Sunlight passes through the atmosphere and warms the Earth's surface. The warm surface gives off infrared radiation. Greenhouse gases such as carbon dioxide, methane and water vapour absorb some of this infrared and send it back, keeping the planet warm enough for life. This is the greenhouse effect.
Since about 1850, burning fossil fuels and cutting down forests have raised the amount of carbon dioxide in the air from about 280 to over 420 parts per million. Methane has risen too, from farming and landfill. More greenhouse gas traps more energy, so the average global temperature has risen by more than 1 °C. Evidence includes temperature records, shrinking glaciers and sea ice, and rising sea levels.
Worked example
Why does cutting down forests add to global warming?
- Trees take in carbon dioxide for photosynthesis.
- Fewer trees means less carbon dioxide removed; burning them releases more.
Answer:So more carbon dioxide stays in the air, trapping more heat.
Key words
- greenhouse gas
- a gas that absorbs infrared, such as CO₂ or methane
- global warming
- the rise in Earth's average temperature
- fossil fuel
- coal, oil and gas
- deforestation
- cutting down forests
New questions every time.
NGSS-referenced: MS-ESS3-5