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.
Tropical rainforests cover just 6% of the earth's surface but they account for 30-50% of global photosynthesis. Amazon rainforest cycle changes Estimates show that the Amazon basin has lost over 17% of it's primary forest in the last 50 years. This is mainly die to the use of cattle ranching (80%). This is mostly in the South and East parts of the basin (brazil) and in the north-western arc (Columbia and Ecuador). The Amazon basin produced roughly 1/3 of its precipitation by recycling evapotranspiration (EVT), the other 2/3's from moisture laden air from the Atlantic Ocean. As land is cleared of primary forest, the hydrological and carbon cycles are being effected.
Reduced EVT from cleared areas mean the air is less moist, reducing cloud cover. More solar radiation is reflected cleared land and temperatures increase.
Greater radiation reflectivity and increased ascention of warm air reduce the low pressure system over the amazon, that draws in water from the Atlantic, leading the lesson precipitation.
'Vegetation breezes' occur over cleared land as air above crop/ranch land warms faster, rises more quickly and creates low pressure, drawing in moist air from nearby forests. This leads to increased cloud cover and rain over cleared land.
River discharge increases on a water shed scale as a result of deforestation as increased surface flow discharges into local river systems.
River discharge increases on a watershed scale as a result of deforestation as increased surface flow discharges into local river systems.
River discharge decreases on a continental basin scale as decreasing precipitation leads to a reduction in river discharge.
Particles resulting from deliberate burning of forests to clear for agriculture or the production of charcoal, increases the density of airborne aerosols around which water vapour condenses. As a result, smaller droplets occur in clouds which are too small to precipitate, resulting on less localised rain.
Selective logging of mahogany and ipe often damages neighbouring trees, leading to open gaps in the forest which are more exposed to sunlight and winds, therefore they dry easier and are more prone to wildfires. forest scrub that replaces trees absorbs 40% less carbon.
When burned for land clearance, between 30-60% of the carbon in tropical rainforests is released into the atmosphere. The size of the Brazilian Amazon Rainforest has been decreasing at an average rate of 0.03% per year snce 2000.
In a typical year the Amazon absorbs 2.2 billion tons of CO2, while emitting 1.9 billion tons through decomposition and organism respiration. This makes it a major carbon sink. However, this ability to absorb carbon is rapidly declining, in 1990 it absorbed 2 billion tons and by 2015 it absorbed only 1 billion. The reason for the decline is due to plant death. Some believe that increased carbon being released into the atmosphere can promote biomass growth carbon fertilisation) , however it also triggers them to die sooner. This increased metabolic stress has been compounded by recurrent drought, unusually high temperatures, continues illegal logging and conversion to agriculture. The rainforest is under threat from direct human activity and climate change arising from indirect action in releasing stored hydrocarbons in massive quantities.