C31: Greenhouse Gases and Climate Change
Greenhouse gases and the greenhouse effect, human activities increasing greenhouse gas concentrations, evidence for climate change, carbon footprints and strategies to reduce environmental impact.
The Greenhouse Effect
The greenhouse effect is a natural process that keeps the Earth warm enough to support life. Without it, the average temperature of the Earth would be about −18 °C instead of the current average of about 15 °C.
How the greenhouse effect works:
- Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface.
- The Earth's surface warms up and radiates longer-wavelength infrared radiation back out.
- Greenhouse gases in the atmosphere absorb some of this infrared radiation and re-radiate it in all directions, including back towards the Earth's surface.
- This trapped radiation warms the atmosphere and the Earth's surface, maintaining a temperature that supports life.
Greenhouse Gases
The main greenhouse gases in the atmosphere are:
- Carbon dioxide (CO₂) — the most significant greenhouse gas in terms of human contribution to climate change.
- Methane (CH₄) — a much more potent greenhouse gas than CO₂ (about 80 times more effective at trapping heat over 20 years), but present in lower concentrations.
- Water vapour (H₂O) — the most abundant greenhouse gas naturally, but human activity has little direct effect on its concentration.
| Greenhouse Gas | Source | Relative Potency | Atmospheric Lifetime |
|---|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels, deforestation, cement production | Moderate (reference level) | Hundreds to thousands of years |
| Methane (CH₄) | Livestock, rice paddies, landfill, natural gas leaks | ~80× CO₂ (over 20 years) | ~12 years |
| Water vapour (H₂O) | Natural evaporation from oceans and water bodies | Moderate | Days to weeks |
The Enhanced Greenhouse Effect
The natural greenhouse effect is essential for life on Earth. However, human activities have increased the concentrations of greenhouse gases in the atmosphere, causing an enhanced greenhouse effect. This means more infrared radiation is trapped, causing the Earth's average temperature to rise — this is called global warming or climate change.
Concentrations of CO₂ and CH₄ have increased significantly since the Industrial Revolution (around 1750) due to human activities. The current CO₂ concentration is over 420 ppm, compared to about 280 ppm before the Industrial Revolution.
Human Activities Increasing Greenhouse Gases
| Human Activity | Greenhouse Gas Released | Explanation |
|---|---|---|
| Burning fossil fuels (coal, oil, natural gas) | CO₂ | Fossil fuels contain carbon that was locked up millions of years ago. Burning releases this stored CO₂ back into the atmosphere. |
| Deforestation | CO₂ | Trees absorb CO₂ through photosynthesis. Cutting down and burning trees releases stored CO₂ and reduces future CO₂ absorption. |
| Agriculture (livestock) | CH₄ | Cattle and sheep produce methane during digestion (enteric fermentation). Manure also releases methane. |
| Rice paddies | CH₄ | Anaerobic conditions in flooded rice fields promote methane-producing bacteria. |
| Landfill waste | CH₄ | Organic waste decomposes anaerobically in landfill sites, producing methane. |
| Extraction of fossil fuels | CH₄ | Methane leaks during the mining and transport of natural gas and from oil wells. |
| Cement production | CO₂ | Calcium carbonate is heated to produce calcium oxide, releasing CO₂ as a by-product. |
Be careful to distinguish between the natural greenhouse effect (which keeps Earth warm and habitable) and the enhanced greenhouse effect (caused by human activities increasing greenhouse gas levels). The exam may test your understanding of this distinction.
Evidence for Climate Change
Scientists use a range of evidence to study climate change:
- Temperature records — direct measurements show that global average temperatures have increased by about 1.1 °C since pre-industrial times.
- Ice cores — air bubbles trapped in Antarctic ice provide samples of ancient atmospheres. Analysis shows that CO₂ and CH₄ concentrations are now higher than at any time in at least 800,000 years.
- Sea level measurements — global sea levels have risen by about 20 cm since 1900 due to thermal expansion of seawater and melting ice sheets.
- Glacier retreat — glaciers around the world are shrinking, providing visual evidence of warming.
- Satellite data — satellites measure changes in ice cover, sea surface temperature and atmospheric composition.
Peer-reviewed evidence and scientific consensus: the overwhelming majority of climate scientists agree that the current warming trend is primarily caused by human activities that increase greenhouse gas concentrations. This conclusion is supported by the Intergovernmental Panel on Climate Change (IPCC).
Effects of Climate Change
Increasing global temperatures are predicted to cause significant changes:
- Rising sea levels — caused by thermal expansion of ocean water and melting of ice sheets and glaciers. This threatens coastal communities and low-lying island nations.
