13.1 showed what the data reveals. This lesson explains why — separating natural causes (ice-age cycles, volcanoes) from human causes (disrupting the carbon cycle, the enhanced greenhouse effect). The key exam skill is showing why natural causes can't explain the recent, rapid change.
Climate has always changed — so the real question is what is driving the recent, rapid change.
Earth's climate has fluctuated over tens of thousands to millions of years, driven by natural forces. But the rate and magnitude of the change since ~1950 is unlike anything in the natural record — and it coincides exactly with industrialisation.
The biggest natural climate driver — and why it doesn't fit today's warming.
Slow, regular changes in Earth's orbit and axial tilt that alter how much solar radiation reaches the surface, pacing the ice ages over tens of thousands to 100,000+ years.
There have been at least five major ice ages, with glaciation and warming phases set by these orbital cycles. When more solar radiation reaches Earth, glaciation ends and a warming phase begins. These cycles are slow — the current warming is far too fast, and out of step with the orbital timing, to be one of them.
To argue humans are the cause, you must rule out the natural ones. Ice-age cycles are ruled out by timing and speed: today's warming is orders of magnitude faster and doesn't match the orbital schedule.
Volcanoes can shift the climate — but the wrong way, and by too little.
Large eruptions inject sulphur dioxide and ash high into the atmosphere, reflecting sunlight and causing short-term cooling. They also release CO₂ — but far less than humans do.
Pinatubo (Philippines) released about 20 million tonnes of sulphur dioxide, which cooled global temperatures by roughly 0.5 °C for about one to two years. Yet the annual CO₂ released by all the world's volcanoes is far less than human emissions.
Shows volcanoes cause short-term cooling (not warming) and are dwarfed by human CO₂ — so they can't explain sustained warming. Concepts: environment, change, interconnection.
EnvironmentChangeThe Pinatubo cooling is commonly cited as about 0.5 °C (some sources 0.4–0.6 °C). Use “about 0.5 °C” and attribute to a reputable source (e.g. NASA/USGS).
The single biggest driver of the crisis — humans breaking a balanced cycle.
The natural movement of carbon between the atmosphere, land (plants, soil) and oceans. Photosynthesis and ocean uptake normally keep atmospheric CO₂ roughly in balance.
Fossil-fuel burning releases carbon that was locked underground for millions of years, while deforestation removes the plants that would absorb it. The result is a net rise in atmospheric CO₂ — the single biggest cause of current climate change.
More greenhouse gas traps more heat — the mechanism that turns CO₂ into warming.
The greenhouse effect is natural and essential — it keeps Earth warm enough to live on. The problem is the enhanced greenhouse effect: extra greenhouse gases (CO₂, methane, nitrous oxide) trap more outgoing heat, raising surface temperatures.
The single most persuasive piece of evidence for a human cause.
Ice cores let scientists read atmospheric CO₂ back 800,000 years. Across that whole span, CO₂ never rose above about 300 ppm — until the industrial era. It then shot past 300, reached about 410 ppm by 2019, and is now over 420 ppm.
This graph is the strongest single line of evidence: today's CO₂ is outside the entire natural range of the last 800,000 years, and the spike lines up exactly with industrialisation. Natural cycles cannot produce it.
Check you can do these before moving to 13.3 (Impacts).
13.2 established the causes. 13.3 Impacts examines the consequences (environmental and human), then 13.4 Responses looks at challenges, opportunities and responses — with Costa Rica as a case study.
Everything in this chapter traces to a source you can check. Watch the explainer, read the primary sources, follow the news, and practise the geographical skills this chapter uses.