased UV-B.
• Response: Montreal Protocol and chemical substitution (plus supportive policies like tariffs and appliance turn-in programs).

Evaluating solutions using tradeoffs

APES often asks you to evaluate solutions. Strong evaluation includes effectiveness, time scale, cost/feasibility, side effects, and equity.

For example, widespread air conditioning can reduce heat deaths (adaptation) but may increase electricity demand and emissions if powered by fossil fuels (mitigation challenge). A nuanced response might pair cooling centers with efficiency upgrades and cleaner electricity.

Example: a combined mitigation + adaptation plan for wildfire risk

If a region faces increasing wildfire risk linked to hotter, drier conditions:

• Adaptation: defensible space around homes, improved evacuation planning, restoring fire-resilient ecosystems.
• Mitigation: reduce greenhouse gas emissions to limit future warming, and manage forests to reduce catastrophic fire intensity.

The point is that layered strategies reduce near-term harm and long-term drivers.

Exam Focus

• Typical question patterns:
  • Link a human activity to a global change mechanism and then to an impact.
  • Evaluate a proposed solution by discussing at least one benefit and one drawback.
  • Compare and contrast climate change and ozone depletion (cause, mechanism, impacts, solutions).
• Common mistakes:
  • Writing “laundry lists” of impacts without causal links.
  • Proposing solutions that don’t match the problem (for example, ozone-layer solutions for climate change).
  • Ignoring tradeoffs and constraints (cost, time, equity) when asked to evaluate.