The causes of climate change
FoundationThe greenhouse effect is natural: the surface absorbs shortwave sunlight and re-emits longwave heat that greenhouse gases partly retain. Burning coal, oil and gas plus deforestation have lifted CO2 from about 280 ppm past 420 ppm, warming the world about 1.2 degrees Celsius.
- The greenhouse effect is natural; the enhanced effect is the problem
- CO2 up from about 280 ppm to over 420; about 1.2 degrees of warming so far
- Emission sources: coal power, transport, cement and steel, agricultural methane
文體小知識：原理講解課：先弄懂溫室效應的物理原理（短波入、長波留），再分類人類活動如何加強它——記住中一天氣與氣候課的基礎正好用得上。
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1The greenhouse effect itself is a blessing: without it, Earth would average about -18 degrees Celsius and be lifeless. The mechanism: the sun sends energy to the surface as shortwave radiation; the surface re-emits it as longwave infrared heat; greenhouse gases (carbon dioxide, methane, water vapour, nitrous oxide) absorb and re-radiate part of that heat, like glass in a greenhouse, keeping Earth at a liveable 15 degrees or so.
2The trouble is the 'enhanced' part. Since the Industrial Revolution, burning coal, oil and gas has lifted carbon dioxide from about 280 ppm to over 420 ppm today — the highest in 800,000 years — and the global average temperature is now about 1.2 degrees Celsius above pre-industrial times, rising far faster than natural rhythms. Air bubbles trapped in Antarctic ice cores preserve ancient air — hard evidence of this history.
3Human activities fall into groups. Energy and power: coal-fired electricity is the single largest source, and coal emits the most carbon per unit of energy. Transport: vehicles, ships and planes burning fuel make up about a fifth of global emissions. Industry: making cement and steel releases carbon dioxide directly. Agriculture: rice paddies and cattle belch methane, which traps about 28 times more heat than CO2 over a century. And deforestation: felled and burned forests both stop absorbing CO2 and release their stored carbon.
4So the causes can be summarised as more emissions plus less absorption: burning coal and oil adds carbon to the air while cutting forests weakens the sinks. Next lesson: where all this extra heat is pushing the planet.
1Summary 重點摘要
The greenhouse effect is natural: the surface absorbs shortwave sunlight and re-emits longwave heat that greenhouse gases partly retain. Burning coal, oil and gas plus deforestation have lifted CO2 from about 280 ppm past 420 ppm, warming the world about 1.2 degrees Celsius.
2Vocabulary cards Vocabulary
The natural trapping of outgoing longwave heat by greenhouse gases that keeps Earth warm; without it the average would be about -18 degrees Celsius.
The human-made strengthening of the effect as gas concentrations rise — the direct cause of global warming.
CO2, methane and nitrous oxide, which absorb longwave radiation; methane traps about 28 times more heat than CO2 over a century.
Coal, oil and gas, formed from ancient organisms; burning them releases CO2.
The amount of CO2 in the air, measured in ppm (parts per million) — up from about 280 to over 420.
Large-scale clearing of forests, which releases stored carbon and removes trees that absorb CO2.
Systems that absorb and store CO2, such as forests, oceans and soils.
The greenhouse gas CH4, from paddies, cattle and landfills — a powerful heat-trapper.
The long-term rise in global average temperature — about 1.2 degrees Celsius since the Industrial Revolution.
3Step by step Steps
- Para 1: how the greenhouse effect worksThe greenhouse effect is a natural process: shortwave radiation from the sun passes through the atmosphere and warms the surface; the surface re-emits the heat as longwave radiation; and greenhouse gases such as carbon dioxide and water vapour absorb part of it, keeping the lower atmosphere warm. Without this effect Earth would average about minus 18 degrees Celsius, a frozen world; thanks to it, the surface stays at a liveable 15 degrees. Remember: shortwave in, longwave kept.
- Para 2: the enhancement in dataHuman activity has over-amplified this effect. Carbon dioxide has climbed from about 280 ppm before the industrial revolution past 420 ppm today — the highest in 800,000 years — and the world has warmed about 1.2 degrees Celsius. Air bubbles trapped in Antarctic ice cores are samples of ancient air; from them scientists reconstruct hundreds of thousands of years of change, proving today's concentration is unprecedented and rising far faster than natural cycles.
- Para 3: energy and power generationBurning fossil fuels is the largest source of new emissions, and coal-fired power is the most carbon-intensive of all — a single large plant emits millions of tonnes of carbon dioxide a year. Hong Kong illustrates the point: power generation accounts for about 60% of local emissions and is the biggest source, now switching gradually from coal to cleaner natural gas, though net-zero electricity still needs renewables and regional cooperation.
- Para 3: transport and industryTransport contributes about a fifth of global emissions: cars burn petrol, and planes and ships rely on fossil fuels that are hard to replace soon. Industrial processes emit directly too — cement making heats limestone in kilns, releasing large amounts of carbon dioxide, and steelmaking reduces iron ore with coke. This is why clean electricity alone is not enough; technological reform of industrial processes matters just as much.
- Para 3: agriculture and deforestationAgriculture is the main source of methane: flooded rice paddies and belching cattle both release it, and molecule for molecule it warms the planet far more than carbon dioxide. Deforestation strikes twice — Amazon forest cleared for soy fields and pasture releases carbon as it burns and decays, while the planet loses a sink that absorbs carbon dioxide. The world loses about 10 million hectares of forest a year, an area the size of Iceland.
- Para 4: the summary formulaThe causes of climate change fit one formula: rising emissions (burning coal and oil, industrial processes, farming) plus falling absorption (large-scale deforestation). Write answers in two layers: the natural mechanism, which keeps Earth liveable, then the human enhancement, which has raised gas concentrations and trapped excess heat. Both layers are essential — answering either alone loses marks for an incomplete argument.
4Diagrams Diagrams
5Worked example Example
Example: 'The greenhouse effect is bad and should be eliminated.' What is wrong with this claim? Correct it and explain the real causes of climate change.
- Unpack the error: the greenhouse effect is natural, lifting Earth from -18 to a liveable 15 degrees Celsius; removing it would freeze the planet
- Right concept: the problem is the enhanced greenhouse effect — humans have raised gas concentrations so too much heat is kept
- Cause chain: burning fossil fuels (energy, transport, industry) and agricultural methane raise emissions; deforestation cuts absorption
- Evidence: CO2 rose from about 280 ppm past 420, and warming of about 1.2 degrees far outpaces natural cycles
- Answer: Say instead that the enhanced greenhouse effect is the problem: the natural effect keeps Earth liveable, but burning fossil fuels and clearing forests have raised concentrations so much that excess heat is trapped. Tip: present the natural mechanism first, then the human enhancement, backed by ppm and temperature data.
6Common mistakes Mistakes
7Quick check Quick check
Q1How does the greenhouse effect work?
Q2How has atmospheric CO2 changed?
Q3List three human activities that emit greenhouse gases.
Q4Why does methane draw special attention?
Q5How does deforestation both add carbon and shrink sinks?
8Recap Recap
- The greenhouse effect is natural; the enhanced effect is the problem
- CO2 up from about 280 ppm to over 420; about 1.2 degrees of warming so far
- Emission sources: coal power, transport, cement and steel, agricultural methane
- Deforestation deals a double blow: releasing carbon and losing a sink
- Motto: shortwave in, longwave kept; more emissions plus less absorption