6.12 Net zero and the importance of trees

Net zero does not only mean the deployment of technology options to reduce the emissions of greenhouse gases caused by human activities. Alongside these actions is the requirement to enhance the uptake of CO₂ from the atmosphere and thus reduce atmospheric CO₂ concentrations. Trees and the world’s forests are a key element of this.

As explained in Section 2, the carbon dioxide in the Earth’s atmosphere is part of an overall carbon cycle. CO₂ is continuously being emitted into the atmosphere by processes such as the burning of fossil fuels, but it is also continuously being removed. One of the most important mechanisms is the absorption of CO₂ during photosynthesis by plants on land.

When a plant grows, it absorbs CO₂ from the air and the carbon is chemically incorporated into the plant as carbohydrates. In trees (Figure 24) some carbon will be turned into wood and remain locked up in it for the whole of its life. The world’s forests contain about the same amount of carbon as is present as CO₂ in the atmosphere.

The simplest (and cheapest) ways to increase the rate of removal of CO₂ from the atmosphere are to:

• stop deforestation
• restore the world’s forests by planting more trees.

The areas required are large. In the UK, converting one square kilometre of grassland to forest with broadleaved trees, such as oak or beech, is likely to result in the uptake of about 600 tonnes of CO₂ per year (Matthews et al., 2022). This value will vary depending on the type of forest. Thus, at the global scale, absorbing a gigatonne of CO₂ per year would require reforesting an area roughly seven times the total area of the UK.

Wood is also a very useful material. If it is used as a construction material, for example in housing, its carbon content will remain in storage out of the atmosphere for the life of the building, which in some cases can be considerable. The Horyu-ji temple in Japan (Figure 25) is thought to be the oldest surviving wooden structure in the world. It was built 1300 years ago and was constructed from trees that were 2000 years old when felled.

So how much difference would it make if everyone in the world planted a tree? This video gives you a chance to consider what would happen.