HL only: Subtopic 6.2: Climate change: causes and impacts. Global climate models predict future climate scenarios by representing Earth’s climate system mathematically, changing inputs such as greenhouse gas emissions, and calculating the resulting long-term climate outcomes. They produce conditional projections, not one certain forecast.
How the Models Work
The atmosphere, oceans, land, ice and biosphere are divided into three-dimensional grid cells. Mathematical equations represent processes within and between cells, including atmospheric circulation, ocean heat transfer, radiation, the carbon cycle and feedback mechanisms.
The model repeatedly calculates how conditions change over time:
- Scientists enter initial climate conditions and external influences, called forcings, such as greenhouse gas concentrations, aerosols, volcanic activity and solar radiation.
- The model calculates interactions and transfers of energy and matter between components.
- Scientists run the model under different emissions scenarios, each based on assumptions about future population, energy use, technology and policy.
- The outputs show possible changes in temperature, precipitation, sea level and other climate variables.
| Model stage | Purpose |
|---|---|
| Scenario selection | Establishes possible future greenhouse gas emissions and concentrations |
| Simulation | Calculates interactions, including ice-albedo and water-vapour feedbacks |
| Multiple model runs | Produces a range of outcomes and indicates uncertainty |
| Hindcasting | Runs the model for a past period and compares its output with observed climate data |
Hindcasting tests model validity: a model that reproduces historical patterns more accurately gives greater confidence in its projections. However, uncertainty remains because future human choices are unknown, grid cells simplify local processes, and some feedbacks are difficult to represent.
A common misconception is that climate models predict exact future weather. They instead project long-term statistical patterns under stated assumptions. These projections inform mitigation and adaptation decisions, although unequal vulnerability means the same projected change can affect societies differently.
Exam Technique
For an IB explain question, link inputs, modelled interactions and outputs in a clear causal sequence. For evaluate, include strengths such as hindcasting and multiple runs, limitations such as scenario uncertainty and simplification, and a supported judgement about reliability.