Carbon dioxide and methane let sunlight in but slow the escape of heat. More of them means a warmer Earth and a changing climate. Plants pull carbon dioxide back out by photosynthesis, and the strategies to cut emissions follow directly from the sources.
Carbon dioxide and methane are greenhouse gases: higher levels cause increased global warming, which leads to climate change. Extended: the Earth's surface absorbs energy from the Sun and emits thermal energy as infrared radiation; greenhouse gases absorb this infrared and re-emit it in all directions, including back towards the Earth, so less thermal energy is lost to space. Photosynthesis removes carbon dioxide: carbon dioxide + water → glucose + oxygen, using light energy in the presence of chlorophyll (Extended: 6CO2 + 6H2O → C6H12O6 + 6O2). Strategies to reduce climate change include planting trees, reducing livestock farming, decreasing the use of fossil fuels and increasing the use of hydrogen and renewable energy such as wind and solar.
Extended. The explanation has three steps. Use the words absorb, emit and infrared and you will gain the marks.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which two gases are the greenhouse gases named in the syllabus?
What are the products of photosynthesis?
How does planting trees help to reduce climate change?
Why does reducing livestock farming help to reduce global warming?
Burning fuels, respiration and decay add carbon dioxide to the air; photosynthesis takes it out.
| Equation | |
|---|---|
| word equation | carbon dioxide + water → glucose + oxygen (light energy, chlorophyll) |
| Extended: symbol equation | 6CO2 + 6H2O → C6H12O6 + 6O2 |
Worked example (Extended): what mass of carbon dioxide is absorbed to make 90 g of glucose? Mr(C6H12O6) = 180, so moles of glucose = 90 ÷ 180 = 0.500 mol. The equation gives 6 mol CO2 per mol glucose, so 3.00 mol CO2, and mass = 3.00 × 44 = 132 g. Cutting down forests removes this sink as well as releasing carbon dioxide when the wood is burned.
Each strategy either cuts the release of a greenhouse gas or removes carbon dioxide from the air.
| Strategy | How it helps |
|---|---|
| planting trees | more photosynthesis removes carbon dioxide from the air |
| reducing livestock farming | fewer animals release less methane from digestion |
| decreasing use of fossil fuels | less carbon dioxide from combustion |
| increasing use of hydrogen and renewable energy (wind, solar) | energy without releasing carbon dioxide; hydrogen burns to form only water |
The greenhouse effect is not the hole in the ozone layer, and acid rain does not cause global warming. Do not write that greenhouse gases "reflect" heat back or "trap the Sun's rays": they absorb and re-emit infrared given off by the warm Earth. And the greenhouse effect itself is natural and keeps the Earth habitable; the problem is the increase in greenhouse gases.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) Which statement explains how carbon dioxide causes global warming?
(Extended) Which is the balanced symbol equation for photosynthesis?
(Extended) What mass of carbon dioxide is used to make 90 g of glucose by photosynthesis? (Mr: CO2 = 44, C6H12O6 = 180)
Which change would do most to reduce carbon dioxide emissions from electricity generation?
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