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IB DP ESS — Topic 6: Atmosphere and Climate Change | SL & HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define the greenhouse effect, enhanced greenhouse effect, and climate change using IB-aligned ESS terminology.
- Explain the natural greenhouse effect and how it maintains Earth's energy balance, distinguishing it from the enhanced greenhouse effect driven by human activity.
- Describe the major greenhouse gases (CO₂, CH₄, N₂O, water vapour, CFCs), their sources, and their relative contributions to radiative forcing.
- Explain how deforestation, fossil fuel combustion, agriculture, and land-use change amplify the enhanced greenhouse effect at local, regional, and global scales.
- Analyse positive and negative feedback mechanisms in the climate system, including the ice-albedo feedback, water vapour feedback, and permafrost methane release.
- Distinguish between natural and anthropogenic drivers of climate change, including Milankovitch cycles, solar variability, and volcanic forcing.
- Evaluate the reliability and uncertainty of climate models and proxy data in reconstructing past and projecting future climate change.
- Discuss contrasting worldviews — ecocentric, technocentric, and anthropocentric — in relation to the causes and human responsibility for climate change.
Real-World Context & Relevance
In May 2023, the global monthly average concentration of carbon dioxide at the Mauna Loa Observatory in Hawaiʻi reached 424 parts per million (ppm) — the highest level recorded in at least 800,000 years of ice-core proxy data. This figure, tracked continuously since Charles Keeling began measurements in 1958, has risen by over 50% since pre-industrial levels of approximately 280 ppm. The trajectory is not a gradual drift: the rate of increase has itself accelerated, from roughly 0.7 ppm per year in the 1960s to over 2.4 ppm per year in the 2020s. Behind this single number lies a cascade of interconnected human activities — the burning of coal, oil, and gas; the clearing of tropical forests; the expansion of industrial agriculture — each releasing greenhouse gases that alter the energy balance of the entire atmosphere.
The consequences are already measurable. The IPCC Sixth Assessment Report (AR6, 2021) concluded that global average surface temperature has risen by approximately 1.1°C above pre-industrial levels, with the last decade (2011–2020) being the warmest on record. Yet climate change is not simply a story of rising temperatures: it is a story of disrupted systems. The Arctic is warming at more than twice the global average rate — a phenomenon known as Arctic amplification — triggering feedbacks that accelerate further warming. Meanwhile, in the Sahel region of sub-Saharan Africa, shifting rainfall patterns linked to changing atmospheric circulation are intensifying drought cycles, threatening food security for hundreds of millions of people. Understanding the causes of climate change means understanding how human activities interact with natural Earth systems to produce these cascading, often irreversible, effects.
🌍 Why This Matters Climate change is the defining environmental challenge of the twenty-first century. Understanding its causes — both natural and anthropogenic — is essential for evaluating mitigation strategies, assessing international policy frameworks such as the Paris Agreement, and making informed judgments about the sustainability of current human development pathways. ESS students who can explain the mechanisms driving climate change are equipped to critically analyse the most consequential environmental issue of their generation.
Core Concepts & Definitions
To understand why Earth's climate is changing, we must first understand why Earth has a climate suitable for life at all. Without any atmosphere, Earth's average surface temperature would be approximately −18°C — far too cold to support liquid water or complex life. The fact that the actual average is around +15°C is entirely due to a natural atmospheric process that has operated for billions of years. Appreciating this natural baseline is essential before we can understand how human activities are disrupting it.
📖 Key Definition — The Natural Greenhouse Effect The natural greenhouse effect is the process by which certain gases in Earth's atmosphere — primarily water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) — absorb outgoing longwave infrared radiation emitted by Earth's surface and re-emit it in all directions, including back toward the surface. This process raises Earth's average surface temperature by approximately 33°C above what it would otherwise be, making the planet habitable.
The mechanism works through a