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IB DP ESS — Topic 2: Ecology (HL Extension) | HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define the lithospheric carbon store and distinguish it from other carbon reservoirs (atmosphere, biosphere, hydrosphere) using IB-aligned ESS terminology.
- Explain how carbon is sequestered into and released from deep geological stores, including fossil fuels, carbonate rocks, and marine sediments.
- Describe the formation, sources, and atmospheric fate of methane (CH₄) as a greenhouse gas, including its global warming potential relative to CO₂.
- Explain the role of permafrost, methane clathrates, and peatlands as carbon stores and the conditions under which they release carbon to the atmosphere.
- Analyse positive feedback mechanisms linking deep carbon store destabilisation to accelerated climate change, applying ESS analysis parameters of scale, uncertainty, and sustainability.
- Evaluate the significance of human extraction of lithospheric carbon (fossil fuel combustion) as a perturbation to the global carbon cycle.
- Discuss the implications of tipping points in deep carbon stores for global climate projections and the limits of current climate models.
Real-World Context & Relevance
In August 2020, researchers working across Siberia reported an alarming observation: the Batagaika crater — already the world's largest permafrost thaw slump at over 1 km wide and 100 m deep — was expanding at a rate of approximately 10–30 metres per year. As the frozen ground melted, ancient organic matter that had been locked away for tens of thousands of years was being exposed to microbial decomposition for the first time. The result was the release of both carbon dioxide and methane into the atmosphere — gases that had been effectively removed from the active carbon cycle since the last ice age.
This single site illustrates a broader pattern that climate scientists find deeply concerning. The Arctic permafrost region stores an estimated 1,500 Gt of organic carbon — roughly twice the amount currently in the atmosphere. Meanwhile, the ocean floor holds vast reserves of methane clathrates (ice-like structures trapping methane molecules), and human industrial activity has spent over 200 years extracting and burning fossil fuels — carbon that took hundreds of millions of years to accumulate in the lithosphere. Understanding these deep carbon stores, how they formed, and what destabilises them is essential to understanding the full scope of climate risk.
🌍 Why This Matters The deep carbon stores of the lithosphere and cryosphere represent the largest reservoirs of carbon on Earth. Their destabilisation — whether through human fossil fuel extraction or climate-driven permafrost thaw — has the potential to trigger self-reinforcing feedback loops that push the climate system past tipping points beyond human control. Understanding these stores is not merely academic: it defines the upper boundary of what is climatically possible in the 21st century and beyond.
Core Concepts & Definitions
To understand why deep carbon stores matter, we must first appreciate the difference between the active and inactive (or slow) components of the carbon cycle. Most introductory treatments of the carbon cycle focus on the fast cycle — the rapid exchanges between atmosphere, biosphere, and surface ocean that operate on timescales of days to centuries. But Earth also operates a slow cycle, in which carbon is locked away in geological formations for millions of years, effectively removed from biological and atmospheric circulation.
It is this slow cycle — the lithospheric carbon store — that humans have dramatically short-circuited through fossil fuel combustion, and it is this same slow cycle that climate change now threatens to destabilise through permafrost thaw and clathrate release. The distinction between fast