G9: Climate Change
The evidence for climate change, natural causes (orbital changes, volcanic activity, solar output) and human causes (fossil fuels, deforestation, agriculture).
📋 Climate Change vs. Global Warming
Climate change refers to long-term changes in average weather patterns (temperature, rainfall, wind) across the whole planet or a region. Global warming specifically refers to the increase in Earth's average surface temperature. Global warming is one aspect of climate change — other aspects include changing rainfall patterns, more extreme weather events, rising sea levels and shifting seasons.
The Earth's climate has changed naturally throughout its history — there have been ice ages and warmer periods. However, the current period of climate change is different because it is happening much faster and is largely caused by human activities.
📊 Evidence for Climate Change
Historical Records (since ~1850)
- Global temperature records — show that the average global temperature has risen by approximately 1.1°C since the pre-industrial period (1850–1900)
- The 10 warmest years on record have all occurred since 2005
- 2023 was the warmest year on record globally
- UK temperature records — the 10 warmest UK years have all occurred since 2002
Ice Cores
Ice cores are cylinders of ice drilled from ice sheets in Antarctica and Greenland. They contain trapped air bubbles that preserve samples of the ancient atmosphere. By analysing these bubbles, scientists can determine past CO₂ concentrations and temperatures going back over 800,000 years. Ice cores show that current CO₂ levels (over 420 ppm) are higher than at any point in the last 800,000 years, and that they have risen extremely rapidly since the Industrial Revolution.
Other Evidence
- Glacial retreat — glaciers worldwide are shrinking. The Mer de Glace in the French Alps has retreated over 2 km since 1850
- Sea level rise — global sea level has risen by about 20 cm since 1900, and the rate is accelerating
- Arctic sea ice decline — Arctic sea ice extent has decreased by approximately 13% per decade since satellite records began in 1979
- Seasonal changes — spring events (leaf unfolding, bird migration) are happening earlier
- Temperature proxies — tree rings, ocean sediments and pollen records confirm the warming trend
- Satellite data — since 1979, satellites have measured warming of the lower atmosphere, declining ice sheets and rising sea levels
Example: The Keeling Curve
Since 1958, scientists at the Mauna Loa Observatory in Hawaii have continuously measured atmospheric CO₂. The resulting "Keeling Curve" shows CO₂ rising from 315 ppm in 1958 to over 420 ppm today. It also shows a seasonal zigzag pattern — CO₂ drops in summer when Northern Hemisphere plants absorb it through photosynthesis, and rises in winter when decaying vegetation releases it. The overall upward trend is unmistakable.
🌊 Natural Causes of Climate Change
1. Orbital Changes (Milankovitch Cycles)
Milankovitch cycles are long-term changes in the Earth's orbit and tilt that affect how much solar radiation reaches the planet. There are three cycles: (1) Eccentricity — the shape of Earth's orbit varies from circular to elliptical over ~100,000 years; (2) Axial tilt — the angle of Earth's axis varies between 21.5° and 24.5° over ~41,000 years; (3) Precession — the wobble of Earth's axis over ~26,000 years. These cycles explain the timing of past ice ages but are too slow to explain current rapid warming.
