Part 3.5: Amazon rainforest
The Amazon rainforest is the largest rainforest in the world. It stretches across nine countries in South America and is twice the size of India. The lush vegetation is a paradise for millions of species of plants, insects, birds and animals.
As the name suggests, a rainforest is sustained by very humid conditions. But the forest itself also plays a crucial role in the local climate. When the forest is saturated by heavy rainfall, much of the moisture is released back into the atmosphere through evaporation. In addition, transpiration of moisture from plant leaves transfers water from the soil into the atmosphere. These two processes together are called “evapotranspiration”. This keeps the atmosphere moist, but also contributes to convection — a strong upward movement of air — that ultimately leads to clouds and more precipitation. Research published in the 1970s shows that the Amazon rainforest produces about half of its own precipitation.
Why is the Amazon rainforest a tipping point?
As we saw above, the Amazon rainforest is a self reinforcing system. Reducing the amount of rainfall or the amount of forest can shift the local climate into a drier state that cannot support a rainforest. There are three potential causes of how this could happen:
- Deforestation: Fewer trees means less evapotranspiration and less moisture entering the atmosphere.
- Reduced transpiration in response to higher CO2 levels: The microscopic pores of plant leaves open less wide at higher CO2 concentrations in the atmosphere. The plants therefore lose less water, and less transpiration means less water going back into the atmosphere.
- Decline in precipitation due to warming climate: Model projections suggest that this would be a result of “specific patterns of sea surface temperature (SST) change in the tropical Atlantic and Pacific oceans,” but there is wide variation among models in how severe this impact on the Amazon rainforest would be. (Source)
Where is the tipping point of the Amazon rainforest?
In absence of other contributing factors such as deforestation, climate models predict that the tipping point is around 3–4 °C warming above pre-industrial levels. But we can’t ignore deforestation. Without global warming, at ~ 40% deforestation about 60–70% of the Amazon rainforest would turn into a dry savannah. When climate change and the widespread use of fire are taken into account, the tipping point of deforestation in the eastern, southern, and central Amazon approaches 20–25%(Source). Currently, we have already cleared 17% of the Amazon rainforest.
What would happen if we crossed the tipping point?
Beyond the tipping point, large-scale forest dieback would occur and the rainforest would turn into a savannah — a drier ecosystem dominated by open grasslands with few trees, resulting in a massive loss of biodiversity. The reduced evaporation and convection would alter atmospheric circulation globally, affecting weather patterns around the world. Increased release of CO2 through forest fires and tree death would accelerate global warming even more. Additionally, with the forest gone, we would also have lost an important carbon sink. This acceleration of global warming could also trigger other tipping points.
Back to the overview of the tipping points.
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