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IB DP ESS — Topic 6: Atmosphere and Climate Change | HL Extension | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define ozone-depleting substances (ODSs) and explain their chemical mechanism of ozone destruction using chlorine and bromine radical chain reactions.
- Explain why polar stratospheric clouds (PSCs) accelerate ozone depletion over Antarctica and the Arctic, and describe the seasonal pattern of the ozone hole.
- Distinguish between the ozone hole and general global ozone thinning, and describe the spatial and temporal patterns of each.
- Analyse the role of HFCs as replacements for CFCs, including their contribution to radiative forcing and climate change.
- Evaluate the effectiveness of the Montreal Protocol and the Kigali Amendment in addressing ozone depletion and climate forcing by ODSs and HFCs.
- Discuss the trade-offs and unintended consequences of transitioning from CFCs to HCFCs to HFCs, and the implications for both ozone recovery and climate change mitigation.
- Discuss contrasting worldviews (ecocentric, technocentric, anthropocentric) in relation to the regulation of ODSs and the search for sustainable refrigerant alternatives.
Real-World Context & Relevance
In September 2023, the Antarctic ozone hole reached an area of approximately 26 million km² — one of the largest on record since monitoring began in the 1970s. Scientists at the Copernicus Atmosphere Monitoring Service noted that while the long-term trend shows gradual recovery attributable to the Montreal Protocol, year-to-year variability remains significant, driven partly by stratospheric temperature anomalies linked to the 2022 Hunga Tonga volcanic eruption. This single data point encapsulates a remarkable story: humanity identified a global atmospheric crisis, negotiated an international treaty, and began reversing the damage — yet the system remains fragile and subject to feedbacks we did not anticipate.
The ozone layer story is also a cautionary tale about technological substitution. When chlorofluorocarbons (CFCs) were phased out under the Montreal Protocol, industry replaced them largely with hydrofluorocarbons (HFCs). HFCs do not deplete ozone — but they are potent greenhouse gases, with global warming potentials (GWPs) hundreds to thousands of times greater than CO₂. The Kigali Amendment (2016) was required to address this unintended consequence, illustrating how solving one environmental problem can inadvertently create another. For HL ESS students, this topic is a masterclass in systems thinking: atmospheric chemistry, international governance, industrial economics, and climate feedbacks are all entangled.
🌍 Why This Matters The stratospheric ozone layer absorbs 97–99% of the Sun's harmful ultraviolet-B (UV-B) radiation. Its depletion increases UV-B reaching Earth's surface, elevating rates of skin cancer, cataracts, and immune suppression in humans, while also damaging phytoplankton — the base of marine food webs — and reducing crop yields. The ozone depletion story is simultaneously a success story of international environmental governance and a warning about the complexity of atmospheric systems and the unintended consequences of industrial substitution.
Core Concepts & Definitions
To understand ozone