The Role of Human Activity in Permanent Changes to Both the Carbon and Water Cycles in Tropical Rainforests - Izzy

Human activity can be categorised as the actions of humans having an impact on a process in this case, the water or carbon cycles. It can take many forms such as deforestation, with food based drivers such as palm oil plantations or soya beans for livestock feed. Mining can also occur in tropical rainforests which will affect the water and carbon cycles. Human activity can contribute to the symbiotic relationship between rainforests and the climate which affects the water and carbon cycles. As deforestation occurs, there is a reduction in evapotranspiration which leads to less atmospheric humidity and changing rainfall patterns, in some cases drought. As well as this, global warming can lead to drought and a contributor to this is human activity such as the slash and burn technique. This interrelationship between the climate and tropical rainforests is greatly due to human activity which can alter the magnitude of stores and flows. There are many examples of rainforests where humans have had an impact on the   carbon and water cycle in many different ways such as releasing carbon stores into transfers and increasing flows in the water cycle. Humans can have a larger impact on the water and carbon cycle in rainforest environments because of how the rainforest is affected by the cycles for example, the high levels of precipitation.

Rainforests have characteristics which are specific to that environment such as the high levels of biodiversity as 200 million species of plants and animals live in the rainforest. Some features which are specific to rainforests include a very high annual rainfall of 2000mm per year and the absorbance and emittance of large quantities of oxygen. There is much opportunity for humans to affect the water cycle of the rainforest as the dense canopy intercepts up to 75% of the rainfall. The remaining 25% of this rainfall, dome drips on to the ground or through trees as stemflow. Evaporation rates are high so 25% is evaporated with half of the other 75% is returned to the atmosphere. Because of this, the human removal of trees has a huge impact on the water cycle in the rainforest. Deforestation can cause a decrease in the humidity of the atmosphere and consequently supressed precipitation and a reduction in evapotranspiration. There is also increased overland flow as the number of trees is reduced. The higher rates of runoff encourage flooding. The large number of impacts of deforestation which alter the water cycle. The potential of change to the water cycle by human activity is large particularly in rainforest environments. Similarly, the effect of human activity on the carbon cycle is significant with photosynthesis reducing until the area becomes colonised by new plants. This is the most significant impact of deforestation on the carbon cycle as rainforests become sources of carbon dioxide rather than sinks which happened after the flood in the Amazon basin in 2005 where 8 billion tonnes of carbon dioxide were released due to human activity. An increase of carbon in the atmosphere is also caused by the burning of trees which releases a large quantity of carbon as trees are a carbon store and 50% carbon. Plant and animal respiration decreases as there is less oxygen available as trees are deforested. This does reduce the amount of carbon given off as does the ash washed into the ground by rainwater. There are significant impacts of human activity on both the water and carbon cycles as actions such as deforestation can have knock on effects which can be permanent.

In the Amazon Basin, 17% of the primary rainforest has been lost in the last 50 years which is due to human activity. This has repercussions on both the water and carbon cycles. Some effects of this can be in the form of reduced evapotranspiration which causes decreased cloud cover. The cleared area encourages the reflection of solar radiation and a rise in temperature which in turn promotes evaporation. The level of precipitation in the Amazon Basin is altered through human activity as the radiation reflectivity reduces the influence of the low pressure system, reducing rainfall. The discharge of rivers is changed by human activity. On a watershed level, the increased surface runoff adds to the total discharge of a river. However, on a continental basin scale, there is a reduction in river discharge due to the decreased precipitation. These effects on the water cycle however, although they are significant, there are other processes involved which may lead to them being temporary. Processes which impact the carbon cycle include more frequent wildfires which are due to open gaps in the forest from deforestation. The wildfires cause the destruction of trees as well as releasing carbon dioxide into the atmosphere and when the primary forest is replaced, the forest scrub stores 40% less carbon. This is an indirect impact of human activity on the carbon cycle in the Amazon basin. There have also been impacts on the carbon cycle in Madagascar where biomass carbon is quickly transferred into the atmosphere through the slash and burn method of clearing land. These impacts on both the water and carbon cycle are evidence of the long-lasting impacts humans can have on tropical rainforests. Despite this, the permanency of their impacts is not determined as many of the fluxes and stores affected by humans can be counteracted.

However, there are ways in which the actions of humans can start to be reversed. The impact that humans have on the water and carbon cycles such as deforestation which promotes surface runoff and releases a store of carbon dioxide into the atmosphere can be counteracted by human processes such as afforestation which will result in more carbon being absorbed and the interception of rainfall. The crops planted to replace rainforest plants such as soy bean plants do photosynthesise and although not as much carbon dioxide is taken in as would be by primary rainforest, there is not a permanent reduction in the absorption of carbon dioxide from the atmosphere due to human activity. In Indonesia, the bureaucracy is showing attempts to reduce land being cleared outside of their allocated areas meaning that extra carbon dioxide is not released into the atmosphere. This takes place in the form of fines and arrests. An individual called Mr Teguh is attempting to return the carbon stores and flows to their previous magnitude by planting rows of native hardwood saplings to help reforest peatlands in Kalimantan and Sumatra. As well as this, an example of human activity taking place with minimal impact on the carbon cycle is in Malaysia where there are strict selective logging management and replanting schemes. This is sustainable and ensures that humans have little impact on the water and carbon cycles. This shows that as well as the large impact of humans, which can take place in certain areas of the tropical rainforest there is little change to the water and carbon cycles in other areas.

Human activity in tropical rainforests can take on many different forms and affect different cycles in different ways. Therefore, it is difficult to determine the extent of the impact on the water and carbon cycles. In two cycles which are constantly changing and the fluxes and stores are altering in magnitude continuously, humans have an impact alongside many other factors. There is evidence to support humans having an impact on both cycles but there are also processes taking to offset these alterations. Human activity in the present is arguably the leading cause of change to the carbon and water cycles in the rainforest but there are other causes such as climate change.