AP Environmental Science Global Change — Worked Answer Explanations

Unit 9 · 12 questions explained

Below is a complete answer key for our AP Environmental Science Global Change practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Global Change practice test and come back here to review, or head back to the Global Change unit overview.

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Question 1 · Easy
Global average surface temperatures have increased approximately 1.1°C since the pre-industrial period. Which of the following is a documented consequence of this warming?
- A
  Expansion of glaciers and polar ice sheets globally
  Why not A: Warming causes glacial retreat and ice sheet mass loss; glaciers are shrinking worldwide, not expanding.
- B
  Sea level rise due to thermal expansion of seawater and melting of land iceCorrect
- C
  Decreased intensity of tropical storms due to warmer sea surface temperatures limiting convection
  Why not C: Warmer seas provide more energy for tropical cyclones; evidence suggests storm intensity (not necessarily frequency) is increasing.
- D
  A slowdown in the global water cycle as warmer air holds less moisture
  Why not D: Warmer air holds MORE moisture (Clausius-Clapeyron: ~7% more water vapor per °C warming), intensifying the water cycle.
Explanation
Global sea level has risen ~20 cm since 1900, with the rate accelerating. Two causes: (1) thermal expansion — warmer water occupies more volume; (2) land ice melt — glaciers, Greenland ice sheet, and Antarctic ice contribute meltwater to the ocean. Coastal flooding, erosion, and saltwater intrusion into freshwater aquifers are observed consequences already affecting low-lying coastal regions and island nations.

Key takeaway
Climate change → sea level rise (thermal expansion + land ice melt) + glacier retreat + permafrost thaw + shifting species ranges.
Question 2 · Easy
The Intergovernmental Panel on Climate Change (IPCC) reports conclude that global warming since 1950 is 'unequivocally' caused primarily by:
- A
  Increased solar output from the sun
  Why not A: Solar output has been flat or slightly declining since the 1980s while temperatures have risen — solar cannot explain observed warming.
- B
  Human activities, primarily burning fossil fuels and deforestation, increasing greenhouse gas concentrationsCorrect
- C
  Natural volcanic eruptions releasing large quantities of CO₂ and aerosols
  Why not C: Volcanic eruptions typically cause short-term cooling (aerosols reflect sunlight) and emit far less CO₂ per year than fossil fuel combustion.
- D
  Natural Milankovitch orbital cycles shifting Earth's position relative to the sun
  Why not D: Milankovitch cycles operate over 10,000–100,000-year timescales; they cannot explain the rapid warming observed over decades.
Explanation
The IPCC Sixth Assessment Report (2021) concluded with 'unequivocal' certainty (the strongest scientific language) that human activities are causing observed warming. The fingerprints: CO₂ concentration tracks industrialization; troposphere is warming while stratosphere cools (consistent with greenhouse effect, not solar); and climate models match observations only when human forcings are included. 97%+ of climate scientists agree on anthropogenic cause.

Key takeaway
Scientific consensus (IPCC): climate change is 'unequivocally' human-caused, primarily from fossil fuels. Solar and volcanic causes cannot explain observed patterns.
Question 3 · Easy
The concept of carbon sequestration refers to:
- A
  The release of stored carbon from soils and vegetation into the atmosphere
  Why not A: Release of stored carbon is the opposite of sequestration; it describes processes like deforestation and permafrost thaw.
- B
  The long-term storage of carbon dioxide in biological or geological reservoirsCorrect
- C
  The conversion of CO₂ to ozone in the stratosphere
  Why not C: CO₂ is not converted to ozone; ozone is formed from O₂ via UV photolysis. These are entirely different chemical processes.
- D
  The combustion of biomass to release stored solar energy
  Why not D: Combustion releases stored carbon (as CO₂); sequestration stores it. These are opposite processes.
Explanation
Carbon sequestration removes CO₂ from the atmosphere and stores it in reservoirs: biological (forests, soils, wetlands — especially peatlands), oceanic (phytoplankton sink carbon to the seafloor), and geological (injecting compressed CO₂ into deep saline aquifers or basalt — carbon capture and storage, CCS). Reforestation and wetland restoration are natural sequestration strategies; CCS is a technological approach. Both are considered as climate mitigation tools.

