Section 1 of 8Where the extra gases come from
| Gas | Human source | Why levels rise |
|---|---|---|
| Carbon dioxide | Burning coal, oil and gas for electricity, transport and industry | Combustion releases carbon stored for millions of years |
| Carbon dioxide | Deforestation | Fewer trees remove less CO2 by photosynthesis |
| Methane | Landfill sites | Rotting waste releases methane as the population and rubbish grow |
| Methane | Farming (cattle, rice paddies) | Livestock digestion and waterlogged fields produce methane |
Section 2 of 8How greenhouse gases warm the planet
1Sunlight arrives
- Sunlight arrives Short-wavelength radiation passes through the atmosphere and warms the surface.
2Heat is re-radiated
- Heat is re-radiated The warm surface emits longer-wavelength infrared radiation back towards space.
3Gases absorb it
- Gases absorb it Carbon dioxide, methane and water vapour absorb some of this infrared and send it in all directions, including back down.
4Temperature rises
- Temperature rises More greenhouse gas traps more heat, so the average temperature of the Earth increases.
Section 3 of 8Evidence for human-caused change
More greenhouse gas
- More greenhouse gas Carbon dioxide concentration has risen strongly over about 150 years.
Higher temperature
- Higher temperature The average surface temperature has risen over the same period.
More than correlation
- More than correlation Scientists agree the rise in gases has caused the warming, as greenhouse gases trap heat.
Checked by peers
- Checked by peers The consensus comes from reviews of many peer-reviewed studies.
Section 4 of 8Climate models
What they do
- What they do Computer models use data and equations to predict future climate.
Why predictions are uncertain
- Why predictions are uncertain The climate system is complex, and human behaviour is hard to forecast.
- Future methane levels depend on choices such as how much meat is eaten, how much rice is grown and how much waste goes to landfill.
Improving confidence
- Improving confidence Models are tested against past data and refined as more information is gathered.
Section 5 of 8Evaluating a claim about climate
1
Check whether the data come from many sources and a long time period.
2
Look for peer review rather than a single unchecked opinion.
3
Ask whether the writer has a reason to be biased, such as being paid by an industry.
4
Decide if the data show only a correlation or also a mechanism, such as heat absorption by greenhouse gases.
5
Be cautious about conclusions drawn from a short run of years, because weather varies naturally from one year to the next.
Section 6 of 8Possible consequences
Global warming effects
Sea level
- Sea level Melting ice sheets and the expansion of warmer water raise sea levels and increase flooding of coasts.
Weather
- Weather More extreme weather such as storms, droughts and heatwaves may occur.
Wildlife
- Wildlife Habitats change, so some species move or die out, and migration patterns alter.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Greenhouse gas | A gas such as carbon dioxide or methane that absorbs heat radiated from Earth. |
| Deforestation | The large-scale removal of trees. |
| Fossil fuel | A fuel formed from the remains of ancient organisms, such as coal, oil and gas. |
| Peer review | Checking of scientific work by other experts before it is accepted. |
| Climate model | A computer simulation used to predict future climate. |
| Global warming | The gradual rise in the average temperature of the Earth's surface. |
Section 8 of 8Exam technique
1
Saying deforestation adds CO2 only when trees are burnt. It also removes the trees that would absorb it.
2
Calling the link between CO2 and temperature a mere coincidence; the consensus says it is causal.
3
Explaining unpredictable methane levels with only technical faults. Include changes in human behaviour.
4
Confusing global warming with the ozone layer problem.
5
Claiming that one cold winter disproves global warming.