Case study of a tropical rainforest setting to illustrate and analyse key themes in water and carbon cycles and their relationship to environmental change and human activity.
The Amazon is the largest tropical rainforest on Earth. It sits within the Amazon River basin, covers some 40% of the South American continent and as you can see on the map below includes parts of eight South American countries: Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Guyana, and Suriname. The actual word “Amazon” comes from river.
Amazing Amazon facts;
• It is home to 1000 species of bird and 60,000 species of plants
• 10 million species of insects live in the Amazon
• It is home to 20 million people, who use the wood, cut down trees for farms and for cattle.
• It covers 2.1 million square miles of land
• The Amazon is home to almost 20% of species on Earth
• The UK and Ireland would fit into the Amazon 17 times!
The Amazon caught the public’s attention in the 1980s when a series of shocking news reports said that an area of rainforest the size of Belgium was being cut down and subsequently burnt every year. This deforestation has continued to the present day according to the Sao Paulo Space Research Centre. In 2005 they had lost 17% of Amazon rainforest or 650,000 square kilometres. Their satellite data is also showing increased deforestation in parts of the Amazon.
Tropical forests are very important stores of carbon, and in their untouched state act as carbon sinks. It is thought that there are approximately 100 PgC in aboveground biomass (AGB) in Amazonia. 1 The Amazon forest covers an estimated 5.3 million sq km and holds 17% of the global terrestrial vegetation carbon stock.5
A study by Fauset (2015) showed that around 1% of all the tree species in the Amazon account for half of the carbon locked in the vast South American rainforest. Despite the fact that the region is home to an estimated 16,000 tree species, just 182 species dominated the carbon storage process. 6
Untouched Amazon forests take in more carbon dioxide than they put back into the atmosphere. This shows that the Amazon forests help reduce global warming by lowering the planet's greenhouse gas levels.
Dead Amazonian trees (which account for around 20% of above ground biomass5) emit an estimated 1.9 billion tons (1.7 billion metric tons) of carbon to the atmosphere each year. In a normal year, the Amazon rainforest absorbs about 2.2 billion tons (2 billion metric tons) of carbon dioxide. This means that untouched tropical forests act as a SINK for carbon. 2
However, a study from 2015 confirms that Amazon forests have acted as a long-term net biomass sink, but also found a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s. This means tropical forests are becoming less efficient at trapping carbon. 7
The water cycle is very active within the Amazon rainforest and it interlinks the lithosphere, atmosphere and biosphere. The basin is drained by the Amazon River and its tributaries. The average discharge of water into the Atlantic Ocean by the Amazon is approximately 175,000 m3 per second, or between 1/5th and 1/6th of the total discharge into the oceans of all of the world's rivers. 3
The Rio Negro, a tributary of the Amazon, is the second largest river in the world in terms of water flow, and is 100 meters deep and 14 kilometers wide near its mouth at Manaus, Brazil.
Rainfall across the Amazon is very high. Average rainfall across the whole Amazon basin is approximately 2300 mm annually. In some areas of the northwest portion of the Amazon basin, yearly rainfall can exceed 6000 mm. 3
Only around 1/3 of the rain that falls in the Amazon basin is discharged into the Atlantic Ocean. It is thought that;
1. Up to half of the rainfall in some areas may never reach the ground, being intercepted by the forest and re-evaporated into the atmosphere.