Key takeaway
Carbon sequestration: removing CO₂ from atmosphere → stored in forests, soils, oceans, or geological formations. Opposite of carbon emission.
Question 4 · Medium
Ice core data from Antarctica and Greenland are used as evidence for past climate change. What property of ice cores provides the most direct record of past atmospheric CO₂ concentrations?
- A
  Isotopic ratios of oxygen (¹⁸O/¹⁶O) in the ice crystals
  Why not A: Oxygen isotope ratios in ice record past temperatures, not CO₂ concentrations directly.
- B
  Air bubbles trapped in the ice that preserved ancient atmosphere samplesCorrect
- C
  Volcanic ash layers that correlate with carbon-rich periods
  Why not C: Volcanic ash layers date the ice but don't record CO₂ concentration.
- D
  The depth of annual snow layers, which correlates with CO₂ concentration
  Why not D: Annual layer thickness records precipitation/snowfall, not CO₂ concentration.
Explanation
As snow accumulates on glaciers and compacts into ice, tiny air bubbles are trapped — preserving samples of the ancient atmosphere. Scientists extract these bubbles and analyze their gas composition, providing a direct record of past CO₂, CH₄, and N₂O concentrations going back ~800,000 years. This data shows that current CO₂ levels (~420 ppm) are unprecedented in at least 3 million years.

Key takeaway
Ice core air bubbles = direct record of ancient atmospheric CO₂ and other gases. Oxygen isotopes = past temperatures.
Question 5 · Medium
The oceans have absorbed approximately 25–30% of anthropogenic CO₂ emissions. The primary chemical consequence of this is:
- A
  Increased ocean productivity because CO₂ fertilizes phytoplankton photosynthesis
  Why not A: While some CO₂ fertilization occurs, ocean acidification reduces carbonate ions critical for calcifying organisms — the net effect is harmful, not simply beneficial.
- B
  Ocean acidification — lowered pH that reduces carbonate ion availability, threatening calcifying organismsCorrect
- C
  Increased ocean oxygen content as dissolved CO₂ displaces nitrogen
  Why not C: CO₂ absorption doesn't displace nitrogen or increase oxygen; it reacts with water to form carbonic acid, reducing pH.
- D
  Formation of methane hydrates at the ocean surface from dissolved CO₂
  Why not D: Methane hydrates form from methane at high pressure and low temperature in deep seafloor sediments; CO₂ does not directly produce them at the surface.
Explanation
CO₂ + H₂O → H₂CO₃ (carbonic acid) → H⁺ + HCO₃⁻. The released H⁺ ions lower ocean pH (now ~8.08, down from pre-industrial ~8.18 — a 30% increase in H⁺ concentration). Crucially, H⁺ also reacts with carbonate ions (CO₃²⁻), reducing their availability. Corals, oysters, sea urchins, and pteropods use CO₃²⁻ to build calcium carbonate (CaCO₃) shells/skeletons — acidification makes this increasingly difficult or impossible below certain pH thresholds.

Key takeaway
Ocean acidification: CO₂ + H₂O → H₂CO₃ → lower pH + fewer carbonate ions → coral bleaching/dissolution + shell-builder stress.
Question 6 · Medium
Positive feedback loops amplify the initial effect of climate warming. Which of the following is an example of a positive climate feedback?
- A
  Increased cloud cover reflecting more solar radiation, reducing surface warming
  Why not A: If warming → more clouds → more reflection → less warming, this is a negative (stabilizing) feedback.
- B
  Melting Arctic sea ice exposes darker ocean water, absorbing more heat and accelerating warmingCorrect
- C
  Ocean absorption of CO₂ from the atmosphere, slowing the greenhouse effect
  Why not C: Ocean CO₂ absorption reduces atmospheric CO₂ — this is a negative feedback that dampens warming.
- D
  Increased vegetation in the Arctic as temperatures rise, absorbing more CO₂
  Why not D: Increased Arctic plant growth that absorbs CO₂ would be a negative feedback; however, in reality, permafrost thaw releases more carbon than new vegetation absorbs.
Explanation
The ice-albedo feedback is a classic positive (amplifying) feedback: warming → ice melts → lower albedo (white ice replaced by dark water/land that absorbs more radiation) → more heating → more melting. This is one reason the Arctic is warming ~3–4× faster than the global average. Other positive feedbacks: permafrost thaw releasing CH₄, increased water vapor (itself a greenhouse gas), and reduced forest cover from bark beetle outbreaks.