- More extreme weather — including more frequent and intense heatwaves, storms, floods and droughts.
- Changes to ecosystems — species may migrate or face extinction if they cannot adapt to changing conditions. Coral reefs are particularly vulnerable to warming oceans.
- Impact on agriculture — changing rainfall patterns and temperatures affect crop yields. Some regions may benefit, but many will face food insecurity.
- Water shortages — changing precipitation patterns and melting glaciers that supply freshwater to millions of people.
- Spread of disease — warmer temperatures allow disease-carrying organisms (like mosquitoes) to expand their range.
Carbon Footprint
A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, event or activity. It is usually expressed as a mass of CO₂ equivalent (CO₂e).
Actions that increase a carbon footprint:
- Driving petrol or diesel cars
- Flying in aeroplanes
- Heating homes with fossil fuels
- Eating large amounts of meat and dairy
- Buying products with high manufacturing and transport emissions
- Using electricity generated from fossil fuels
Actions that reduce a carbon footprint:
- Walking, cycling or using public transport
- Using renewable energy sources (solar, wind, hydroelectric)
- Improving home insulation and energy efficiency
- Eating less meat and more plant-based foods
- Reducing, reusing and recycling materials
- Offsetting emissions by planting trees
Limiting the Impact of Climate Change
| Strategy | Description | Example |
|---|---|---|
| Renewable energy | Replace fossil fuels with energy sources that do not release CO₂ | Solar panels, wind turbines, hydroelectric power |
| Energy efficiency | Reduce energy demand by using less energy for the same outcome | Insulation, LED lighting, efficient appliances |
| Carbon capture and storage (CCS) | Capture CO₂ from power stations and industry and store it underground | Injecting CO₂ into depleted oil and gas fields |
| Afforestation | Plant trees to absorb CO₂ from the atmosphere through photosynthesis | Reforestation programmes, urban tree planting |
| Alternative fuels | Use fuels that produce fewer or zero greenhouse gas emissions | Hydrogen fuel cells, electric vehicles, biofuels |
| International agreements | Countries agree to reduce emissions collectively | Paris Agreement, net-zero commitments |
| Changes to agriculture | Reduce emissions from farming | Fewer livestock, better manure management, modified rice cultivation |
Challenges in Reducing Greenhouse Gas Emissions
There are significant challenges to reducing emissions:
- Economic reliance on fossil fuels — many industries and economies depend heavily on coal, oil and gas. Transitioning to alternatives requires major investment.
- Increasing energy demand — as populations grow and developing countries industrialise, global energy demand continues to rise.
- Incomplete scientific understanding — there are still uncertainties about exactly how much temperatures will rise and what the full effects will be.
- International cooperation — all countries need to work together, but some are more willing or able to act than others. Developing countries may argue they have a right to economic growth.
- Carbon offset limitations — planting trees helps, but trees take time to grow and may release CO₂ if they die or are burned.
In the exam, you may be asked to evaluate different strategies for reducing greenhouse gas emissions. Consider both the advantages and limitations of each approach. For example, renewable energy reduces CO₂ but requires investment and may have environmental impacts of its own.
Practice Questions
1. Describe how the greenhouse effect warms the Earth's surface.
Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface. The Earth warms up and emits longer-wavelength infrared radiation. Greenhouse gases in the atmosphere absorb some of this infrared radiation and re-radiate it in all directions, including back towards Earth. This trapped radiation warms the atmosphere and surface.
2. Explain the difference between the natural greenhouse effect and the enhanced greenhouse effect.
The natural greenhouse effect is the process by which naturally occurring greenhouse gases (water vapour, CO₂, CH₄) trap infrared radiation and keep the Earth warm enough for life. The enhanced greenhouse effect is the additional warming caused by increased concentrations of greenhouse gases from human activities (burning fossil fuels, deforestation, agriculture), which trap more infrared radiation and raise global temperatures further.
3. Give two human activities that increase methane concentrations in the atmosphere and explain how each produces methane.
Livestock farming: cattle and sheep produce methane during digestion (enteric fermentation). Landfill waste: organic waste decomposes anaerobically in landfill sites, producing methane as bacteria break down the material without oxygen.
4. What is a carbon footprint? State three ways an individual could reduce their carbon footprint.
A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or activity. Three ways to reduce it: use public transport or walk instead of driving; switch to renewable energy sources; reduce meat consumption and eat more plant-based foods.
5. Explain why international cooperation is needed to tackle climate change effectively.
Greenhouse gases mix throughout the atmosphere regardless of where they are emitted, so climate change is a global problem. One country reducing its emissions alone will not solve the problem if others continue to increase theirs. All countries need to work together to make a significant impact, but this is challenging because countries have different economic needs and priorities.
