Chapter 13.1 established what the data shows — the world is warming, fast. This lesson answers the harder question: why, and how scientists know the cause is us, not a natural cycle. That distinction — natural vs human forcing — is the backbone of the whole climate-change topic and of every extended response you'll write on it.
By the end you can:
Every explanation of climate change sorts into one of two buckets. Natural forcings have operated for billions of years and work slowly — over thousands to millions of years. Human (anthropogenic) forcings have been added only since the Industrial Revolution, and they act fast — in decades. The scientific skill in this topic is telling the two apart, and then showing why the recent warming can only be the human kind.
Orbital (Milankovitch) cycles, solar output, volcanic eruptions, ocean circulation. Slow — thousands to millions of years.
Carbon-cycle disruption, fossil-fuel burning, deforestation, the enhanced greenhouse effect. Fast — the last ~250 years.
These orbital wobbles — Milankovitch cycles — play out over 10,000 to 100,000+ years, and have driven at least five major ice ages. Three overlapping cycles set the rhythm: eccentricity (the orbit's shape, ~100,000 yrs), obliquity (axial tilt, ~41,000 yrs) and precession (axial wobble, ~23,000 yrs).
But they are ruled out for today's warming for two reasons: the timing is wrong (on this clock Earth should be slowly cooling right now), and they act far too slowly to explain a change measured in decades. A natural cycle cannot turn on a dime — the current spike can.
When Mount Pinatubo erupted in the Philippines in June 1991 it injected about 20 million tonnes of sulfur dioxide into the stratosphere. The haze reflected sunlight and cooled the whole planet by roughly 0.5°C for one to two years.
This is the key point for the exam: large eruptions cause short-term cooling, not warming. And although volcanoes do emit CO₂, all the world's volcanoes together release far less than human fossil-fuel burning. Put in numbers: humans emit roughly 40 billion tonnes of CO₂ a year; every volcano on Earth combined manages barely 0.3–0.4 billion — about 100 times less.
Carbon constantly moves between the atmosphere, plants and the oceans — the land and sea act as carbon sinks that absorb roughly half of what is emitted. Humans unbalance this two ways at once: burning fossil fuels releases carbon locked away for millions of years, while deforestation removes the sinks that would soak it up. The result is a net rise in atmospheric CO₂, tracked live at Mauna Loa (NOAA) — the single biggest cause of modern warming.
The greenhouse effect itself is natural and essential: gases such as water vapour and CO₂ trap outgoing heat and keep Earth about 33°C warmer than it would otherwise be — without it the planet would be frozen. The problem is the human enhancement: extra CO₂, methane and nitrous oxide thicken that heat-trapping blanket, so more energy stays in the system and the surface warms. The IPCC's Sixth Assessment concludes this human enhancement is unequivocally the cause of observed warming.
Air bubbles trapped in Antarctic ice preserve samples of the atmosphere going back 800,000 years. Across that entire record — through every ice-age cycle — CO₂ never once rose above about 300 ppm. Today it is over 420 ppm and climbing, tracked live on the Keeling Curve. On a geological timescale the rise is near-vertical, and it lines up precisely with industrialisation.
Before you write, watch this short overview from National Geographic. It ties the whole causal chain together — greenhouse gases, the human activities that release them, and the consequences that follow — in one three-minute narrative. Treat it as a model of how to link cause to effect in a paragraph.
▶▶ Watch: Causes and Effects of Climate Change — National Geographic (click → opens on YouTube)As you watch, note: (1) which greenhouse gases are named; (2) which human activities are identified as sources; (3) how each cause connects to a real-world effect.
Every claim in this lesson traces back to a primary source — a dataset, a measurement record or an assessment report. Use these to check the data yourself and to build a research task. Being able to name and cite a source is exactly what lifts a Band 5–6 response above a Band 3–4 one.