Greenhouse effect and climate changeIB MYP Chemistry: Revision notes
Section 1
The greenhouse effect
The Sun gives out radiation that passes through the atmosphere and warms the Earth's surface. The warm surface then gives out infrared radiation (heat). Some gases in the atmosphere absorb this infrared radiation and re-emit it in all directions, so some of it returns to the surface. This is the greenhouse effect.
The greenhouse effect is natural and useful. Without it the average surface temperature would be about −18 °C instead of about 15 °C, and most life could not survive.
Greenhouse gases do not reflect sunlight back to space. They absorb infrared radiation from the Earth and re-emit it, sending part of it back to the surface.
Section 2
Greenhouse gases
The main greenhouse gases are:
- Carbon dioxide (CO₂): from burning fossil fuels, respiration and volcanoes, and removed by photosynthesis.
- Methane (CH₄): from cattle, rice paddies, rubbish in landfill and leaks from natural gas supplies. Per molecule it traps much more heat than CO₂, but there is far less of it.
- Water vapour (H₂O): from evaporation of seas and lakes.
- Nitrous oxide (N₂O): from fertilisers and some industrial processes.
Nitrogen and oxygen make up about 99% of dry air but are not significant greenhouse gases.
Section 3
The enhanced greenhouse effect
Human activities are adding extra greenhouse gases to the air. Burning fossil fuels (coal, oil, gas) releases carbon dioxide that was locked underground for millions of years. Deforestation removes trees that would absorb CO₂, and farming releases methane and nitrous oxide.
More greenhouse gases mean more infrared radiation is trapped. This is the enhanced greenhouse effect, and it causes global warming: a rise in the Earth's average surface temperature. Possible consequences include melting ice and rising sea levels, more heatwaves, droughts and floods, and habitats changing so that some species struggle to survive.
Section 4
Evidence for climate change
Scientists use several independent types of evidence:
- Ice cores: air bubbles trapped in polar ice show past carbon dioxide levels. About 280 ppm in 1750 has risen to about 420 ppm today.
- Temperature records: thermometers, ocean buoys and satellites show the average global temperature has risen by about 1.1 °C.
- Physical changes: glaciers and sea ice shrinking, sea level rising, and more frequent extreme weather.
- Living things: plants flowering earlier and animals moving towards the poles.
Two trends rising together show a correlation. To show cause, scientists also need a mechanism (CO₂ absorbs infrared radiation) and evidence that other causes, such as changes in the Sun, do not explain the rise.
When asked about evidence, name different types (ice cores, temperature records, melting ice) rather than giving three examples of the same thing.
Section 5
Reducing emissions (mitigation)
Mitigation means reducing the amount of greenhouse gases in the atmosphere. Strategies include:
- Using renewable energy (wind, solar, hydroelectric) instead of fossil fuels.
- Improving energy efficiency and using public transport, cycling and electric vehicles.
- Planting trees (afforestation) and protecting forests, so more CO₂ is absorbed.
- Carbon capture and storage: trapping CO₂ from power stations and storing it underground.
- International agreements to set emission targets.
Every strategy has limits. Renewables can be intermittent and costly to build, trees take decades to grow and release their carbon if they burn, and agreements depend on countries keeping their promises.
Section 6
Evaluating scientific claims
Claims about climate change appear in the news and on social media. To evaluate a claim, ask:
- Who made it, and are they an expert with no reason to be biased?
- What evidence supports it? Is it from several independent sources?
- Does it confuse correlation with cause?
- Is the sample or time period large enough, or has data been picked selectively?
- Has it been checked by other scientists (peer review)?
A good evaluation weighs strengths and weaknesses and reaches a justified conclusion.
"Correlation proves cause" is a common error. Say the data are consistent with the claim, and give extra evidence such as a known mechanism.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Greenhouse effect and climate change
- A science teacher in Nairobi explains that, without certain gases in the atmosphere, the average surface temperature of the Earth would be about −18 °C instead of the present 15 °C. Her class is learning which atmospheric gases cause this warming and how they work.Explain how greenhouse gases keep the Earth warmer than it would be without them.2 marks
- Scientists in Antarctica drill ice cores and analyse tiny air bubbles trapped in the ice. Air trapped around the year 1750 contained about 280 parts per million (ppm) of carbon dioxide. A mountain observatory in Hawaii now measures about 420 ppm. Over the same period, global average temperature records show a rise of about 1.1 °C.Describe two other types of evidence, apart from the data given, that scientists use to show that the climate is changing.2 marks
- A student in Singapore investigates whether carbon dioxide traps heat. She fills one clear plastic bottle with air and an identical bottle with air containing extra carbon dioxide from a gas cylinder, and puts a thermometer in each. She shines the same 60 W lamp on both bottles from 30 cm away for 10 minutes and records the temperature each minute. She predicts that the bottle with extra carbon dioxide will warm up more. After 10 minutes, the temperature of the air-filled bottle has risen by 4.0 °C and that of the carbon dioxide bottle by 5.5 °C.Identify the independent variable, the dependent variable and one control variable in the student's investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).