Human disruption of the carbon cycle, the climate consequences and tipping points it triggers, the uncertainty in projecting future change, and the mitigation and adaptation responses available.

What this dot point is asking

Edexcel wants you to explain how humans disrupt the carbon cycle, explain the climate consequences and tipping points that follow, explain the uncertainty in projecting future change, and evaluate the mitigation and adaptation responses available.

Human disruption of the carbon cycle

Atmospheric CO2 has risen from a pre-industrial level of about 280 parts per million to over 420 parts per million, the fastest rise in the geological record. Because the oceans and biosphere absorb only part of the surplus, the remainder accumulates in the atmosphere and drives warming.

Climate consequences

Impacts fall unevenly. Low-lying and low-income countries face the worst sea-level and storm impacts with the least capacity to cope, while shifting rainfall threatens agriculture and water supply across the tropics and subtropics, linking back to water insecurity.

Tipping points and uncertainty

The gravest risks are tipping points, thresholds beyond which a system shifts abruptly and often irreversibly through self-reinforcing positive feedback. Key tipping elements include the Greenland and West Antarctic ice sheets (committing the world to metres of sea-level rise), Arctic sea ice (loss lowers albedo and amplifies warming), Amazon dieback (forest to savanna, flipping a sink to a source), permafrost thaw (releasing methane) and thermohaline circulation shutdown.

The future is uncertain because of unknown climate sensitivity (warming per doubling of CO2), the strength of feedbacks, and the path of emissions, captured in scenarios such as the RCPs (representative concentration pathways) and SSPs (shared socio-economic pathways) that span low to high emission futures.

Mitigation and adaptation

Mitigation attacks the cause by cutting emissions: decarbonisation of energy with renewables and nuclear, efficiency, carbon capture and storage (CCS) and afforestation, coordinated through treaties such as the UNFCCC, the Kyoto Protocol and the Paris Agreement 2015, which aims to hold warming well below 2 degrees Celsius. Adaptation manages the consequences: coastal defences, drought-resistant crops, water management and resilient planning. The two are complementary, since mitigation is the only long-term cure while adaptation is unavoidable for warming already locked in, and climate justice governs who should pay, given that the biggest emitters are rarely the worst affected.

Examples in context

Example 1: the Paris Agreement 2015. Nearly every country agreed to hold warming well below 2 degrees Celsius and pursue 1.5 degrees through nationally determined contributions, with five-yearly reviews and finance for developing nations. It marks the leading global mitigation framework, but its voluntary targets and the gap between pledges and action expose the difficulty of cooperative mitigation and the role of climate justice.

Example 2: Bangladesh coastal adaptation. Highly exposed to sea-level rise, storm surges and cyclones in the Ganges-Brahmaputra delta, Bangladesh has invested in embankments, raised cyclone shelters, early-warning systems and salt-tolerant rice. It shows effective, low-cost adaptation in a low-income, high-vulnerability country, while underlining that adaptation has limits if mitigation fails and warming accelerates.

Try this

Q1. Define a climate tipping point. [2 marks]

• Cue. A threshold beyond which part of the climate system shifts abruptly and often irreversibly through positive feedback.

Q2. Explain one difference between mitigation and adaptation. [4 marks]

• Cue. Mitigation cuts emissions to tackle the cause (renewables, CCS); adaptation manages the consequences (coastal defences, resilient crops).