The Amazon rainforest and the Carbon cycle (2)
A question and an answer (Edexcel/Pearson)
As promised here is the Carbon cycle essay question, together with an answer, from the Edexcel (Pearson) A Level Geography paper in June 2023. Rainforests are not mentioned in the question (unlike the AQA one previously published) though I have included a reference to them in the answer.
There will only be one post a week for the next few weeks as I am busy doing other things.
The processes of the carbon cycle operate at longer and slower (geological) and shorter and faster (biological) timescales.
Evaluate the view that human activities are having a greater impact on shorter term biological processes than on longer term geological processes.
It is true that biological processes operate over shorter time scales than geological processes. It is also true that most of the Earth’s carbon is stored in the lithosphere, over 100 million PgC, in the form of fossil fuel rocks and limestones. These are a function of geological processes. Other carbon stores are in the atmosphere, the oceans, the biosphere, and the cryosphere and these are more the outcome of biological processes. These latter stores are perhaps more at risk of human activities than the lithosphere.
The main longer-term geological process is out-gassing of CO2 from volcanoes and via the heating of limestone through subduction in mountain belts. The amount emitted from these processes is only 0.2/3PgC per year. These are natural and generally thought unconnected to human activity. However, there is a view that human activity might be tenuously involved in volcanic outgassing. With human-induced climate change, increased seasonal rainfall increases the eruptions of volcanoes because of changing the pore pressure build-up of gases, triggering the eruptive cycle.
Through the human activity of burning of fossil fuels, anthropogenic carbon emissions are up to 100 times greater than all volcanic emissions. Since 1850 there has been 2,500 billion tonnes of CO2 released into the atmosphere. Some might argue that this having a great impact on the longer-term geological processes as it involves the burning of rocks. The counterview is that such burning adds to the impact of shorter-term processes in the atmosphere.
The ocean store of carbon is very large – many times more than that of the atmosphere. The ocean carbon pump operates in several ways. Firstly, phytoplankton near the sea surface take CO2 from the atmosphere by absorbing it into their cells and using energy from the sun, they form carbohydrates (CH2O). Animals (zooplankton and larger creatures) in the sea then consume the phytoplankton. Secondly, downwelling and upwelling ocean currents move dissolved CO2 around the ocean. In general, cold waters sink and warmer waters rise. As phytoplankton and other animals (fish, crustaceans, and coral) die they sink and form sediments on the ocean floor, adding to geological processes.
As CO2 levels rise due to the burning of fossil fuels and deforestation (both human activities) ocean temperatures rise and ocean acidification occurs. These have impacts on the ocean carbon pump. Warmer conditions reduce the abundance of phytoplankton and so disrupts the food chain impacting on the shorter-term biological processes. Changes in ocean temperature and acidification will also impact on the carbonate pump. Decreasing the amount of carbonates in the water makes conditions more difficult for both phytoplankton, coral, and shellfish.
On land, the processes of deforestation and conversion of grasslands to farmland have impacted the shorter-term biological processes of photosynthesis and respiration. Deforestation has increased in tropical rain forests especially in developing countries such as Brazil. Millions of acres of primary forest have been lost due to practices initiated in the colonial era, such as the mechanised logging of forest reserves, the establishment of state-owned agricultural plantations (e.g. cocoa and oil palm), and mining. Also on land, poor agricultural practices in semi-arid areas have led to increases in desertification. This reduces the organic content of the soil and so reduces the subsequent release of this carbon through the respiration of soil biota.
In conclusion, a response to this question depends on whether you think the burning of fossil fuels has more of an impact on geological processes than atmospheric processes. Crucially, perhaps, it is fair to state that the impacts of human activity have had a greater impact on all aspects of the shorter-term biological processes. [596]