Causes of Climate Change
FoundationWeather is short-term atmospheric condition while climate is the long-term average; rising global temperatures, melting glaciers and sea level rise show the climate is changing. Human activities such as burning fossil fuels and felling forests strengthen the greenhouse effect, while volcanoes and solar activity provide natural causes — the heart of the 'fluctuation or irreversible trend' debate.
- Weather is short, climate is long: judge climate change by decades of trend, never single events
- Five pieces of evidence: rising global mean temperature, heatwaves, sea level rise, melting glaciers, more extreme weather
- Greenhouse mechanism: shortwave in, longwave out of the surface, gases absorb and radiate back — natural and vital, but the strengthened version causes warming
文體小知識：自然系統說明文——講解天氣與氣候的概念、氣候轉變的證據，以及溫室效應的機制與人為和自然成因。
§Text Passage
1Weather is the atmospheric condition of a place over a short period — a day's temperature, rainfall and wind; climate is the average weather pattern over a long time, usually thirty years. The two must not be confused: one cold day does not mean the climate is not warming. Judging climate change means looking at long-term trends, just as gauging sea level means reading multi-year mean water levels rather than a single wave.
2Several lines of evidence show the climate is changing: over recent decades the global mean temperature has risen sharply, about 1.1 degrees Celsius above pre-industrial levels; heatwaves and abnormally warm weather have grown more frequent; glaciers are retreating and melting; sea level rise floods low-lying coasts; and extreme weather events are multiplying. Long-term observations of the Hong Kong Observatory likewise show rising local temperatures and more variable rainfall.
3The greenhouse effect is the atmosphere's natural warming mechanism: shortwave radiation from the sun passes through the atmosphere and warms the surface, which then emits longwave radiation; greenhouse gases such as water vapour and carbon dioxide absorb part of this longwave radiation and radiate heat back, keeping the surface warm. Without the natural greenhouse effect the Earth would be far colder; the problem is that human activities add greenhouse gases and strengthen the effect.
4Human activities strengthen the greenhouse effect in five main ways: burning fossil fuels such as coal, oil and natural gas releases large amounts of carbon dioxide; felling forests reduces the plants that absorb it; incinerating waste emits greenhouse gases; refrigerants and aerosols release chlorofluorocarbons; and agriculture — rice cultivation and livestock rearing — releases methane. Added together, these keep raising the concentration of greenhouse gases in the atmosphere.
5Climate change also has natural causes: volcanic ash from eruptions reflects sunlight and can cool the planet briefly; solar output varies in cycles; and long-term changes in the Earth's orbit and the tilt of its axis alter the solar radiation the surface receives. Those who argue for 'long-term fluctuation' note that climate has always swung between warm and cold periods; opponents reply that recent warming is far faster than natural variability and closely matches emissions since the Industrial Revolution, pointing to human dominance. This is precisely the core of the unit's driving question: long-term fluctuation, or an irreversible trend?
1Summary 重點摘要
Weather is short-term atmospheric condition while climate is the long-term average; rising global temperatures, melting glaciers and sea level rise show the climate is changing. Human activities such as burning fossil fuels and felling forests strengthen the greenhouse effect, while volcanoes and solar activity provide natural causes — the heart of the 'fluctuation or irreversible trend' debate.
2Vocabulary cards Vocabulary
The atmospheric condition of a place over a short period — temperature, rainfall and wind — changing quickly and widely.
The average weather pattern over a long period, usually thirty years, reflecting a place's long-term atmospheric character.
The sustained rise in global mean temperature, mainly linked to the human strengthening of the greenhouse effect.
The natural warming mechanism by which greenhouse gases absorb longwave radiation from the surface and radiate heat back; human activity has made it excessive.
Gases that absorb longwave radiation, including carbon dioxide, methane, water vapour and chlorofluorocarbons.
High-energy radiation from the sun, most of which passes through the atmosphere to reach the surface.
