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IB DP ESS — Topic 6: Atmosphere and Climate Change | HL Extension | Paper 1 & Paper 2A
🎯 Learning Intentions By the end of this note, you will be able to:
- Describe the structure and composition of the major layers of Earth's atmosphere, including the troposphere, stratosphere, mesosphere, and thermosphere.
- Explain the temperature profile of each atmospheric layer and the physical processes responsible for heating or cooling within each zone.
- Outline the evolution of Earth's atmosphere through three major stages, from the primordial reducing atmosphere to the present oxygen-rich composition.
- Define Milankovitch cycles and identify the three orbital parameters — eccentricity, axial tilt (obliquity), and precession — that drive long-term climate variability.
- Explain how Milankovitch cycles influence the distribution of solar radiation received at Earth's surface and their relationship to glacial–interglacial cycles.
- Analyse the limitations of Milankovitch cycles as a sole explanation for past climate change, considering additional forcing factors such as greenhouse gas feedbacks and ocean circulation.
- Evaluate the significance of natural climate variability (Milankovitch cycles) in the context of current anthropogenic climate change, distinguishing natural from human-driven forcing.
- Discuss the role of the evolving atmosphere in enabling the development of life on Earth, including the formation of the ozone layer and the rise of photosynthetic organisms.
Real-World Context & Relevance
In August 2023, the Copernicus Climate Change Service reported that global average surface temperatures had exceeded 1.5 °C above pre-industrial levels for the first time on a monthly basis. This milestone — long discussed as a critical threshold in the Paris Agreement — prompted urgent questions about whether current warming is truly unprecedented. To answer that question rigorously, scientists look far beyond the instrumental record, reaching back hundreds of thousands of years into ice cores drilled from Antarctica and Greenland. Those cores reveal a striking pattern: Earth's climate has oscillated between glacial and interglacial periods with remarkable regularity, driven in part by predictable changes in Earth's orbit around the Sun.
The Vostok ice core from Antarctica, which extends back approximately 420,000 years, shows four complete glacial–interglacial cycles, each corresponding to periods when atmospheric CO₂ ranged from roughly 180 ppm during glacials to 280 ppm during interglacials. By 2023, atmospheric CO₂ had reached 421 ppm — a concentration not seen in at least 3 million years, and rising at a rate approximately 100 times faster than any natural transition recorded in the ice core record (IPCC, "Climate Change 2021: The Physical Science Basis"). Understanding why Earth's climate has changed naturally over geological time — through the structure of the atmosphere and the mechanics of orbital forcing — is therefore not merely historical curiosity. It is the essential baseline against which the scale and speed of anthropogenic change can be properly judged.
🌍 Why This Matters Understanding the natural architecture of the atmosphere and the orbital drivers of past climate change allows scientists and policymakers to distinguish natural variability from human-forced warming. Without this baseline, it is impossible to attribute current climate change to anthropogenic greenhouse gas emissions with scientific confidence — a distinction that underpins all international climate policy from the UNFCCC to the Paris Agreement.
Core Concepts & Definitions
Before examining how Earth's climate changes over orbital timescales, it is essential to understand the physical