Unit 4: Earth's Resources, Change and Human Impact
Resources, cycles and change over time
Unit 4 shifts from Earth processes to how those processes supply resources and how human activity alters them. Candidates must understand the formation and distribution of non-renewable resources (fossil fuels, mineral ores, groundwater), renewable resources and their limits, and the biogeochemical cycles - carbon, nitrogen, phosphorus and water - that govern availability and environmental impact. You should be able to analyse data on resource extraction, emissions and land use, and to evaluate management strategies against criteria such as sustainability, cost and equity.
Traps that appear in questions
- Calling a resource renewable without qualification. Groundwater in a slowly recharged aquifer is effectively non-renewable at extraction timescales; the key is the rate of replenishment relative to use.
- Confusing a reservoir with a flux. A reservoir stores carbon (ocean, atmosphere, biomass); a flux moves it (respiration, combustion, dissolution). Questions often ask which has changed, not how much is stored.
- Treating correlation as causation. Two rising graphs do not prove one causes the other; look for a mechanism and for data that could falsify the claim.
- Ignoring spatial and temporal scale. Local impacts differ from global ones, and short-term trends may not reflect long-term change.
- Overstating mitigation. Evaluate strategies on evidence, including limitations and unintended consequences.
Worked example: a rule of thumb for resource questions
A question gives extraction rates and recharge rates for an aquifer. Compare them directly: if extraction exceeds recharge, storage falls and the resource is being mined even if recharge occurs. Express the deficit as a percentage of recharge to quantify pressure, and state the assumption (constant rates, no recycling) explicitly. This habit - compare rates, quantify, state assumptions - earns marks across resource and impact questions.