Low-energy radiation emitted by the warmed surface, absorbed by greenhouse gases and partly radiated back.
Coal, oil and natural gas; burning them releases large amounts of carbon dioxide, the largest human carbon source.
Man-made chemicals once widely used in refrigerants and aerosols; they are greenhouse gases and also damage the ozone layer.
A greenhouse gas that traps heat more strongly than carbon dioxide, mainly from rice paddies, livestock digestion and decomposing waste.
3Step by step Steps
- Step 1: Weather versus climateWeather is short-term atmospheric condition — a day's temperature, rainfall, humidity and wind — shifting within hours; climate is the average pattern over a long period, usually thirty years, and far more stable. For example, a December cold surge pushing urban Hong Kong down to 10 degrees Celsius is a weather event, while Hong Kong's mild subtropical winter is climate — the two do not contradict. Writing both definitions first secures the basic marks in explanation questions; judging climate change requires long-term observations such as climographs and decades of temperature records, never a single cold snap or heatwave — the methodological starting point of the whole unit.
- Step 2: Evidence of a changing climateThe curriculum lists five types of evidence: global mean temperature has risen sharply in recent decades, about 1.1 degrees Celsius above pre-industrial levels; heatwaves and abnormally warm weather have grown more frequent; glaciers are retreating and melting; sea level rise floods low-lying coasts; and extreme weather is multiplying. For example, comparing photos of the same glacier decades apart shows the ice tongue visibly retreating — the most intuitive evidence of warming. In Hong Kong, Observatory records show higher annual mean temperatures, more hot nights and more variable rainfall. List at least three items of evidence and explain how each points to the same conclusion.
- Step 3: The mechanism of the greenhouse effectThe mechanism has four stages: first, shortwave radiation from the sun passes through the atmosphere and mostly reaches the surface; second, the surface absorbs it, warms up and emits longwave radiation; third, greenhouse gases such as water vapour, carbon dioxide and methane absorb part of the longwave radiation; fourth, the gases radiate the heat back, keeping the surface and lower atmosphere warm. For example, a car parked in the summer sun with windows shut lets sunlight in but keeps heat from escaping, so the cabin keeps warming — a good analogy. The key: greenhouse gases intercept longwave radiation from the surface, not shortwave from the sun; the natural effect makes Earth habitable, and it is the strengthening that causes trouble.
- Step 4: Human causes (1): fossil fuels and deforestationBurning coal, oil and natural gas is the largest source of carbon dioxide: power generation, industry and transport each year release into the atmosphere carbon stored underground for hundreds of millions of years. Deforestation attacks the carbon balance from the other end: plants absorb carbon dioxide by photosynthesis, so felling and burning forests both lowers that uptake and releases the stored carbon directly. For example, the sustained rise in atmospheric carbon dioxide since the Industrial Revolution closely tracks the curve of fossil fuel use — among the strongest evidence for the human cause of global warming. Last lesson's rainforest felling is the concrete case on this causal chain; the two units interlock here.
- Step 5: Human causes (2): waste, CFCs and agricultureThree further human sources must be memorised precisely. Incinerating waste burns rubbish into carbon dioxide and other greenhouse gases, while waste decomposing in landfills releases methane. Chlorofluorocarbons: man-made chemicals once widely used as refrigerants in fridges and air conditioners and in aerosols; they trap heat strongly and also destroy the ozone layer, and although now controlled, the stock already in the atmosphere still acts. Agriculture: flooded rice paddies let microbes release methane as they decompose organic matter, and the digestion of cattle and sheep emits large amounts of methane. For example, equal masses of methane trap far more heat than carbon dioxide over a century, so even smaller sources matter. In answers, match each activity to its gas and link both back to the greenhouse effect.