Key takeaway
Positive feedback amplifies warming. Ice-albedo: ice melts → dark ocean absorbs more heat → more warming. Permafrost thaw → CH₄ release is another key example.
Question 7 · Medium
An invasive species is introduced to a new habitat. Which combination of factors MOST typically explains its population explosion in the new environment?
- A
  It encounters more competitors in the new habitat than in its native range.
  Why not A: Invasive species typically succeed because they face FEWER competitors and natural enemies, not more.
- B
  It lacks natural predators, parasites, and pathogens in the new environment, and finds abundant resources.Correct
- C
  Its reproductive rate is inherently higher than all native species.
  Why not C: Many invasive species are K-selected (slow reproducers); release from natural enemies, not inherent fecundity, primarily drives their success.
- D
  The new habitat's climate is identical to its native range, ensuring immediate adaptation.
  Why not D: Climate match is a condition for survival but is not the key factor explaining population explosions over native species.
Explanation
The 'enemy release hypothesis' explains invasive success: in native ranges, predators, parasites, pathogens, and competitors keep populations in check. In a new environment without these controls, and with prey/resources that haven't co-evolved defensive behaviors, the species can grow exponentially (biotic resistance is absent). Examples: European starlings (U.S.), cane toads (Australia), kudzu vine (U.S. Southeast), zebra mussels (Great Lakes).

Key takeaway
Invasive species succeed via 'enemy release': no natural predators/parasites + naive prey/competitors → population explosion.
Question 8 · Medium
The Paris Agreement (2015) established a goal of limiting global warming to well below 2°C above pre-industrial levels, with efforts to limit to 1.5°C. This agreement operates primarily through:
- A
  Legally binding, enforced carbon taxes on all signatory nations
  Why not A: The Paris Agreement is largely voluntary — nations submit Nationally Determined Contributions (NDCs), but there are no enforcement penalties.
- B
  Nationally Determined Contributions (NDCs) — voluntary pledges by each nation to reduce emissionsCorrect
- C
  A global cap-and-trade system distributing emissions allowances to each country
  Why not C: No global cap-and-trade system was established; individual nations may have domestic carbon markets, but Paris didn't mandate a global one.
- D
  Mandatory phase-out of fossil fuels within 10 years of ratification
  Why not D: No mandatory fossil fuel phase-out was included; the COP28 agreement (2023) referenced transitioning 'away from' fossil fuels, but Paris itself set no such mandate.
Explanation
The Paris Agreement (ratified by 194 parties) operates on voluntary nationally determined contributions (NDCs) — each country sets its own emission reduction targets. Nations must submit increasingly ambitious NDCs every 5 years and report on progress, but there are no legal penalties for failing to meet pledges. This differs from the Kyoto Protocol (1997) which had legally binding targets for developed nations (but the U.S. never ratified it).

Key takeaway
Paris Agreement: voluntary NDCs, updated every 5 years, goal of <2°C (ideally 1.5°C). No enforcement mechanism — 'pledge and review' approach.
Question 9 · Medium
Rising global temperatures are shifting the geographic ranges of many species poleward and to higher elevations. A species living on a mountaintop that cannot move higher as temperatures rise faces:
- A
  Climate-driven extinction due to range compression with no suitable habitat remainingCorrect
- B
  Competitive exclusion by species migrating downslope to the valley
  Why not B: Range shifts are moving species upslope, not downslope; mountaintop species are being pushed off the top, not invaded from below.
- C
  Enhanced fitness because warmer temperatures increase metabolic rates
  Why not C: Species adapted to cool mountaintop conditions are physiologically stressed by warming, not benefited by it.
- D
  Rapid evolution that allows it to adapt to the new temperature regime within a generation
  Why not D: Evolutionary adaptation typically requires many generations; the pace of climate change outstrips the evolutionary capacity of most species.
Explanation
Mountaintop species (pikas, alpine plants, cold-water fish) are experiencing 'climate squeeze': warming pushes suitable conditions upslope, but the mountain top acts as a barrier. With no cooler habitat to colonize, populations contract and face local extinction. This is particularly acute for islands, mountaintops, and peninsulas where species cannot shift their range to track suitable climate. The American pika is a well-documented example.

