The biosphere is the part of the Earth that is occupied by living organisms, encompassing all ecosystems at the Earth’s surface. The terrestrial biosphere is made up of all ecosystems on land, including the six primary biomes: tundra, taiga (boreal or coniferous forest), temperate deciduous forest, grassland, desert, and tropical rainforest. Together this makes up approximately 28.3% of Earth’s surface. Whilst this number seems small, the relative influence of vegetation on weather and climate is huge. Vegetation, particularly rainforests due their high Net Primary Productivity (NPP = photosynthesis – respiration), acts as one of the world’s major carbon sinks. Like all organisms, plants release CO₂ in respiration, but this is outweighed by the amount of CO₂ that is taken in during photosynthesis (the process by which plants use sunlight to manufacture nutrients using CO₂ and water):
In doing this, plants take in CO₂ from the atmosphere, thereby partly counteracting the effects of human activity and minimising human-induced climate change. Plants also carry out a process called evapotranspiration where water is absorbed by osmosis at the roots and transported up to the leaves through the xylem (a tube in the stem that transports water from root to leaf). In the leaves, the presence of lots of water in the cells makes them turgid (swollen) causing the stomata to open (tiny pores on the underside of a leaf that allow gas exchange to occur). This allows water vapour to pass out of the leaf by transpiration and evaporate into the atmosphere. This is vital in the process of cloud formation. For example, the frequent cloud cover in the rainforest is caused by the hot temperatures at the Equator and the ready availability of water that both contribute to the increase in the rate of evapotranspiration. Water vapour rises rapidly and condenses to form clouds resulting in the high levels of precipitation that the rainforest takes its name from. Vegetation also changes the reflectivity of the Earth’s surface (albedo) as vegetation has a very low albedo of around 0.02 leading to strong absorption of solar radiation. However, the heat stored by this process is counteracted by the cooling effect of evapotranspiration.
The ability of vegetation to act as effective carbon sinks could be affected by changing climate as a result of human activity. On one hand, higher levels of atmospheric CO₂ will increase photosynthesis within ecosystems to a point, but eventually increased temperatures and irregular rainfall could encourage forest dieback or an increased frequency in wildfires which will instead contribute further to rising atmospheric CO₂ concentrations. Changing human land use has also resulted in deforestation and reduced productivity further reducing the ability of ecosystems to act as effective carbon sinks. The way that terrestrial and atmospheric processes are intrinsically tied together means it is vital to build the terrestrial biosphere into our climate models.