Human Sources of Greenhouse Gases

Explained

Which human activities release which gas

Three greenhouse gases are examinable, and each comes from a different set of activities. Matching the gas to its source is what most questions here are testing, so learn them as three separate lists rather than as one idea about pollution.

Carbon dioxide

The largest source is the combustion of fossil fuels, meaning coal, oil and natural gas burned in power stations, vehicles, homes and industry.

Deforestation adds to it twice over. Fewer trees means less photosynthesis removing carbon dioxide from the atmosphere, and the felled wood is often burned or left to decompose, which releases more.

Cement manufacture is a smaller but real source, because heating limestone drives carbon dioxide out of it chemically, quite apart from the fuel used to do the heating.

Methane

Methane is released by cattle and other livestock, as a product of digestion by the microorganisms in their guts.

Flooded rice fields release it too, because the waterlogged soil has no oxygen and the bacteria decomposing material there produce methane rather than carbon dioxide.

Landfill sites work the same way, with buried waste decomposing without oxygen. And natural gas is mostly methane, so leaks during extraction and transport release it directly.

There is less methane in the atmosphere than carbon dioxide, but each molecule traps far more energy, which is why it is worth naming separately.

Nitrous oxide

Nitrogen fertilisers are the main source. Bacteria in the soil convert some of the added nitrogen compounds to nitrous oxide, which escapes into the air. Some industrial processes and vehicle engines release it too.

How they warm the planet

Radiation from the Sun arrives at short wavelengths and passes through the atmosphere. The Earth's surface absorbs it, warms, and re-radiates energy at longer wavelengths, as infrared.

Greenhouse gases absorb that outgoing infrared and re-radiate some of it back towards the surface, so less energy escapes to space. Raising their concentration traps more, and the average temperature rises. That is the enhanced greenhouse effect.

The word enhanced is doing useful work. The greenhouse effect itself is natural and necessary; without it the Earth would be far too cold. The problem is the increase.

Reductions that match the source

Because the sources differ, so do the solutions, and a question asking how to reduce emissions wants the one that fits the gas named.

For carbon dioxide: renewable or nuclear electricity instead of fossil fuels, better insulation and more efficient vehicles, planting trees, and capturing and storing carbon dioxide from power stations.

For methane: capturing landfill gas and burning it for energy, changing livestock diets, draining rice fields periodically, and repairing gas leaks.

For nitrous oxide: applying fertiliser more precisely and only when the crop needs it, so less is left in the soil for bacteria to convert.

Notice that burning captured methane counts as a reduction even though it produces carbon dioxide, because carbon dioxide traps far less energy per molecule than methane does. Explaining that trade off is a strong answer to an evaluation question.