What are the 10 effects of deforestation

Deforestation (tropics)

Geographical location of the tropical rainforests

In contrast to deforestation in Europe, the deforestation (deforestation) of tropical forests is a relatively new development that did not begin until the middle of the 20th century, when modern chainsaws and other equipment became available. Since then, ever larger areas have fallen victim to clearing. Apart from the loss of biodiversity and human culture, which should not be discussed here, the deforestation of the rainforests also has consequences for the climate, especially on site.

1 water cycle and its modeling

Since the beginning of the 1980s, numerous model simulations of extensive deforestation in the tropics have been carried out, most of them limited to the Amazon. It turns out that the uncertainties are greater here than with boreal deforestation. The reason for this is the lack of description of the water cycle, which is strongly influenced by the vegetation. Only through the use of models of the terrestrial biosphere in the climate system that explicitly describe photosynthesis and the water turnover of plants has it been possible to estimate these processes at all since the end of the 1990s. As in the high latitudes, deforestation also increases the albedo, but the direct influence on the water cycle is of greater importance for the temperature. Due to the good availability of water, the many surfaces and the high temperature, the tropical rainforest evaporates a lot of water, which in turn can fall to the ground again as precipitation. It is estimated that 25 - 50% of the precipitation in the Amazon is recycled - the forest itself partly provides the precipitation that it needs to live. This constant evaporation and "sweating" of the forest has a cooling effect, so that in the event of deforestation there would be a high probability of local warming between 0 and 3 ° C (the result depends, among other things, on the vegetation replacing the forest - including many types of grass and cereals have a high transpiration - at least before harvest). Temperature measurements in deforested areas also show a warming, in particular due to stronger extreme values. Without the trees, the soil would dry out because the precipitation would run off immediately. Evaporation from the ground would be even higher (because it is more likely to be prevented due to the slowed down wind by the foliage), but the provision of large surfaces and the tapping of ground water by the trees has a far greater effect. Especially in the dry season, the difference in evapotranspiration and therefore also in temperature is large, since a forest can then continue to release water into the atmosphere, but a deforested area cannot. This is at least suggested by numerous observations, but the appropriate consideration of the water cycle with consideration of the vegetation is still a weak point of climate models, so that not all simulations can confirm this finding.

2 Changes in precipitation due to deforestation?

Deforestation of tropical rainforest in Panama

Despite the reduced evapotranspiration and reduced water recycling, the change in the amount of precipitation is uncertain. As expected, the majority of the models show a decrease. However, it is also conceivable that instead more moisture convergence takes place, i.e. water is brought in from other regions, which compensates for the lack of evapotranspiration. This approach of humid air masses is facilitated by the fact that the treetops offer a resistance to the wind, which is much less after a large-scale clearing. In the Amazon, the high Andes mountain range is also a barrier for moisture transport. Therefore, the water, which is mainly supplied to the area from the east, is difficult to transport in the west and is returned to the Amazon basin via the rivers.[1] This natural recycling is of course not affected by deforestation. On the other hand, if the ground roughness is lower (and thus higher speed), the wind does not turn so strongly in the direction of the low air pressure due to the Coriolis force, which in turn prevents moisture convergence. This effect could predominate in Africa and Asia. Since precipitation in the tropics is mainly generated by deep convection (i.e. thunderstorms), a lot depends on how much energy is available on the ground for convection. Both latent and sensible heat are relevant here. The latent heat in the form of moisture is so important that, despite the increased temperature, there may no longer be enough energy available for convection. The rule of thumb "warm air rises, cold air sinks" turns out to be misleading! Whether enough moisture can be brought in seems to depend on the size of the deforested area. In small areas of the order of a few kilometers (as they have been deforested up to now) there is enough water available from the surrounding forest areas and more cloud cover and precipitation occur because the local increase in temperature favors convection. An increase in cloud cover and convection over the Amazon in the last few decades can also be seen from direct measurements and simulations of the past. In contrast, precipitation has not increased.