2. Volcanic Activity
- Major volcanic eruptions release large volumes of ash and sulfur dioxide into the stratosphere, forming aerosols that reflect sunlight back into space
- This causes a temporary cooling effect lasting 1–3 years — e.g. the 1991 Mount Pinatubo eruption cooled global temperatures by about 0.5°C for two years
- Volcanoes also release CO₂, which causes warming, but human CO₂ emissions are over 60 times greater than all volcanic emissions combined
- Extended periods of intense volcanic activity (e.g. the Deccan Traps 66 million years ago) can cause significant long-term warming through CO₂ release
3. Solar Output Variations
- The Sun's energy output varies over an approximately 11-year sunspot cycle
- During periods of high sunspot activity, solar output increases slightly, and during low activity it decreases
- The Maunder Minimum (1645–1715) was a period of very low sunspot activity that coincided with the "Little Ice Age" in Europe
- However, satellite measurements since 1979 show that solar output has NOT increased enough to explain the rapid warming of recent decades — the warming trend continues even during periods of declining solar activity
| Natural Cause | Effect on Climate | Timescale | Can it explain recent warming? |
|---|---|---|---|
| Orbital changes | Alters distribution of solar energy | Tens of thousands of years | No — far too slow |
| Volcanic eruptions | Short-term cooling (aerosols); long-term warming (CO₂) | 1–3 years per eruption | No — temporary effect only |
| Solar output changes | Varies solar energy reaching Earth | 11-year cycle | No — output has not increased enough |
🏭 Human Causes of Climate Change
The greenhouse effect: Greenhouse gases (CO₂, methane, water vapour, nitrous oxide) in the atmosphere trap heat that would otherwise escape into space. This natural process keeps Earth about 33°C warmer than it would be without an atmosphere. However, human activities are increasing the concentration of greenhouse gases, enhancing the greenhouse effect and causing additional warming.
1. Burning Fossil Fuels
- The single largest contributor to increased greenhouse gas emissions
- Burning coal, oil and gas for energy and transport releases CO₂ that was stored underground for millions of years
- Fossil fuels account for approximately 75% of global CO₂ emissions
- Global CO₂ emissions from fossil fuels have risen from ~2 billion tonnes per year in 1900 to over 36 billion tonnes per year today
- The main sources are: electricity generation (coal and gas power stations), transport (petrol and diesel vehicles), industry (manufacturing processes), and heating (domestic and commercial)
2. Deforestation
- Trees absorb CO₂ through photosynthesis — they are carbon sinks. When forests are cut down, this stored carbon is released
- If trees are burned (slash and burn), the carbon is released immediately as CO₂
- If trees decay, the carbon is released more slowly
- Deforestation also reduces the future capacity of the forest to absorb CO₂
- Tropical deforestation accounts for approximately 10% of global CO₂ emissions
- About 18 million acres of forest are lost each year — equivalent to 27 football pitches per minute
3. Agriculture
- Methane (CH₄) — livestock (especially cattle) produce methane through digestion (enteric fermentation). Paddy rice farming also releases methane. Methane is 80 times more potent as a greenhouse gas than CO₂ over a 20-year period
- Nitrous oxide (N₂O) — synthetic fertilisers release nitrous oxide, which is nearly 300 times more potent than CO₂
- Agriculture accounts for approximately 10–12% of global greenhouse gas emissions
- Land use change for agriculture (clearing forest for pasture or cropland) adds to emissions
Other Human Causes
- Cement production — chemical process of making cement releases CO₂ (about 8% of global emissions)
- Waste disposal — landfill sites produce methane as organic waste decomposes
- Refrigerants and industrial gases — some synthetic gases (e.g. HFCs) are extremely potent greenhouse gases
| Greenhouse Gas | Source | Global Warming Potential (vs CO₂) | % of Emissions |
|---|---|---|---|
| Carbon dioxide (CO₂) | Fossil fuels, deforestation, cement | 1 (reference) | ~76% |
| Methane (CH₄) | Livestock, rice, landfill, fossil fuel extraction | 80× (over 20 years) | ~16% |
| Nitrous oxide (N₂O) | Fertilisers, industrial processes | ~300× | ~6% |
| Fluorinated gases | Refrigeration, industry | Up to 23,000× | ~2% |
⚖️ Natural vs. Human Causes: The Balance of Evidence
Scientific consensus: The Intergovernmental Panel on Climate Change (IPCC) states that it is "unequivocal" that human influence has warmed the atmosphere, ocean and land. The IPCC's Sixth Assessment Report (2021) concluded that human activities have caused approximately 1.1°C of global warming above pre-industrial levels, with the warming likely reaching 1.5°C between 2030 and 2052.
