Greenhouse gases at Higher Level: The same gases and aerosols as SL, with different real cases: satellites seeing less infrared escape, the Mekong Delta's rice paddies, the Hunga Tonga eruption and Mount Pinatubo. At HL, compare the gases: how common each is, how strongly it warms, and whether it can be cut.
Practise this as you read
- Rank the gases by amount and by warming power.
- Contrast an aerosol that warms with one that cools.
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Catch and throw back: Some gases in the air let sunlight through but catch the heat rising from the ground and send part of it back down. They are the greenhouse gases.
The points to remember
- Sunlight passes through the air and warms the ground.
- The warm ground gives off infrared (long-wave) radiation: heat you cannot see.
- Greenhouse gases (GHGs) and some aerosols absorb some of this outgoing infrared ...
- ... then re-emit it in all directions, some of it back down towards the ground.
- So less infrared escapes to space, and heat is held in the lower atmosphere.
- Evidence CO2 is a GHG: it absorbs outgoing infrared, and rising CO2 matches rising global temperature.
Remember it as: Light in, infrared out: the gases catch it and throw some back.
Real example: satellites measured the infrared leaving the Earth in 1970 and again in 1997. In 1997 less escaped to space at exactly the wavelengths that carbon dioxide and methane absorb, as their amounts in the air had risen.
Absorb, not reflect: GHGs absorb outgoing infrared and re-emit it. They do not 'reflect' heat, and they let most incoming sunlight pass.
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Several gases do this job. Some are common, some are rare but very strong, and people have added more of each. Each one adds to radiative forcing.
The greenhouse gases
- The four GHGs to learn: water vapour, carbon dioxide, methane and nitrous oxide (N2O).
- Carbon dioxide and water vapour are the most abundant GHGs.
- Methane is scarce but warms a lot: each kg traps about 28 times as much heat as CO2 over 100 years.
- Nitrous oxide traps about 265 times as much per kg; it comes from fertilisers and soil bacteria.
- Other gases add warming too (radiative forcing): ozone near the ground, CFCs and HFCs.
- Asked for 'a greenhouse gas'? Write one: only the first gas you name is marked.
Real example: flooded rice paddies in Vietnam's Mekong Delta give off methane, made by microbes in the airless mud. Draining the fields for a few days at a time, called 'alternate wetting and drying', can cut this methane by a third or more.
Name the main four: When a question asks for the main greenhouse gases, give water vapour, carbon dioxide, methane and nitrous oxide. CFCs and ozone near the ground are human-made extras.
Water vapour: strong but different: Water vapour absorbs more of the outgoing infrared than any other gas, yet it is treated differently from carbon dioxide and methane. The reason: the temperature controls how much there is.
Why water vapour is a special case
- Water vapour is a significant GHG: it absorbs a large share of the outgoing infrared.
- Its amount depends on temperature: warmer air can hold more water vapour.
- It is dynamic: water evaporates and falls as rain within days.
- So it changes as a result of warming, and climate models usually leave it out as a cause.
- It is essential for life and people cannot cut it, so it cannot be mitigated.
Remember it as: Water vapour follows the warming; it does not lead it.
Real example: in January 2022 the Hunga Tonga volcano erupted under the Pacific and blasted about 146 million tonnes of water vapour high into the stratosphere, about 10% more than was there before. Scientists are still tracking how long it stays.
Do not say 'cut water vapour': Water vapour cannot be mitigated: it is part of the water cycle and essential for life. Cuts aim at carbon dioxide, methane, nitrous oxide and black carbon.
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Not everything that holds heat is a gas. An aerosol can do it too, and the guide names one: black carbon, the soot from smoky fires and engines.
Aerosols and black carbon
- An aerosol is a mass of tiny solid or liquid particles floating in the air.
- Black carbon (soot) comes from burning diesel, wood, coal or forests incompletely.
- It absorbs sunlight and infrared, warming the air around it.
- When it settles on snow and ice, it darkens them: they reflect less and melt faster.
- It stays in the air for only days to weeks, so cutting it slows warming quickly.
- Other aerosols, such as sulfate from volcanoes, reflect sunlight and cool the Earth.
Black carbon (soot)
- Absorbs sunlight and infrared
- Warms the air
- Darkens snow and ice
Sulfate (from volcanoes)
- Reflects sunlight
- Cools the surface
- Lasts a year or two high up
Real example: in 1991 Mount Pinatubo, in the Philippines, threw about 15 million tonnes of sulfur dioxide into the stratosphere. The sulfate aerosol it formed reflected sunlight and cooled the world by about 0.5 °C for about two years: the opposite of black carbon.
How this comes up: Paper 2, Section A: an explain of three marks, one linked point per mark.
Diesel trucks and wood fires release black carbon, while a large volcanic eruption releases sulfur dioxide that forms sulfate aerosol.
Explain why black carbon warms the Earth while sulfate aerosol cools it.
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