Key takeaway
Climate squeeze: mountaintop/island species pushed toward shrinking habitat edges → extinction risk when no cooler refuge remains.
Question 10 · Medium
Which of the following actions most directly addresses climate change by reducing atmospheric CO₂ concentrations (mitigation), as opposed to adapting to climate change impacts?
- A
  Building sea walls to protect coastal cities from rising sea levels
  Why not A: Sea walls protect against sea level rise impacts but do not reduce atmospheric CO₂ — this is adaptation.
- B
  Developing drought-resistant crop varieties for farming in hotter, drier conditions
  Why not B: Drought-resistant crops help communities cope with climate effects but do not reduce CO₂ — this is adaptation.
- C
  Replacing coal power plants with utility-scale solar and wind energyCorrect
- D
  Relocating coastal communities threatened by storm surge
  Why not D: Relocating communities is a managed retreat strategy for adapting to sea level rise impacts — not CO₂ reduction.
Explanation
Climate mitigation reduces greenhouse gas emissions or enhances carbon sinks — directly addressing the cause of climate change. Replacing fossil fuel power plants with renewables reduces CO₂ emissions at source. Climate adaptation involves adjusting to unavoidable impacts: seawalls, drought-resistant crops, early warning systems. Both are necessary, but only mitigation addresses root cause. Other mitigation examples: electrifying transport, improving efficiency, protecting forests, reducing methane from agriculture.

Key takeaway
Mitigation = reducing GHG emissions (source of problem). Adaptation = adjusting to climate impacts (consequences). Both are needed; mitigation is preferred to limit long-term damage.
Question 11 · Hard
Which conservation strategy is most likely to protect the greatest number of species across a landscape?
- A
  Captive breeding programs for individually endangered species
  Why not A: Captive breeding can prevent extinction of specific species but does not protect ecosystems or the thousands of undocumented species in a landscape.
- B
  Protecting large, contiguous core reserves connected by habitat corridorsCorrect
- C
  Focusing protection on charismatic megafauna as umbrella species
  Why not C: Umbrella species protection can help but may miss habitat requirements of many non-charismatic species not covered by the megafauna's range.
- D
  Banning all human activities across a small, intensively managed reserve
  Why not D: Small reserves suffer from edge effects and species-area limitations; connectivity and size matter more than intensity of management.
Explanation
Conservation design principles from island biogeography and landscape ecology: large reserves > small reserves (more interior habitat, lower extinction rates); connected reserves (via corridors) > isolated fragments (allow recolonization after local extinctions); contiguous reserves > fragmented ones; and multiple reserves in different regions provide redundancy. This 'large-landscape conservation' approach protects ecosystem processes and the full range of species that depend on them.

Key takeaway
Best conservation design: large, connected core reserves with corridors > many small isolated fragments. Size and connectivity are the key variables.
Question 12 · Hard
Methane (CH₄) contributes to global warming despite being present at far lower atmospheric concentrations than CO₂. The reason methane is such a significant greenhouse gas concern is that:
- A
  Methane lasts longer in the atmosphere than CO₂ (thousands of years vs. ~12 years)
  Why not A: It is the opposite: methane has an atmospheric lifetime of ~12 years (short-lived) while CO₂ persists for centuries to millennia.
- B
  Methane's global warming potential (GWP) is ~80× that of CO₂ over a 20-year timeframe, so even small mass emissions cause significant warmingCorrect
- C
  Methane blocks more ultraviolet radiation than CO₂, protecting stratospheric ozone
  Why not C: Methane is a greenhouse gas that affects infrared radiation, not UV. CFCs affect stratospheric ozone, not methane.
- D
  Methane is heavier than air and accumulates at ground level, trapping heat more efficiently near the surface
  Why not D: Methane (MW = 16) is lighter than air (MW ≈ 29), not heavier. Greenhouse gases trap heat through molecular infrared absorption, not gravitational settling.
Explanation
Methane's short atmospheric lifetime (~12 years) and very high warming potential (GWP 80 over 20 years; 27–30 over 100 years) make it a high-priority mitigation target. Sources: livestock (enteric fermentation), rice paddies, natural gas leaks, coal mines, landfills, wetlands. Because methane degrades relatively quickly, reducing methane emissions produces near-term climate benefits faster than CO₂ cuts — making it critical for near-term climate targets.

Key takeaway
Methane: ~80× GWP vs CO₂ over 20 years, but short-lived (~12 yr). Reducing methane gives fastest near-term climate benefit. Sources: livestock, natural gas leaks, landfills.