Lesson 28: Stratospheric Ozone Depletion & the Greenhouse Effect

Warm-Up

Here is the single most confused pair of ideas in all of AP Environmental Science: ozone depletion and the greenhouse effect are different problems, with different chemicals, in different atmospheric layers, causing different harms. Ozone depletion is a hole in our UV sunscreen high in the stratosphere; the greenhouse effect is heat-trapping down in the troposphere. Mixing them up is the fastest way to lose points on this heaviest unit (15–20% of the exam). This lesson nails the distinction, then builds the foundation of climate science: which gases trap heat, and the feedback loops that can amplify or dampen change. Get this lesson right and Unit 9 falls into place.

Core Concept

Stratospheric ozone depletion

The ozone layer in the stratosphere (Lesson 12) absorbs most incoming ultraviolet (UV) radiation, protecting life. In the late 20th century, scientists discovered it was thinning, especially a seasonal "ozone hole" over Antarctica.

Cause: Chlorofluorocarbons (CFCs) — synthetic chemicals once used in refrigerants, aerosol propellants, and foam. CFCs are stable and drift up to the stratosphere, where UV breaks them apart, releasing chlorine atoms. Each chlorine atom catalytically destroys many ozone molecules (a chlorine atom is not consumed, so it keeps destroying O₃).

Effects of ozone depletion (more UV reaching the surface): - Increased skin cancer and cataracts in humans. - Damage to phytoplankton (base of marine food webs) and crops. - Harm to some organisms' immune systems and DNA.

Solution — the Montreal Protocol (1987): an international treaty that phased out CFCs and other ozone-depleting substances, replacing them with safer alternatives (HCFCs, then HFCs — though HFCs are potent greenhouse gases, a wrinkle). It is widely regarded as the most successful international environmental agreement — the ozone layer is slowly recovering.

[DIAGRAM: Ozone depletion. CFCs released at surface → drift to stratosphere → UV splits CFC, freeing Cl atom → Cl + O₃ → ClO + O₂ (ozone destroyed) → Cl regenerated to destroy more ozone. Result: thinner ozone layer, more UV reaching the surface.]

The greenhouse effect (a DIFFERENT problem)

The greenhouse effect is the natural process by which certain gases in the troposphere absorb and re-emit infrared (heat) radiation, keeping Earth warm enough for life (~33 °C warmer than it would be otherwise). Incoming solar (mostly visible) radiation warms the surface; the surface re-emits infrared, which greenhouse gases trap.

The problem is the enhanced (anthropogenic) greenhouse effect: humans have increased greenhouse-gas concentrations, trapping more heat and driving global warming/climate change (Lesson 29).

[DIAGRAM: Greenhouse effect. Sunlight (shortwave) passes through atmosphere, warms surface. Surface emits infrared (longwave). Greenhouse gases (CO₂, CH₄, H₂O, N₂O) absorb and re-emit infrared back toward surface, trapping heat. More greenhouse gas = more heat trapped.]

Major greenhouse gases

Gases differ in global warming potential (GWP) — heat-trapping ability per molecule relative to CO₂ over a time period — and atmospheric lifetime. CO₂ dominates total warming because of its sheer quantity and longevity, even though methane and N₂O are more potent per molecule.

Climate feedback loops

• Positive feedback amplifies change:
• Ice-albedo feedback: warming melts reflective ice → darker ocean/land absorbs more heat → more warming → more melting.
• Permafrost thaw: warming thaws permafrost → releases CO₂ and methane → more warming.
• Water-vapor feedback: warming increases evaporation → more water vapor (a greenhouse gas) → more warming.
• Negative feedback dampens change (e.g., more CO₂ can increase plant growth, removing some CO₂ — generally weaker than the positive feedbacks).

Positive feedbacks are a major concern because they can accelerate warming beyond the initial forcing.