Maths Skills
Interpreting Climate Data and Carbon Footprint Calculations
Climate data is often presented as line graphs showing CO₂ concentration or global temperature over time.
Reading graphs: Identify the trend (increasing, decreasing, or stable). Look for correlation between variables — e.g. as CO₂ concentration increases, does global average temperature also increase?
Carbon footprint calculation: A carbon footprint is the total amount of CO₂ (and other greenhouse gases) emitted by a person, organisation or product, usually measured in tonnes of CO₂ equivalent per year.
Example: A household uses 3,500 kWh of electricity per year. Electricity produces 0.233 kg CO₂ per kWh.
CO₂ from electricity = 3,500 x 0.233 = 815.5 kg = 0.82 tonnes CO₂
If they also drive 10,000 miles (0.29 kg CO₂/mile): CO₂ from driving = 10,000 x 0.29 = 2,900 kg = 2.9 tonnes CO₂
Total carbon footprint = 0.82 + 2.9 = 3.72 tonnes CO₂ per year
Interpreting data tables: When given data for different countries or activities, compare values and calculate percentage differences. For example, if Country A emits 5.2 tonnes CO₂ per capita and Country B emits 0.6 tonnes, Country A emits (5.2 - 0.6)/0.6 x 100 = 767% more per person.
Common Misconceptions
Misconceptions About the Greenhouse Effect
The greenhouse effect is entirely human-made and unnatural.
The natural greenhouse effect is essential — without it, Earth would be around -18 degrees C instead of +15 degrees C. Greenhouse gases like water vapour, CO₂ and methane naturally trap heat. What humans are doing is enhancing the greenhouse effect by adding more greenhouse gases, which traps more heat and causes additional warming.
The ozone hole causes global warming.
The ozone hole and the greenhouse effect are separate problems. The ozone hole is caused by CFCs breaking down ozone in the stratosphere, allowing more UV to reach Earth. The greenhouse effect is caused by greenhouse gases trapping infrared radiation in the troposphere. They are different phenomena, though some gases (like CFCs) contribute to both.
Carbon dioxide is the only greenhouse gas that matters.
While CO₂ is the most significant greenhouse gas from human activity, methane (CH₄) is about 80 times more potent as a greenhouse gas per molecule over 20 years, and water vapour is the most abundant greenhouse gas. All greenhouse gases contribute to warming.
6-Mark Question
Evaluate the evidence for human-induced climate change and discuss strategies to reduce its impact.
Evidence for human-induced climate change includes: a sharp rise in atmospheric CO₂ concentration since the Industrial Revolution (from 280 ppm to over 420 ppm), closely correlating with increased fossil fuel burning; a rise in global average temperature of about 1.1 degrees C since pre-industrial times; melting of glaciers and polar ice; and rising sea levels. Computer models that include human greenhouse gas emissions match observed temperature changes, whereas models with only natural factors do not. Strategies to reduce impact include: reducing fossil fuel use by switching to renewable energy sources (solar, wind, nuclear); increasing energy efficiency in homes, transport and industry; carbon capture and storage to remove CO₂ from power station emissions; planting trees to absorb CO₂ through photosynthesis; reducing methane emissions from agriculture and landfill; and international agreements like the Paris Agreement to set binding emission targets. Limitations include the cost of transitioning to renewable energy, the need for global cooperation, and the fact that some climate change is already locked in due to past emissions.
AO3: Analysis and Evaluation
Evaluating Strategies to Reduce Carbon Footprints
The table below shows the carbon footprint of different energy sources and their costs:
- Coal: 820 g CO₂/kWh, cheap but polluting
- Natural gas: 490 g CO₂/kWh, moderate cost
- Solar: 41 g CO₂/kWh (manufacturing only), high initial cost but free fuel
- Nuclear: 12 g CO₂/kWh, high initial cost, long-term waste issues
Evaluate which energy source a country should invest in to reduce its carbon footprint while maintaining a reliable energy supply. Consider economic, environmental and social factors.
Answer: Coal must be phased out — it has the highest CO₂ emissions and also produces SO₂ and particulates. Natural gas is a transition fuel with half the CO₂ of coal, but is still a fossil fuel and will eventually need replacing. Solar produces very little CO₂ but is intermittent (no power at night) and requires energy storage or backup. Nuclear is the lowest carbon option and provides reliable baseload power, but is expensive to build, produces radioactive waste, and carries public safety concerns. The best strategy is a mix: invest in solar and wind for the majority of generation, use nuclear for reliable baseload, and keep some gas as backup during low-wind/low-sun periods while developing better battery storage. This balances carbon reduction with reliability and cost.
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