Greenhouse gases and global warming
Key idea: Carbon dioxide and methane absorb and re-emit infrared radiation from Earth's surface. Increasing these gases reduces energy loss to space and warms the Earth.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Follow sunlight in and infrared radiation out.
- Match each gas to its sources.
- Use the table and greenhouse model to explain warming.
- Match each source to carbon dioxide or methane.
- Choose the effect linked to each gas.
- Explain how each gas causes its effect.
- Predict what happens when only methane increases.
- Explain why extra methane can increase warming.
- Give sources of carbon dioxide and methane.
- Decide whether the greenhouse effect continues after sunset.
- Explain why carbon dioxide still affects energy loss at night.
- Describe two possible effects of increased greenhouse gases.
- Find and compare the average rise in each period.
- Explain what the two averages tell you.
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ContinueConnect gases, energy and warming
By the end of this lesson you can:
- state that carbon dioxide and methane are greenhouse gases that contribute to global warming;
- give the main sources of carbon dioxide and of methane;
- describe the potential effects of increased levels of these gases, including more extreme weather events and melting of polar ice.
You need the carbon cycle lesson in this topic: burning fossil fuels and cutting down forests make the amount of carbon dioxide in the air rise.
The greenhouse effect
Energy from the Sun passes through the atmosphere and warms the Earth’s surface. The warm surface gives off energy as infrared radiation. Some of this infrared escapes to space. Greenhouse gases in the atmosphere absorb some of it and give it out again in all directions, including back towards the surface. Less energy escapes, so the Earth stays warmer.
The natural greenhouse effect keeps the surface warmer than it would be without greenhouse gases. It helps make Earth suitable for life.
When the amounts of greenhouse gases in the atmosphere increase, more infrared is absorbed and less energy escapes. This extra warming is the enhanced greenhouse effect. It causes global warming: a rise in the average temperature of the Earth over many years. As the surface warms, it emits more infrared radiation. Warming continues until energy leaving Earth once again balances energy arriving from the Sun: energy is not trapped forever.
Carbon dioxide and methane: sources
The two greenhouse gases in this course are carbon dioxide and methane.
| Gas | Main sources |
|---|---|
| carbon dioxide, CO₂ | burning fossil fuels in power stations, vehicles and factories; burning forests; respiration |
| methane, CH₄ | cattle and other farm animals (from digesting food); flooded rice fields; waste rotting in landfill sites; leaks from natural gas wells and pipes |
Human activities have raised the levels of both gases. Burning fossil fuels adds carbon dioxide. Cutting down forests reduces photosynthesis; burning or rotting the felled trees also releases carbon dioxide. More cattle, more rice fields and more landfill add methane.
Carbon dioxide measured at the Mauna Loa observatory in Hawaii shows the rise:
| Year | Annual mean carbon dioxide / parts per million (ppm) |
|---|---|
| 1960 | 316.9 |
| 2000 | 369.7 |
| 2020 | 414.2 |
Source: NOAA Global Monitoring Laboratory annual means, rounded.
Potential effects of increased greenhouse gases
As the average temperature rises:
- extreme weather risks change: heatwaves become more frequent and intense; heavy rainfall becomes more intense in many regions, increasing flood risk; some regions face more severe droughts. The effects vary by place and by type of event;
- polar ice melts, together with glaciers on land. Water from melting land ice raises sea level and increases coastal flood risk. Floating sea ice already displaces water, so its melting has little direct effect on sea level;
- plants and animals lose their habitats, and crops may fail in places that become hotter or drier.
A single hot day or storm is weather. Global warming is a change in the average over many years, so one event cannot prove or disprove it. See the IPCC summary of climate-change effects for the evidence behind these risks.
Read evidence, then explain it
Worked example 1
From a data table to global warming
Problem
Use the Mauna Loa table to describe the change in carbon dioxide from 1960 to 2020. Then explain how this change can cause global warming, and give one potential effect.
Show solution step by step
Describe the data
Method
Quote the first and last values and find the change.
Reason
A description should say what the data show, with numbers.
Working
Answer
414.2 − 316.9 = 97.3 ppm. The level rose by 97.3 ppm, from 316.9 ppm to 414.2 ppm, and rose in each period shown.
Explain the warming
Method
Use the greenhouse model.Reason
The table shows the rise but not why it matters; the model explains the link.
Working
Answer
More carbon dioxide absorbs more of the infrared given off by the Earth, so less energy escapes to space. This enhanced greenhouse effect raises the average temperature.
Give an effect
Working
Answer
Polar ice melts, and water from melting land ice raises the sea level.
Quick check
414.2 ppm is 0.04142% of dry air, or about 0.04%. A small fraction can still have a large effect, because the gas absorbs infrared.
Match sources to gases
Guided practice 2
Match the source to the gas
Problem
Match each source to carbon dioxide from burning or methane from decay without air and animal digestion.
Choose carbon dioxide or methane
Hints
Hint 1
burning
Which sources involve burning carbon-containing material?
Hint 2
decay and digestion
Methane comes from decay without air, digestion in cattle, rice fields and gas leaks.
Show full solution
Answer
Working
Answer
Landfill: methane. Power station: carbon dioxide. Cattle: methane. Burning forest: carbon dioxide.
Quick check
Burning points to carbon dioxide. Rotting waste without air, cattle, rice fields and gas leaks point to methane.
Keep acid rain separate from warming
Spot the mistake 3
Separate acid rain from global warming
Learner claim
A learner writes: “Sulfur dioxide is a greenhouse gas that causes global warming, and methane causes acid rain.” First correct the two pairings. Explain the mechanisms and give a specific effect of each in the next activity.