2. Additional evaporation occurs from ground and river surfaces, or is released into the atmosphere by transpiration from plant leaves (in which plants release water from their leaves during photosynthesis)
3. This moisture contributes to the formation of rain clouds, which release the water back onto the rainforest. In the Amazon, 50-80 percent of moisture remains in the ecosystem’s water cycle. 4
This means that much of the rainfall re-enters the water cycling system of the Amazon, and a given molecule of water may be "re-cycled" many times between the time that it leaves the surface of the Atlantic Ocean and is carried by the prevailing westerly winds into the Amazon basin, to the time that it is carried back to the ocean by the Amazon River. 4
It is thought that the water cycle of the Amazon has global effects. The moisture created by rainforests travels around the world. Moisture created in the Amazon ends up falling as rain as far away as Texas, and forests in Southeast Asia influence rain patterns in south eastern Europe and China. 4
When forests are cut down, less moisture goes into the atmosphere and rainfall declines, sometimes leading to drought. These have been made worse by deforestation. 4
Change to the water and carbon cycles in the Amazon
The main change to the Amazon rainforest is deforestation. Deforestation in the Amazon is generally the result of land clearances for;
1. Agriculture (to grow crops like Soya or Palm oil) or for pasture land for cattle grazing
2. Logging – This involves cutting down trees for sale as timber or pulp. The timber is used to build homes, furniture, etc. and the pulp is used to make paper and paper products. Logging can be either selective or clear cutting. Selective logging is selective because loggers choose only wood that is highly valued, such as mahogany. Clear-cutting is not selective. Loggers are interested in all types of wood and therefore cut all of the trees down, thus clearing the forest, hence the name- clear-cutting.
3. Road building – trees are also clear for roads. Roads are an essential way for the Brazilian government to allow development of the Amazon rainforest. However, unless they are paved many of the roads are unusable during the wettest periods of the year. The Trans Amazonian Highway has already opened up large parts of the forest and now a new road is going to be paved, the BR163 is a road that runs 1700km from Cuiaba to Santarem. The government planned to tarmac it making it a superhighway. This would make the untouched forest along the route more accessible and under threat from development.
4. Mineral extraction – forests are also cleared to make way for huge mines. The Brazilian part of the Amazon has mines that extract iron, manganese, nickel, tin, bauxite, beryllium, copper, lead, tungsten, zinc and gold!
5. Energy development – This has focussed mainly on using Hydro Electric Power, and there are 150 new dams planned for the Amazon alone. The dams create electricity as water is passed through huge pipes within them, where it turns a turbine which helps to generate the electricity. The power in the Amazon is often used for mining. Dams displace many people and the reservoirs they create flood large area of land, which would previously have been forest. They also alter the hydrological cycle and trap huge quantities of sediment behind them. The huge Belo Monte dam started operating in April 2016 and will generate over 11,000 Mw of power. A new scheme the 8,000-megawatt São Luiz do Tapajós dam has been held up because of the concerns over the impacts on the local Munduruku people.
6. Settlement & population growth – populations are growing within the Amazon forest and along with them settlements. Many people are migrating to the forest looking for work associated with the natural wealth of this environment. Settlements like Parauapebas, an iron ore mining town, have grown rapidly, destroying forest and replacing it with a swath of shanty towns. The population has grown from 154,000 in 2010 to 220,000 in 2012. The Brazilian Amazon’s population grew by a massive 23% between 2000 and 2010, 11% above the national average.
The WWF estimates that 27 per cent, more than a quarter, of the Amazon biome will be without trees by 2030 if the current rate of deforestation continues. They also state that Forest losses in the Amazon biome averaged 1.4 million hectares per year between 2001 and 2012, resulting in a total loss of 17.7 million hectares, mostly in Brazil, Peru and Bolivia. 12
The impacts of deforestation
Deforestation causes important changes in the energy and water balance of the Amazon. Pasturelands and croplands (e.g. soya beans and corn) have a higher albedo and decreased water demand, evapotranspiration and canopy interception compared with the forests they replace. 9 Lathuillière et al. 10 found that forests in the state of Mato Grosso;
• Contributed about 50 km3 per year of evapotranspiration to the atmosphere in the year 2000.
• Deforestation reduced that forest flux rate by approximately 1 km3 per year throughout the decade.
• As a result, by 2009, forests were contributing about 40 km3 per year of evapotranspiration in Mato Grosso.
