Project Amazonia: Characterization - Abiotic - Climate

Anthropogenic deforestation of the Amazon rain forest is fueling climate change.¹  This change is unlike any previous: it is largely preventable and occurring rapidly.²  The effect of this climate change is the failure of the Amazon rain forest ecosystem, due to complex non-linear interactions between atmosphere, land, and vegetation.¹ The overall picture is simple: the Amazon climate will be thrown into a different climatic equilibrium and the forest below will not survive.¹ 

Two main forms of deforestation create different drives for climatic change, but both forms appear to lead to the same result:

1) Patchy Deforestation

This form creates a landscape alternating between rain forest and grassland/deforested regions.  In this case, variable forcing creates local convection currents due to the uneven heating of the atmosphere.  Deforested areas have two main effects: changes in albedo (reflective properties of the land), and less evaporation/roughness.  The result is a net radiation change.  The direction of the change appears to be negative (a decrease in net radiation), the contrast between forested/deforested areas creates local atmospheric circulation.  These circulation cells force moist air up, producing clouds.¹

2) Mass Deforestation

The driving force of climate change in this case is increase in temperature and decrease in moisture. 

When an area of trees is cleared, the humidity in that region decreases because transpiration no longer occurs.  This loss of precipitation recycling significantly decreases rainfall, resulting in a drier region in the vicinity of the deforestation and wetter regions thousands of miles away.² Normally, rain is formed through water vapor condensation, which releases heat.  Heat causes convection, transferring the heat across the globe.  A decrease in this heat transport will change dynamics of global climate.⁴

Two hypotheses predict a dryer, hotter atmosphere.  Both cite the formation of smaller, thinner clouds as a result of moisture decrease.  Such clouds cover less area and reflect less solar radiation.  Whether the net effect is global cooling or warming is highly debatable.

There are two opposing effects caused by less cloud coverage.  First, fewer clouds in the atmosphere will allow the earth to release heat more efficiently.  At the same time, fewer clouds result in less blocking of the sun’s radiation. 

The Iris hypothesis states that heat loss will outweigh heat gain: global cooling.³  This would be a great negative feedback system for climate control.⁴ However, results from the Tropical Rainfall Measuring Mission (TRMM) satellite launched by NASA suggest the opposite.  Clouds and the Earth’s Radiant Energy System (CERES) has modeled the exchange of energy and has found that the amount of heat let in by these clouds is much more than the amount of heat that escapes.  Thus, the net effect is global warming.⁵

Conclusion

Both deforestation situations create a positive feedback loop due to a decrease in precipitation recycling.  This recycling is a measure of rainfall due to moisture evaporating from the land surface.² Deforestation decreases transpiration.  Transpiration is a large source of total evapo-transpiration from any rain forest region.⁶  As deforestation removes biomass, recycling decreases.  A decrease in recycling reduces rainfall which, in turn, creates the positive feedback system further reducing vegetation.¹

Amazon deforestation is a great danger to the integrity of the rain forest ecosystem.  The region cannot be considered a certain amount of vegetation, but instead as a living, breathing system.  Damage to any parts can and will effect the entire system, and enough damage will destroy the Amazon Basin rainforest.

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