The Amazon rainforest and the Carbon cycle (1)

An essay question and answer

Two recent Substacks have examined the links between the Water cycle and rainforests. This Substack notes that in the summer of 2023 one UK examination board set an essay question on the links between the Carbon cycle and rainforests. Here is the question, and a sample answer to it. Another exam board also set an essay question on the Carbon cycle - I shall provide that question and an answer to it in due course.

[Note: for new subscribers - when I post such questions and answers, I give myself a self-imposed limit of 600 words, as I believe this is the maximum that can be written in the time allowed in the examination.]

Evaluate the potential impact of changes in the carbon budget on a tropical rainforest that you have studied. (20 marks)

The carbon budget refers to the balance of exchanges between the four major stores of carbon: the hydrosphere (oceans), atmosphere, the cryosphere (including the permafrost) and the biosphere (plants and animals). The current carbon budget shows a net gain of 4.4PgC per year in the atmosphere according to the Intergovernmental Panel on Climate Change (IPCC). Carbon dioxide (CO₂) is the most important anthropogenic greenhouse gas (GHG), and rising levels of carbon dioxide and other GHGs, such as methane, in post-industrial times are fuelling fears of climate change through atmospheric warming. Such warming is and will have a significant impact on the Amazon rainforest.

The Amazon rainforest in South America lies within the equatorial climate zone and covers an area of some 8.2 million km², mainly in Brazil. At present, this rainforest acts as a carbon sink and absorbs around 35% of the world’s annual CO₂ emissions and produces more than 20% of the world’s oxygen. It contains the greatest biodiversity on earth, providing a habitat for more than half of the world’s estimated 10 million species of plants, animals, and insects.

Predicted impacts of climate change, caused by changes in the carbon budget, on this region include the following. Firstly, an increase in temperature of 2-3C by 2050, which is likely to result in increased rates of evapotranspiration and a more vigorous hydrological cycle. Sea temperatures are expected to warm too, particularly in the Pacific Ocean. This will have a knock-on effect on the El Nino Southern Oscillation (ENSO), which is likely to occur more frequently.

Secondly, a decrease in precipitation during the dry season (which can last up to four months of the year) is expected. Reduced rainfall and prolonged drought are features of an El Nino year and these could therefore be experienced more frequently. It is also possible that there will be more intense rainfall during the wet season.

Thirdly, there will be changes to the nature of the vegetation here. Sea levels are currently rising by some 5mm per annum along the delta of the Amazon. Increased erosion and flooding are likely to have a substantial impact on low-lying areas and will destroy the coastal mangrove forests. Up to 40% of plant species may become unviable in the Amazon rainforest by 2080. Large areas of the evergreen tropical rainforest may be succeeded by mixed forest and savanna grassland vegetation. As the dry season lengthens, trees will have more time to dry out so there is likely to be an increased incidence of wild forest fires. This would add to CO₂ emissions.

By 2050, forest die-back because of changing vegetation succession and fire is predicted to result in the Amazon region becoming a net source of carbon dioxide, rather than a carbon sink, by the year 2050, exacerbating the rate of atmospheric warming.

The Amazon rainforest is therefore changing. One fifth of it has been destroyed by human activity unrelated to the carbon budget. While there are some optimistic signs that deforestation can be controlled by the new government of President Lula in Brazil, the destruction is undeniable. But there are reasons for hope too.  People have realised the valuable role that rainforests play in the world’s climate – they store huge amounts of carbon, so keeping it out of the atmosphere. Research has also found that the trees in the forests which do remain have been growing bigger, taking up more carbon. Over recent decades tropical forests worldwide have absorbed one fifth of global fossil fuel emissions. In the Amazon rainforest, nature is doing everyone a big favour in helping to slow climate change. [597]