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IB DP ESS — Topic 2: Ecology | SL & HL | Paper 1 & Paper 2
🎯 Learning Intentions By the end of this note, you will be able to:
- Define the carbon cycle and identify the major carbon stores (reservoirs) and fluxes (flows) within it.
- Describe the biological processes of photosynthesis, respiration, decomposition, and combustion as carbon fluxes operating at local and global scales.
- Explain how geological processes — including fossilisation, volcanism, and weathering — contribute to long-term carbon cycling.
- Analyse how human activities, particularly fossil fuel combustion and deforestation, have disrupted the natural carbon cycle and altered atmospheric CO₂ concentrations.
- Evaluate the consequences of carbon cycle disruption for climate systems, ocean chemistry, and biodiversity, using named real-world examples and data.
- Discuss the role of feedback loops — both positive and negative — in amplifying or moderating changes to the carbon cycle.
- Construct a systems diagram of the carbon cycle showing stores, fluxes, and feedback mechanisms.
Real-World Context & Relevance
In May 2023, the Mauna Loa Observatory in Hawaiʻi recorded atmospheric CO₂ concentrations exceeding 424 parts per million (ppm) — the highest level in at least 3.6 million years of Earth's history. This single data point encapsulates one of the most consequential disruptions to a planetary-scale biogeochemical system ever documented. The carbon cycle, which has regulated Earth's climate for hundreds of millions of years, is now being altered faster than at any point in the geological record.
Yet the story of carbon is not simply one of pollution. Carbon is the chemical backbone of all life on Earth. It flows through living organisms, soils, oceans, rocks, and the atmosphere in an intricate web of biological, chemical, and geological processes. Understanding how this cycle functions — and how human activities have disrupted it — is foundational to understanding climate change, ocean acidification, biodiversity loss, and the sustainability of human civilisation itself. From the peat bogs of Siberia to the coral reefs of the Indo-Pacific, the consequences of carbon cycle disruption are already visible and measurable.
🌍 Why This Matters The carbon cycle connects atmospheric chemistry, ocean systems, terrestrial ecosystems, and human societies into a single planetary system. Disrupting it — through fossil fuel combustion, deforestation, and land-use change — drives climate change, ocean acidification, and ecosystem collapse. Understanding the carbon cycle is therefore essential for evaluating every major environmental challenge of the 21st century, from climate policy to biodiversity conservation.
Core Concepts & Definitions
Before examining individual processes, it is useful to think about the carbon cycle as a system of stores and flows. Carbon does not disappear — it moves between different reservoirs, spending varying amounts of time in each. Some reservoirs hold carbon for seconds (a leaf during photosynthesis); others hold it for millions of years (limestone rock). The rate at which carbon moves between stores — the flux — determines whether a reservoir is growing or shrinking at any given time.
📖 Key Definition — Carbon Store (Reservoir) A carbon store (also called a carbon reservoir or carbon pool) is any part of the Earth system that holds carbon for a period of time.