Evidence that current warming is human-caused:
- The rate of warming is far faster than any natural climate change in the past 10,000 years
- The pattern of warming (troposphere warming, stratosphere cooling) matches the enhanced greenhouse effect, not natural factors like solar changes (which would warm both layers)
- Carbon isotope analysis of atmospheric CO₂ shows that the increase comes from fossil fuels (carbon-12 signature), not natural sources
- Natural factors alone (solar, volcanic, orbital) cannot explain the observed warming — climate models only match observations when human greenhouse gas emissions are included
❓ Practice Questions
Q1: Describe three pieces of evidence that show climate change is happening. (3 marks)
Q2: Explain how ice cores provide evidence of past climate change. (3 marks)
Q3: Describe three natural causes of climate change. Explain why none of them can account for the current rapid warming. (6 marks)
Q4: Explain how human activities are increasing the greenhouse effect. Refer to specific greenhouse gases and their sources. (6 marks)
Q5: "Current climate change is entirely natural." To what extent do you agree with this statement? (6 marks)
Q6: Explain why deforestation contributes to climate change. (4 marks)
✅ Answers
- Three pieces of evidence: (1) Global temperature records show an increase of approximately 1.1°C since pre-industrial times, with the 10 warmest years all since 2005; (2) Glacial retreat — glaciers worldwide are shrinking, e.g. Mer de Glace in the Alps has retreated over 2 km since 1850; (3) Arctic sea ice decline — extent has decreased by 13% per decade since 1979. Also: sea level rise (20 cm since 1900), ice core data, and seasonal changes.
- Ice cores are drilled from ice sheets (Antarctica and Greenland) and contain trapped air bubbles that preserve ancient atmosphere samples. Scientists analyse the CO₂ concentration in these bubbles and the ratio of oxygen isotopes (which indicate temperature). This allows reconstruction of past temperatures and CO₂ levels going back over 800,000 years. Ice cores show that current CO₂ levels (over 420 ppm) are higher than at any point in the last 800,000 years, and the recent rate of increase is unprecedented.
- Three natural causes: (1) Orbital changes (Milankovitch cycles) — variations in Earth's orbit, tilt and wobble alter solar energy distribution over tens of thousands of years. These explain past ice ages but are far too slow to explain current rapid warming. (2) Volcanic activity — eruptions release aerosols (cooling) and CO₂ (warming). The cooling effect is temporary (1–3 years per eruption) and human CO₂ emissions are 60× greater than all volcanic emissions. (3) Solar output variations — the 11-year sunspot cycle causes small changes in solar energy. However, satellite data since 1979 shows solar output has NOT increased enough to explain recent warming. The warming trend continues even during periods of declining solar activity.
- Human activities enhance the greenhouse effect by increasing the concentration of greenhouse gases in the atmosphere: (1) Burning fossil fuels (coal, oil, gas) for electricity, transport and industry releases CO₂ — accounting for 75% of global CO₂ emissions (over 36 billion tonnes/year). (2) Deforestation reduces the number of trees absorbing CO₂ through photosynthesis, and burning/decaying trees release stored carbon. (3) Agriculture produces methane from livestock digestion and rice paddies (methane is 80× more potent than CO₂), and nitrous oxide from synthetic fertilisers (300× more potent than CO₂). These increased greenhouse gas concentrations trap more heat in the atmosphere, enhancing the natural greenhouse effect and causing global temperatures to rise.
- I strongly disagree. The scientific consensus (IPCC) is unequivocal that human influence has caused approximately 1.1°C of warming. Natural causes cannot explain the current warming: orbital changes are too slow; volcanic eruptions cause temporary cooling; and solar output has not increased. In contrast, human activities are increasing greenhouse gas concentrations at an unprecedented rate — CO₂ has risen from 280 ppm (pre-industrial) to over 420 ppm. The pattern of warming (troposphere warming, stratosphere cooling) matches the enhanced greenhouse effect, not natural solar changes. Climate models only match observed warming when human emissions are included. However, natural variability does contribute to short-term fluctuations — e.g. El Niño years are warmer than La Niña years. But the long-term warming trend is overwhelmingly human-caused.