Differences such as these can affect atmospheric circulation and rainfall in proportion to the scale of deforestation
The agriculture that replaces forest cover also decreases precipitation. In Rondônia, Brazil, one of the most heavily deforested areas of Brazil, daily rainfall data suggest that deforestation since the 1970s has caused an 18-day delay in the onset of the rainy season.11
SSE Amazon also has many wild fires, which are closely associated with deforestation, forest fragmentation and drought intensity. According to Coe et al (2015) “the increased atmospheric aerosol loads produced by fires have been shown to decrease droplet size, increase cloud height and cloud lifetime and inhibit rainfall, particularly in the dry season in the SSE Amazon. Thus, fires and drought may create a positive feedback in the SSE Amazon such that drought is more severe with continued deforestation and climate change.”9
Contribution to climate change
Tropical forests are very important carbon sinks, but deforestation and degradation are turning these sinks into carbon SOURCES. The degradation (reduction in quality of tropical forests) and deforestation releases the carbon stored within the trees back into the atmosphere. Around 30% of anthropogenic carbon emissions come from burning the rainforests alone. Forests that experienced disturbances such as logging and fires store 40% less carbon than undisturbed forests. 13 This makes climate change alone a major impact of tropical forest destruction with respects to the carbon cycle.
The impacts of climate change on the Amazon
According to the WWF:
• Some Amazon species capable of moving fast enough will attempt to find a more suitable environment. Many other species will either be unable to move or will have nowhere to go.
• Higher temperatures will impact temperature-dependent species like fish, causing their distribution to change.
• Reduced rainfall and increased temperatures may also reduce suitable habitat during dry, warm months and potentially lead to an increase in invasive, exotic species, which then can out-compete native species.
• Less rainfall during the dry months could seriously affect many Amazon rivers and other freshwater systems.
• The impact of reduced rainfall is a change in nutrient input into streams and rivers, which can greatly affect aquatic organisms.
• A more variable climate and more extreme events will also likely mean that Amazon fish populations will more often experience hot temperatures and potentially lethal environmental conditions.
• Flooding associated with sea-level rise will have substantial impacts on lowland areas such as the Amazon River delta. The rate of sea-level rise over the last 100 years has been 1.0-2.5 mm per year, and this rate could rise to 5 mm per year.
• Sea-level rise, increased temperature, changes in rainfall and runoff will likely cause major changes in species habitats such as mangrove ecosystems. 15
Impacts of deforestation on soils
Removing trees deprives the forest of portions of its canopy, which blocks the sun’s rays during the day, and holds in heat at night. This disruption leads to more extreme temperature swings that can be harmful to plants and animals. 8 Without protection from sun-blocking tree cover, moist tropical soils quickly dry out.
In terms of Carbon, Tropical soils contain a lot of carbon. The top meter holds 66.9 PgC with around 52% of this carbon pool held in the top 0.3 m of the soil, the layer which is most prone to changes upon land use conversion and deforestation. 14 Deforestation releases much of this carbon through clearance and burning. For the carbon that remains in the soil, when it rains soil erosion will wash much of the carbon away into rivers after initial deforestation and some will be lost to the atmosphere via decomposition too.
Impacts of deforestation on Rivers
Trees also help continue the water cycle by returning water vapor to the atmosphere. When trees are removed this cycle is severely disrupted and areas can suffer more droughts. There are many consequences of deforestation and climate change for the water cycle in forests;
1. There is increased soil erosion and weathering of rainforest soils as water acts immediately upon them rather than being intercepted.
2. Flash floods are more likely to happen as there is less interception and absorption by the forest cover.
3. Conversely, the interruption of normal water cycling has resulted in more droughts in the forest, increasing the risk of wild fires
4. More soil and silt is being washed into rivers, resulting in changes to waterways and transport
5. Disrupt water supplies to many people in Brazil
1 - Malhi, Y. et al. The regional variation of aboveground live biomass in old-growth Amazonian forests. Glob. Chang. Biol. 12, 1107–1138 (2006).
