International A Level Biology / Environment and climate change

Using the carbon cycle to reduce atmospheric CO₂

Edexcel International A Level Biology (YBI11), chapter 16: Environment and climate change, specification point 5.18, 5.26. Free lesson.

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You can reduce atmospheric CO₂ by lowering combustion or increasing photosynthesis. Forests and biofuels can help, but their benefits depend on how they are managed and on the extra emissions they cause.

How methods change the carbon cycle

CO₂ enters the atmosphere through combustion and respiration, and leaves through photosynthesis. So a method to lower atmospheric CO₂ must either reduce a flow into the air or increase a flow out. To put less CO₂ into the air, burn less fossil fuel. Renewable electricity, insulation and biofuels can reduce fossil fuel combustion. To take more CO₂ out of the air, increase photosynthesis by growing trees. Reforestation replants lost forest, while afforestation plants trees where there was no forest before. Carbon left in coal, oil and gas stays underground. Carbon taken up by…

When a forest is a carbon sink

A carbon sink takes in more carbon than it releases. Trees take CO₂ from the air by photosynthesis and use it to make glucose. Some of this carbon becomes biomass, especially cellulose and lignin in wood and roots. A growing forest is a sink because photosynthesis takes in more carbon than respiration releases. A young forest can be a strong sink as its biomass increases. A mature forest is mainly a carbon store. Photosynthesis is roughly balanced by respiration from trees and decomposers breaking down dead material, so biomass increases little. If trees are felled and burnt, or left…

Why biofuels are carbon neutral in principle

Biofuels are fuels made from recently grown plants. Bioethanol, made from crops such as sugar cane or maize, is a common example. The crop absorbs CO₂ as it grows, then that carbon is released again when the fuel is burnt. For bioethanol, yeast ferments sugars from the crop to produce ethanol. In principle, the CO₂ released by burning the fuel is the same carbon that the crop recently took from the air. The net change in atmospheric CO₂ is zero, which is why it is called carbon neutral. Burning fossil fuel adds carbon that was locked away for millions of years. Biofuel returns carbon that…

Why biofuels are not fully carbon neutral

In practice, making and using biofuels can cause extra emissions and affect resources people need. A biofuel helps only if the emissions it saves are greater than these extra emissions. Land use: clearing forest for crops can release stored carbon, remove a carbon sink and destroy habitats and species. Food competition: land and crops used for fuel cannot be used for food. This can push up food prices, affecting poorer people most. Fertiliser: making nitrogen fertiliser uses a lot of energy, mostly from fossil fuels. Soil bacteria also release nitrous oxide from fertiliser, and…

Managing resources and writing a balanced discussion

Sustainable resource management means meeting people's needs now while keeping the resource and its habitat available for the future. For example, a sustainable forest has as many trees replanted as are felled, helping its carbon store continue. Apply the idea to the resource in the question: explain what is used, what is replaced and what is conserved. A discussion needs advantages and limitations, each supported by a reason, followed by a conclusion that weighs them up. For fast-growing non-native conifers planted on grassland, rapid growth can fix lots of CO₂ into biomass. However, the…

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