Trees and climate change - again

Carbon dioxide, methane, nitrous oxide and chlorofluorocarbons are the principal human-produced greenhouse gases in our atmosphere. Carbon dioxide, CO₂, is our planet’s most important greenhouse gas. CO₂ in the atmosphere is responsible for more than half the rise in global temperatures.

The amount of CO₂ in the atmosphere is still rising rapidly. Atmospheric CO₂ levels have been recorded at the Mauna Loa Observatory in Hawaii for more than 60 years. The latest results from Mauna Loa show that the monthly average of CO₂ in the atmosphere for March 2024 was 425·38 ppm. In March 2023 it was 420·99 ppm.

Atmospheric concentrations of CO₂ are rising mostly because of the fossil fuels, that we are still burning.

All plants absorb CO₂. They use it to produce oxygen (which we breathe) and carbohydrates (which the plants use for energy and growth). It would seem obvious then, that to slow global warming we simply need to plant more trees and that would certainly help but there are some provisos.

First, to plant enough trees globally to absorb enough carbon, it is estimated that an area equivalent to nearly one and half times the size of Australia would be required. It would have to be land that could support the growth of forests – not desert and not snow-covered peaks, etc. Much of the land would have to be arable land that is needed for farming to feed an increasing global population.

When a tree dies, or burns, its carbon is returned to the atmosphere. Planting huge numbers of new trees might mean more trees die, or burn, thus adding to the problem. Different forest types sequester different amounts of carbon. Where the trees are planted becomes important too – generally, tropical forests take up more carbon than forests in temperate zones.

United Nations Climate Action warns: “Ocean habitats such as seagrasses and mangroves, along with their associated food webs, can sequester carbon dioxide from the atmosphere at rates up to four times higher than terrestrial forests can.”

There are many unanswered questions and despite new research that suggest terrestrial plants absorb more CO₂ than we first thought, it is still agreed that we cannot plant enough trees to slow climate change without reducing our emissions.