2 - Fernando D.B. Espírito-Santo et al. Size and frequency of natural forest disturbances and the Amazon forest carbon balance. Nature Communications volume 5, Article number: 3434 (2014) Accessed 3rd of January 2019 retrieved from https://www.nature.com/articles/ncomms4434#ref4
3 - Project Amazonas. Accessed 3rd of January 2019 retrieved from https://www.projectamazonas.org/amazon-facts
4 - Rhett Butler, 2012. IMPACT OF DEFORESTATION: LOCAL AND NATIONAL CONSEQUENCES. Accessed 3rd of January 2019 retrieved from https://rainforests.mongabay.com/0902.htm
5 – Mark Kinver. Amazon: 1% of tree species store 50% of region's carbon. 2015. BBC. Accessed 3rd of January 2019 retrieved from https://www.bbc.co.uk/news/science-environment-32497537
6 - Sophie Fauset et al. Hyperdominance in Amazonian forest carbon cycling. Nature Communications volume 6, Article number: 6857 (2015). Accessed 3rd of January 2019 retrieved from https://www.nature.com/articles/ncomms7857
7- Brienen, R.J.W et al. (2015) Long-term decline of the Amazon carbon sink, Nature, https://www.nature.com/articles/nature14283
8 – National Geographic – Deforestation - Learn about the man-made and natural causes of deforestation–and how it's impacting our planet. Accessed 20th of January 2019 retrieved from https://www.nationalgeographic.com/environment/global-warming/deforestation/
9 - Michael T. Coe, Toby R. Marthews, Marcos Heil Costa, David R. Galbraith, Nora L. Greenglass, Hewlley M. A. Imbuzeiro, Naomi M. Levine, Yadvinder Malhi, Paul R. Moorcroft, Michel Nobre Muza, Thomas L. Powell, Scott R. Saleska, Luis A. Solorzano, and Jingfeng Wang. (2015) Deforestation and climate feedbacks threaten the ecological integrity of south–southeastern Amazonia. 368, Philosophical Transactions of the Royal Society B: Biological Sciences. Accessed 20th of January 2019 retrieved from http://rstb.royalsocietypublishing.org/content/368/1619/20120155
10 - Lathuillière MJ, Mark S, Johnson MS & Donner SD. (2012). Water use by terrestrial ecosystems: temporal variability in rainforest and agricultural contributions to evapotranspiration in Mato Grosso, Brazil. Environmental research Letters Volume 7 Number 2. http://iopscience.iop.org/article/10.1088/1748-9326/7/2/024024/meta
11- Nathalie Butt, Paula Afonso de Oliveira & Marcos Heil Costa (2011). Evidence that deforestation affects the onset of the rainy season in Rondonia, Brazil JGR Atmospheres, Volume 116, Issue D11. https://doi.org/10.1029/2010JD015174
12 – WWF, Amazon Deforestation. Accessed 20th of January 2019 retrieved from http://wwf.panda.org/our_work/forests/deforestation_fronts/deforestation_in_the_amazon/
13 - Berenguer, E., Ferreira, J., Gardner, T. A., Aragão, L. E. O. C., De Camargo, P. B., Cerri, C. E., Durigan, M., Oliveira, R. C. D., Vieira, I. C. G. and Barlow, J. (2014), A large-scale field assessment of carbon stocks in human-modified tropical forests. Global Change Biology, 20: 3713–3726. https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.12627
14 - N.HBatjes, J.ADijkshoorn, (1999). Carbon and nitrogen stocks in the soils of the Amazon Region. Geoderma, Volume 89, Issues 3–4, Pages 273-286. Accessed 20th of January 2019 retrieved from https://www.sciencedirect.com/science/article/pii/S001670619800086X
15 – WWF, Impacts of climate change in the Amazon. Accessed 20th of January 2019 retrieved from http://wwf.panda.org/knowledge_hub/where_we_work/amazon/amazon_threats/climate_change_amazon/amazon_climate_change_impacts/
Written by Rob Gamesby