- Deforestation contributes to climate change because: (1) Trees are carbon sinks — they absorb CO₂ through photosynthesis, removing it from the atmosphere. Cutting down trees removes this carbon-absorbing capacity. (2) When trees are burned (slash and burn) or left to decay, the carbon stored in their wood and leaves is released back into the atmosphere as CO₂. (3) Deforestation reduces the future ability of the forest to absorb CO₂ — a cleared forest area cannot absorb carbon in future years. (4) Tropical deforestation accounts for approximately 10% of global CO₂ emissions. (5) Deforestation can also change the albedo (reflectivity) of the land surface, affecting local and regional climate patterns.
🎯 Exam Tips
- Always distinguish between natural and human causes when discussing climate change
- Know the three main human causes: fossil fuels, deforestation and agriculture
- Use specific data — 1.1°C warming, 420 ppm CO₂, 36 billion tonnes/year emissions
- For "to what extent" questions, consider both sides but reach a clear, evidence-based conclusion
- Understand the greenhouse effect mechanism — it's a common 4-mark explain question
- Remember that methane and nitrous oxide are more potent than CO₂ — mention their global warming potential
📝 Exam Technique
Climate Change Exam Tips:
1. When discussing evidence, always use specific data: CO₂ has risen from 280 ppm (pre-industrial) to over 420 ppm; global temperature has increased by 1.1°C; the Keeling Curve shows continuous rise since 1958. Vague statements like 'CO₂ has increased' score poorly.
2. For natural vs human causes questions, cover three natural causes (orbital changes, volcanic activity, solar output) then explain why NONE can account for current rapid warming — this shows AO3 evaluation.
3. Know the greenhouse effect mechanism step-by-step: solar radiation reaches Earth → surface warms → Earth radiates infrared → greenhouse gases absorb and re-radiate heat → atmosphere warms. Don't confuse this with the ozone hole (a common error).
4. For 'to what extent' questions on causes, always conclude firmly: 'Human causes are overwhelmingly dominant in the current era' — the IPCC's word 'unequivocal' is worth quoting.
1. When discussing evidence, always use specific data: CO₂ has risen from 280 ppm (pre-industrial) to over 420 ppm; global temperature has increased by 1.1°C; the Keeling Curve shows continuous rise since 1958. Vague statements like 'CO₂ has increased' score poorly.
2. For natural vs human causes questions, cover three natural causes (orbital changes, volcanic activity, solar output) then explain why NONE can account for current rapid warming — this shows AO3 evaluation.
3. Know the greenhouse effect mechanism step-by-step: solar radiation reaches Earth → surface warms → Earth radiates infrared → greenhouse gases absorb and re-radiate heat → atmosphere warms. Don't confuse this with the ozone hole (a common error).
4. For 'to what extent' questions on causes, always conclude firmly: 'Human causes are overwhelmingly dominant in the current era' — the IPCC's word 'unequivocal' is worth quoting.
⚠️ Common Errors
Watch Out!
Students often think The greenhouse effect and the ozone layer are the same thing. Wrong: The greenhouse effect and the ozone layer are the same thing Correct: The greenhouse effect is caused by greenhouse gases (CO₂, methane, water vapour) trapping infrared heat in the lower atmosphere, warming the planet. The ozone layer is a separate issue — it blocks harmful UV radiation in the stratosphere. CFCs damage the ozone layer; CO₂ drives the greenhouse effect. They are different processes in different atmospheric layers.
Students often think Because climate has changed naturally before, current change must also be natural. Wrong: Because climate has changed naturally before, current change must also be natural Correct: The RATE of current warming is unprecedented — approximately 1.1°C in just 150 years, compared with natural changes that occurred over thousands or millions of years. Ice cores show current CO₂ levels (420+ ppm) are higher than at any point in 800,000 years, and carbon isotope analysis confirms the increase comes from fossil fuels, not natural sources.