Why this matters

The ozone-vs-greenhouse distinction is arguably the most important single clarification in Unit 9 and shows up in MC and FRQ. Knowing the greenhouse gases, their sources, and feedback loops sets up all of Lesson 29 (climate impacts). The Montreal Protocol is a frequently cited "success story."

Worked Examples

Example 1 (easy): Ozone vs. greenhouse

State two differences between stratospheric ozone depletion and the greenhouse effect.

Solution: (1) Location/chemicals: ozone depletion is in the stratosphere, caused by CFCs/chlorine; the greenhouse effect is in the troposphere, caused by greenhouse gases (CO₂, CH₄, etc.). (2) Effect: ozone depletion lets in more UV (skin cancer); the greenhouse effect traps infrared heat (warming).

Interpretation: Different layer, different chemicals, different harm — never conflate them.

Example 2 (medium): Why one chlorine atom matters

Explain why a single chlorine atom from a CFC can destroy many ozone molecules.

Solution: Chlorine acts as a catalyst: it reacts with ozone to form ClO and O₂ (destroying O₃), but the chlorine is then regenerated and freed to react again. Because it is not consumed, one chlorine atom destroys thousands of ozone molecules before it's eventually removed.

Interpretation: Catalyst = not used up → outsized destruction.

Example 3 (AP-style): Feedback loop

Explain the ice-albedo feedback and classify it as positive or negative.

Solution: Warming melts bright, reflective ice/snow (high albedo), exposing darker ocean/land (low albedo) that absorbs more solar energy, causing more warming and more melting. Because it amplifies the initial warming, it is a positive feedback.

Interpretation: Amplifies change = positive feedback; ice-albedo is the classic example.

Example 4 (AP-style): GWP reasoning

Methane has a much higher global warming potential per molecule than CO₂, yet CO₂ causes more total warming. Explain.

Solution: Per molecule, methane traps far more heat than CO₂, but CO₂ is emitted in vastly larger quantities and persists in the atmosphere much longer. So despite methane's higher potency, CO₂'s sheer abundance and longevity make it the dominant driver of total human-caused warming.

Interpretation: Total warming = potency × quantity × lifetime; CO₂ wins on quantity and lifetime.

Common Mistakes

• Confusing ozone depletion and the greenhouse effect. Stratosphere/CFCs/UV vs. troposphere/greenhouse gases/heat. This is the #1 Unit 9 error.
• Thinking CFCs cause global warming as their main effect (or that CO₂ depletes ozone). CFCs deplete ozone (and are also greenhouse gases, but their headline role is ozone). CO₂ is a greenhouse gas, not an ozone destroyer.
• Calling the greenhouse effect entirely "bad." The natural greenhouse effect is essential for life; the enhanced (human-boosted) version is the problem.
• Getting feedback direction wrong. Positive feedback amplifies; negative feedback dampens. "Positive" doesn't mean "good."
• Assuming methane matters more than CO₂ overall. More potent per molecule, but CO₂'s quantity/longevity make it the dominant total contributor.

Practice Problems

11. (FRQ-style) Clearly distinguish stratospheric ozone depletion from the greenhouse effect in terms of location, cause, and effect, and name the treaty that addressed ozone depletion.

12. (Data) A graph shows Antarctic ozone thinning through the 1980s, then stabilizing after ~1990. (a) What likely caused the turnaround? (b) Explain why recovery is slow.

FRQ Practice — Analyze an Environmental Problem and Propose a Solution (10 pts)

A student's essay claims "the ozone hole is causing global warming by trapping heat." A teacher marks this as conceptually wrong.

(a) Explain why the statement conflates two different problems, distinguishing ozone depletion from the greenhouse effect (location, cause, effect). (4 pts) (b) Identify two major greenhouse gases and a human source of each. (2 pts) (c) Explain one positive climate feedback loop. (2 pts) (d) Name the treaty that successfully addressed ozone depletion and state why it is considered a success. (2 pts)