Match each gas to its effect
Show solution step by step
Correct sulfur dioxide
Method
Link sulfur dioxide to acid rain.Reason
It reacts with water and oxygen in clouds to form sulfuric acid.
Working
Answer
SO₂ → acid rain → limestone buildings wear away.
Correct methane
Method
Link methane to global warming.Reason
Methane absorbs infrared radiation given off by the Earth.
Working
Answer
CH₄ → enhanced greenhouse effect → global warming → more extreme weather.
Common mistake
Both are air problems, but they have different gases and different effects. Acid rain lowers the pH of rain; global warming raises the average temperature.
Check your understanding 1 Explain both mechanisms and their consequences
Correct the sulfur dioxide/methane claim fully. Explain how each gas produces its effect and give a specific environmental consequence of each. Work on paper or in the optional notes.
Show answer
Sulfur dioxide reacts with water and oxygen in clouds to form sulfuric acid: acid rain can react with limestone and damage buildings, and the pollutant harms breathing. Methane is a greenhouse gas; it absorbs and re-emits infrared from Earth, reducing outgoing energy until Earth warms to a new balance. Increased warming can raise heatwave risks or melt land ice, contributing to sea-level rise. Acid rain and global warming have different gases and mechanisms.
Saying greenhouse gases trap sunlight (they absorb the infrared given off by the warm Earth). Saying the greenhouse effect is harmful in itself (the natural effect keeps the Earth warm enough for life; the problem is the increase). Naming only carbon dioxide as a greenhouse gas. Using one hot day as proof of global warming.
What if methane rises?
Check your understanding 2 Predict when only methane increases
In a model atmosphere with other conditions unchanged, carbon dioxide stays constant while methane rises because of gas-pipe leaks. Choose whether the enhanced greenhouse effect can increase and which radiation methane interacts with.
Show answer
Extra methane can enhance the greenhouse effect by absorbing outgoing infrared. Explain the full energy-loss mechanism and source control in the next activity.
Check your understanding 3 Carbon dioxide stays the same, methane rises
Over ten years, the level of carbon dioxide in a model atmosphere stays the same, but the level of methane rises because of leaks from gas pipes. Explain whether global warming would still increase.
Work on paper or in the optional notes. Compare methane’s infrared interaction, reduced energy loss, warming and reducing the leaks.
Show answer
Yes. Methane is a greenhouse gas in its own right. More methane absorbs more of the infrared given off by the Earth, so less energy escapes and the average temperature rises, even though the carbon dioxide level has not changed. Repairing the leaks would reduce this effect.
Try these
Check your understanding 4 Generate sources for both gases
Give one source of carbon dioxide and two sources of methane.
Work on paper or in the optional notes. Compare one valid carbon dioxide source and two distinct methane sources with their mechanisms.
Show answer
Burning fossil fuels releases carbon dioxide. Cattle digestion and food waste decaying without air in landfill release methane. Other valid sources are burning forests or respiration for carbon dioxide, and flooded rice fields or natural-gas leaks for methane.
Check your understanding 5 Check what radiation remains after sunset
The warm ground still emits radiation after sunset. Choose the relevant radiation and whether extra carbon dioxide can still affect energy loss.
Show answer
The warm ground emits infrared, which carbon dioxide can absorb and re-emit after sunset. Explain why the learner’s sunrise claim fails in the next activity.
Check your understanding 6 Explain energy loss after sunset
The Sun has set, but the warm ground still emits infrared radiation. A learner says that extra carbon dioxide cannot affect energy loss until sunrise because there is no incoming sunlight. Is that reasoning correct? Explain which radiation the carbon dioxide interacts with.
Work on paper or in the optional notes. Compare the ground’s infrared emission, absorption/re-emission and reduced escape to space.
Show answer
No. Carbon dioxide absorbs and re-emits infrared radiation from the warm ground, including after sunset. Extra carbon dioxide can reduce the energy escaping to space while the ground continues emitting infrared. Incoming sunlight warms the surface, but sunlight need not arrive at the same time for greenhouse gases to interact with its outgoing radiation.
Check your understanding 7 Describe two potential effects
Describe two potential effects of increased greenhouse-gas levels.
Work on paper or in the optional notes. Compare two specific risks and compare melting land ice and floating sea ice; effects vary by region and event.
Show answer
Heatwaves can become more frequent and intense. Melting land ice adds water to the oceans, raising sea level and coastal flood risk. Other valid effects include greater heavy-rainfall risk in many regions, drought in some regions, habitat loss or crop failure. Floating sea-ice melting has little direct effect on sea level.
Check your understanding 8 Compare rises over unequal periods
Use the historical Mauna Loa table. Find the average annual carbon dioxide rise in 1960–2000 and 2000–2020, rounding each to one decimal place. Which period has the larger average rise?
Show the steps Optional working
Show the steps Optional working
Show answer
The average rises are about 1.3 and 2.2 ppm per year; the later period’s average is larger. Explain what this historical comparison tells you and what it leaves unknown in the next activity.
Check your understanding 9 Interpret the historical comparison
Use the Mauna Loa table to calculate the average rise in carbon dioxide per year from 1960 to 2000, and from 2000 to 2020. What does the comparison show, and do these three observations tell you how the rise changed within each period or what happens next?
Work on paper or in the optional notes. Compare each concentration change with its own duration, then explain what the averages tell you and what they leave unknown.
Show answer
1960–2000: (369.7-316.9)÷40 = 52.8÷40 ≈ 1.3 ppm per year. 2000–2020: (414.2-369.7)÷20 = 44.5÷20 ≈ 2.2 ppm per year. The average annual rise was larger in the later period. Three observations cannot tell you whether the rise became faster throughout each period or predict a future rate. These are changes in concentration per year, not percentage increases.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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