Students often think Volcanic eruptions are a major cause of current global warming. Wrong: Volcanic eruptions are a major cause of current global warming Correct: Individual volcanic eruptions actually cause short-term COOLING by releasing aerosols that reflect sunlight (e.g. Mount Pinatubo 1991 cooled global temperatures by 0.5°C for two years). While volcanoes release CO₂, human emissions are over 60 times greater than all volcanic emissions combined. Volcanoes are not a significant driver of current warming.
✍️ Model Answer
Full-Mark Response
9 marks: Compare the relative importance of natural and human causes of climate change. Use specific evidence to support your answer.
Climate change has both natural and human causes, but their relative importance differs dramatically for the current period of warming. Natural causes operate over different timescales but none can explain recent rapid warming. Orbital changes (Milankovitch cycles) alter Earth's distance and angle to the Sun over 10,000–100,000 year cycles, explaining the timing of past ice ages — but they are far too slow to account for the 1.1°C warming that has occurred in just 150 years. Volcanic eruptions release ash and sulfur dioxide aerosols that reflect sunlight, causing short-term global cooling — the 1991 Mount Pinatubo eruption cooled global temperatures by approximately 0.5°C for two years. While volcanoes also release CO₂, human CO₂ emissions are over 60 times greater than all volcanic emissions combined, so volcanic CO₂ is insignificant. Solar output varies on an approximately 11-year sunspot cycle, and the Maunder Minimum (1645–1715) of low solar activity coincided with the Little Ice Age. However, satellite measurements since 1979 show that solar output has NOT increased enough to explain recent warming, and the warming trend continues even during periods of declining solar activity. In contrast, human causes are overwhelmingly dominant. Burning fossil fuels since the Industrial Revolution has increased atmospheric CO₂ from 280 ppm to over 420 ppm — a 50% increase. The Keeling Curve shows this relentless rise since 1958, with the seasonal zigzag pattern confirming the role of photosynthesis. Fossil fuels account for 75% of global CO₂ emissions (over 36 billion tonnes per year). Deforestation releases stored carbon and removes carbon sinks, contributing approximately 10% of global CO₂ emissions. Agriculture produces methane from cattle and rice paddies (80 times more potent than CO₂ over 20 years) and nitrous oxide from fertilisers (nearly 300 times more potent). The evidence that humans are the dominant cause is conclusive: the pattern of warming (troposphere warming, stratosphere cooling) matches the enhanced greenhouse effect, not natural solar changes; carbon isotope analysis shows the additional CO₂ comes from fossil fuels (carbon-12 signature); and climate models only match observed warming when human emissions are included. The IPCC's Sixth Assessment Report (2021) states it is 'unequivocal' that human influence has warmed the atmosphere. Therefore, while natural causes explain long-term climate cycles over geological time, human causes are responsible for the unprecedented rapid warming of the last 150 years.
Mark scheme: 3 marks for evaluating natural causes with specific evidence showing why each is insufficient, 3 marks for explaining human causes with quantitative data, 3 marks for sustained judgement with IPCC evidence and scientific reasoning
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge): Know the three natural causes (Milankovitch cycles, volcanic activity, solar output), the three main human causes (fossil fuels 75%, deforestation 10%, agriculture 10–12%), and key evidence (ice cores, Keeling Curve, temperature records, glacial retreat, sea level rise). AO2 (Understanding): Explain the greenhouse effect mechanism and distinguish between the enhanced greenhouse effect (human-caused) and the natural greenhouse effect. Understand different timescales of natural vs human causes. AO3 (Analysis/Evaluation): EVALUATE the relative importance of natural vs human causes — this demands a supported judgement using quantitative data. Grade 9 answers quote the IPCC ('unequivocal'), use carbon isotope evidence, and explain the pattern of troposphere warming/stratosphere cooling as proof of human causation.
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