Earth and atmosphere, Year 11: greenhouse gases and climate change

carbon dioxide; methane

提示One is released by burning fuels; the other is the main gas in the natural gas supply.

为什么All three absorb long-wavelength radiation given out by the Earth's surface. They make up only a small part of the air; the main gases, nitrogen and oxygen, are not greenhouse gases.

Short

提示It includes visible light and ultraviolet.

为什么Short-wavelength radiation, such as visible light, mostly passes through the atmosphere and is absorbed by the ground, which warms up. The Earth is far cooler than the Sun, so the radiation it gives out has a longer wavelength (infrared).

They absorb it, and send some of it back towards the surface

提示The gases do not let all of it escape to space.

为什么The gases absorb some of the outgoing infrared radiation and then emit radiation in all directions. Part of it returns to the ground, so the surface and the lower atmosphere are warmer than they would otherwise be.

The surface absorbs the energy, warms, and emits it as longer-wavelength (infrared) radiation

提示The ground is far cooler than the Sun, and that affects what it gives out.

为什么This step is what makes the atmosphere work like a one-way gate: the incoming radiation has a wavelength that the gases let through, while the radiation going out has a wavelength that they absorb.

The greenhouse effect

提示It is named after a glass building used for growing plants.

为什么The effect is natural and keeps the Earth warm enough to support life: without it the Earth would be about 30 °C colder (Met Office). Adding more greenhouse gases makes the effect stronger.

Methane

提示Its formula is CH4, and it forms when material rots or is digested without air.

为什么Cattle release it as they digest grass, and microbes produce it in waterlogged paddy fields and in rubbish rotting in landfill sites. A growing population means more livestock, more rice and more waste.

deforestation

提示The word is built from 'forest' with a prefix that means removal.

为什么Burning coal, oil and gas releases carbon that was locked underground. Clearing forests leaves fewer trees to take in carbon dioxide by photosynthesis, and burning or rotting the wood releases the carbon stored in it.

Peer review

提示The first word means your equals: people of the same standing as you.

为什么It helps to catch errors, weak methods and claims that the data do not support, so published results can be trusted more. The view that human activities are warming the Earth rests on evidence that has been through this process.

The climate is a very complex system, so models of it have to be simplified

提示Think how many things affect the weather, and how they affect one another.

为什么Oceans, clouds, ice, living things and human choices all interact. Models have to simplify this, so scientists report a range of possible outcomes instead of a single figure. Describing that uncertainty openly is part of good science.

It uses only part of the evidence, so it may be biased

提示Consider what the missing studies might have shown.

为什么Reports in the media can simplify, speculate or select evidence to fit an opinion, in either direction. To judge a report, ask who wrote it, whether the evidence was peer-reviewed, and whether all of the evidence was considered.

Water expands as it warms

提示Think about what happens to the liquid in a thermometer.

为什么The oceans take in most of the extra heat, and warmer water takes up more space. Together with meltwater from ice on land, this raised the global average sea level by about 20 cm between 1901 and 2018 (Met Office). Low-lying coasts face more flooding and erosion.

Patterns change: some regions get more rain and flooding, others less rain and drought

提示Not every place is affected in the same direction.

为什么Warmer air can hold more water vapour, so on average rain becomes heavier, but shifting weather systems leave some areas drier. Both changes put water supplies and farming at risk.

They are expected to become more frequent

提示A warmer atmosphere holds extra energy and extra water vapour.

为什么The Met Office reports that heatwaves and heavy rain have already increased worldwide and are expected to increase further. For some other events, such as tropical storms, the evidence so far is less clear.

The temperature and rainfall of its habitat change, so the conditions it needs may shift or disappear

提示Each organism is adapted to one sort of environment.

为什么Some species may spread towards the poles or to higher ground; those that cannot move or adapt quickly enough are at greater risk of extinction. The same changes affect which crops can be grown where, so some regions may produce less food.

temperature

提示It is the quantity measured with a thermometer.

为什么Global warming is the rise in the average; climate change is the wider set of changes that follows, such as shifting rainfall, melting ice and rising seas. The average has risen by more than 1 °C since the 1850s (Met Office).

A carbon footprint

提示The second word is the mark a shoe leaves behind.

为什么It counts carbon dioxide and other greenhouse gases such as methane, and it covers every stage: obtaining the materials, making, transporting, using and disposing. Cutting emissions of carbon dioxide and methane makes it smaller.

Greenhouse gases are emitted in making it, generating its electricity and disposing of it

提示Think about the stages before and after the journey itself.

为什么A carbon footprint covers the full life cycle. Mining the metals, building the car and its battery, and generating the electricity may all involve burning fossil fuels, so the total is never zero.

Less fuel is burned to heat it, so less carbon dioxide is emitted

提示Follow the saving back to the boiler or the power station.

为什么Other actions work in the same way, by cutting emissions at the source: using renewable or nuclear energy in place of fossil fuels, wasting less energy, and reducing the methane given off by farming and landfill.

The changes can be costly and may harm the economy

提示Think about jobs, prices and industries that rely on fossil fuels.

为什么Replacing power stations, vehicles and heating systems costs money, and some industries would shrink. Governments have to weigh these costs against the benefits, which is one reason that action is slower than some people would like.

Greenhouse gases mix through the whole atmosphere, so total global emissions are what matter

提示The air above one nation does not stay there.

为什么Reductions therefore depend on international co-operation, which is hard to achieve and is incomplete. Other limits include a lack of public information and people's reluctance to change the way they live.