Leaf index of the Amazon region

In the case of large-scale deforestation, however, the effect can be reversed due to the lack of moisture. Since the sum of sensible and latent heat is fed by the net radiation (see land use), the albedo change is also relevant in the tropics - but for the precipitation and the local circulation, and less for the temperature. Paradoxically, however, it is not the direct influence of the albedo that leads to less net radiation being absorbed on the ground. When it is less cloudy, more sunlight arrives there, which roughly compensates for the effect of the higher albedo on the earth's surface - seen from space, the albedo remains almost unchanged.[2][3] The net radiation, on the other hand, only falls indirectly as a result of higher long-wave radiation (because the temperature on the ground is higher) and lower counter-radiation (because there is less water vapor in the atmosphere). If the albedo rises above a certain threshold, there is less convection, less cloud cover and less precipitation[4] . In Africa, where the soils are particularly bright, this effect could be strongest and the temperature rise the least. There is also another feedback: dry soils have a higher albedo, so that even less energy is available on the surface, which inhibits convection and thus further exacerbates the lack of precipitation. Overall, it can be seen that precipitation in the tropics depends on all three influencing factors, evapotranspiration, roughness and albedo, and that the end result is very difficult to predict. Exceptions e.g. for the summer monsoon in East India and Bangladesh, where large-scale deforestation would presumably lead to more humidity convergence, since the air comes directly from the ocean. The monsoon circulation would therefore be intensified.

3 Global Consequences

On a global scale, however, influencing the processes in the intra-tropical convergence zone (ITCZ) is particularly relevant. In this area, the high-reaching tropical thunderstorms hit the upper limit of the troposphere, so that the air flows apart and transports moisture and energy to higher latitudes. Several thousand such "hot towers" are active at any given time. A change in their distribution would have global consequences. The subtropical jet, a band of high wind speeds at around 30 °, also receives its energy in this way.

In addition, the change in the convection centers in the atmosphere stimulates waves that, under certain conditions, can propagate to medium latitudes. Since the causes of land use changes persist for a long time, such permanent wave anomalies can lead to a changed climate even in places outside the tropics. Climate models show that the deforestation that has already taken place in the tropics could have led to a northward shift and weakening of both the jet and the ITCZ.[5] Observations suggest that climate changes caused by deforestation interact with natural climate fluctuations.

How the (purely biogeophysical) influence of tropical deforestation is on the global mean temperature has not yet been fully clarified. Although the tropics are warming in the regions of deforestation, due to the severely weakened evapotranspiration, less water vapor is exported to higher latitudes. As a result, the greenhouse effect is weaker all over the world, sea surface temperatures are falling and even less water is evaporating as a result. It is possible that this effect is globally (biogeophysically) dominant; However, it cannot compensate for global warming due to the release of carbon dioxide from the biomass of tropical forests.[6]

4 individual proofs

  1. ↑ Lean, J., Rowntree, P.R. (1997): Understanding the Sensitivity of a GCM Simulation of Amazonian Deforestation to the Specification of Vegetation and Soil Characteristics. Journal of Climate, 10, 1216-1235
  2. ↑ Betts, A.K. (1999): Self-beneficial effects of vegetation on climate in an Ocean-Atmosphere General Circulation Model. Geophysical Research Letters, vol. 26, no.10, 1457-1460
  3. ↑ Zhang, H., Henderson-Sellers, A., McGuffie, K. (1996): Impacts of Tropical Deforestation: Part I: Process Analysis of Local Climate Change. Journal of Climate, vol. 9, 1497-1517
  4. ↑ Mylne, M.F., Rowntree, P.R. (1992): Modeling the Effects of Albedo Change associated with Tropical Deforestation. Climatic Change, vol. 21, 317-343
  5. ↑ Chase, T.N., Pielke Sr., R.A., Kittel, T.G.F., Nemani, R.R., Running, S.W. (2000): Simulated impacts of historical land cover changes on global climate in northern winter. Climate Dynamics, vol. 16, 93-105
  6. ^ Claußen, M., Brovkin, V., Ganopolski, A. (2001): Biogeophysical versus biogeochemical feedbacks of large-scale land cover change. Geophysical Research Letters, vol. 28, no.6, 1011-1014

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