Tropical Rainforests: What Is Their Role In Climate Change? (1/3)

by Beatrice Trascau

Tropical Rainforests and Climate Change

Over the past 50 years, climate change has become an increasingly important issue and is recognized as a major threat to our future. Climate change is defined as a significant change in global and regional climate patterns caused by an increase in the concentration of greenhouse gasses in the atmosphere as a result of human activities [1]. Global climate change has significant effects on ecosystems, biodiversity and organisms, such as surface temperature increases, more frequent and extreme weather events and altered precipitation patterns [1]. 

Despite covering only 6% of the Earth’s surface [6], tropical rainforests are highly important ecosystems that represent hotspots of biodiversity and endemism. Given the varied ecosystem services and functions they provide, it is important to understand the effect that climate change has on tropical rainforests and the role these ecosystems play more broadly in climate change.

The Carbon Cycle

Before discussing the role that tropical rainforests play in climate change, we must first consider the ecosystem services they provide. Firstly, tropical rainforests act as significant carbon sinks by storing 46% of the world’s terrestrial carbon [3]. Furthermore, studies have calculated that tropical trees perform about 60% [4] of the world’s photosynthesis and are therefore able to absorb  between 25-33% of the total carbon emissions [5,6]. However, it is also important to note that animals and plants in tropical rainforests perform respiration which releases similar quantities of carbon back into the atmosphere [6]. Therefore, the balance between the carbon absorbed through photosynthesis and stored by trees, and the carbon released into the atmosphere through respiration, is a key factor contributing to the overall neutral contribution that tropical rainforests have to the global carbon cycle. 

Carbon Sources

However, tropical rainforests across the globe are currently undergoing extensive deforestation driven by an increased need for agricultural land and increased demand for pulp and paper products and mining [5]. As such, increasing quantities of carbon are being released into the atmosphere through the felling and processing of old growth forests. Extensive deforestation in the tropics could also increase carbon emissions through the disturbance of peat forests. As peat is rich in carbon, their disturbance represents an additional source of carbon added to the atmosphere [5]. 

Whilst the effect of deforestation on carbon levels in the atmosphere is concerning, it is also important to account for the effect of extreme weather events caused by climate change, such as wildfires. As wildfires increase in intensity, they are likely to affect more areas and therefore contribute a larger quantity of carbon dioxide (CO₂) to the global carbon cycle, through the burning of organic material. 

Other important and well studied effects of climate change are increased global temperatures and altered precipitation patterns [1]. As temperature and precipitation increase, the rate of plant respiration will also increase, therefore releasing more carbon into the atmosphere [6]. There is however some evidence to suggest that as CO₂ is more easily accessible for trees to photosynthesise, the tropical rainforest trees will be able to absorb this carbon more easily. Furthermore, while rapid deforestation and land conversion in the tropics is of great concern, some studies have also noted that regrowing forests can also act as carbon sinks. Other studies have even noted a tendency of disturbed forests to absorb CO₂ at a higher rate than undisturbed forests [7,8].

Conclusion

Given the rapidly accelerating land use changes and climatic changes in the tropics and the detrimental interaction between the two, there is an urgent need to take action to ensure that tropical rainforests maintain a relatively neutral contribution to the global carbon cycle rather than becoming net emitters of carbon. For this to happen, deforestation and land conversion cannot continue at their current rates. Moreover, given their higher rates of carbon assimilation, degraded forests must be allowed to recover. Lastly, we need to ensure that robust and accurate data regarding land cover and land conversion are collected and compiled to inform effective and science-based policies. 

References:
[1] International Sustainability Unit. (2015). Tropical forests. A review. London, England: Page Bros Ltd
[2] Christina Nunez, Rainforests, Explained, https://www.nationalgeographic.com/environment/article/rain-forests, accessed on 30.05.2022.
[3] Soepadmo, Engkik, 1993, Tropical rain forests as carbon sinks, Chemosphere 27.6: 1025-1039. https://doi.org/10.1016/0045-6535(93)90066-E
[4] Malhi, Yadvinder, The productivity, metabolism and carbon cycle of tropical forest vegetation, Journal of Ecology 100.1: 65-75. https://doi.org/10.1111/j.1365-2745.2011.01916.x
[5] Mitchard, Edward TA, 2018, The tropical forest carbon cycle and climate change. Nature 559.7715: 527-534. https://doi.org/10.1038/s41586-018-0300-2
[6] Grace, John, Edward Mitchard, and Emanuel Gloor, 2014, Perturbations in the carbon budget of the tropics, Global Change Biology 20.10: 3238-3255. https://doi.org/10.1111/gcb.12600
[7] Poorter, Lourens, et al., 2016, Biomass resilience of Neotropical secondary forests, Nature 530.7589: 211-214. https://doi.org/10.1038/nature16512
[8] Bonner, Mark TL, Susanne Schmidt, and Luke P. Shoo, 2013, A meta-analytical global comparison of aboveground biomass accumulation between tropical secondary forests and monoculture plantations, Forest Ecology and Management 291: 73-86. https://doi.org/10.1016/j.foreco.2012.11.024