- Step 6: Natural causesClimate has always been driven by natural factors, mainly three. Volcanic activity: eruptions throw ash and suspended particles into the air, reflecting sunlight and cooling the surface for months to a couple of years. Solar activity: the sun's output varies in roughly eleven-year cycles with small long-term fluctuations. The Earth's orbit and axis: long-term changes in orbital shape and in the tilt and direction of the axis alter the solar radiation different regions receive, linked to the alternation of ice ages. For example, after the great eruption of Mount Pinatubo in the Philippines in 1991, global mean temperature fell visibly for a year or two — a fine demonstration of natural cooling. Natural causes are real, which both sides of the next debate must acknowledge.
- Step 7: Fluctuation or irreversible trendThis is the unit's driving question, and both sides must be writable. Those arguing for 'long-term fluctuation' note that climate has always swung between cold and warm periods, from ice ages to warm epochs; volcanoes, solar cycles and orbital changes still operate, so today's warming may be one more natural swing. Those arguing for an 'irreversible trend' reply that recent decades' warming is far faster than natural variability and closely matches emissions since the Industrial Revolution; the physics of the greenhouse effect is settled science; even if part of the warming is natural, the human strengthening is the dominant cause, and accumulated greenhouse gases cannot disperse quickly. For example, the battleground is the match between speed and causes, not the bare fact that climate fluctuates. In evaluation answers, weigh the reliability of each side's evidence before taking a stance.
4Diagrams Diagrams
5Worked example Example
Data-response practice: the data show that global mean temperature has kept rising in recent decades. Explain the mechanism of the greenhouse effect, and describe how human activities strengthen it, causing global warming.
- Read the question: two tasks — first the natural mechanism, then the human strengthening; outline both before writing
- Mechanism marks: solar shortwave radiation passes through the atmosphere and warms the surface, which emits longwave radiation; greenhouse gases absorb part of it and radiate heat back, keeping the surface warm
- Human strengthening (1): burning fossil fuels emits huge amounts of carbon dioxide while felling forests reduces the plants that absorb it — emissions rise as uptake falls, so concentrations climb twice over
- Human strengthening (2): incineration and landfills release carbon dioxide and methane, refrigerants and aerosols release chlorofluorocarbons, and rice cultivation plus livestock emit methane — match each activity to its gas
- Conclusion: higher greenhouse gas concentrations absorb more longwave radiation, the warming effect grows excessive and global mean temperature rises — close by answering 'causing global warming' directly
- Answer: The greenhouse effect is the atmosphere's natural warming mechanism: most shortwave solar radiation passes through the atmosphere and warms the surface, which then emits longwave radiation; greenhouse gases such as water vapour and carbon dioxide absorb part of it and radiate heat back, keeping the surface warm. Human activities strengthen this at both ends. On the source side, burning coal, oil and natural gas releases vast amounts of carbon dioxide; incinerating waste and decomposing landfill matter emit greenhouse gases; refrigerants and aerosols release chlorofluorocarbons; and rice cultivation and livestock digestion release methane. On the sink side, deforestation removes the plants that absorb carbon dioxide. With sources rising and sinks shrinking, atmospheric greenhouse gas concentrations keep climbing, more longwave radiation is absorbed, the warming effect becomes excessive, and global mean temperature rises — global warming.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the difference between weather and climate?
Q2List three pieces of evidence that the climate is changing.
Q3What is the mechanism of the greenhouse effect?
Q4How do human activities strengthen the greenhouse effect? Give three activities.
Q5Give one argument for and one against 'global warming is just long-term fluctuation'.
8Recap Recap
- Weather is short, climate is long: judge climate change by decades of trend, never single events
- Five pieces of evidence: rising global mean temperature, heatwaves, sea level rise, melting glaciers, more extreme weather
- Greenhouse mechanism: shortwave in, longwave out of the surface, gases absorb and radiate back — natural and vital, but the strengthened version causes warming
- Five human causes: fossil fuels, deforestation, waste incineration, chlorofluorocarbons, agricultural methane
- Natural causes: volcanic cooling, solar cycles, orbital changes; the match between warming speed and emissions is the key argument for human dominance