Deforestation and Feedback Loops

What if the world's largest rainforest became a savanna?

Rainforests can be found on every continent except Antarctica. They come in two types. Temperate rainforests are located away from the equator so are cooler and receive less, though still a lot of, rain. The soil holds a lot of nutrients since decomposition is slow. Examples on the US west coast include Olympic and Redwood National Parks.

Tropical rainforests are hot, humid, ecosystems that can receive 400 inches of rain each year. Much of the biodiversity lives in the canopy. Organic matter decomposes quickly, resulting in a thin layer of soil with few nutrients. If trees are removed the soil becomes even more barren and susceptible to flooding. It is unsustainable beyond a few years for agriculture and ranching use because it quickly is depleted of nutrients. The Amazon jungle is the largest tropical rainforest.

Rainforests are important as habitat, but what’s interesting to me is their role in the water cycle. Water in the Amazon comes inland from the Atlantic Ocean and falls as rain. Roots absorb the water, which then evaporates from leaves to form clouds. A rainforest recycles its own moisture, absorbing it during the hot daylight hours and releasing it into the atmosphere at night.

Rainforests store vast amounts of carbon, but half have been destroyed in the past hundred years due to fire and activities including logging, cattle ranching, agriculture and mining. This destruction continues today as more forest is cleared. Prior to the industrial age there was a balance in the carbon cycle that maintained global temperatures as the sun’s radiation provides energy for warming and photosynthesis for natural carbon capture and oxygen release. This balance was stable for the last 10,000 years, until we began releasing long stored carbon by burning fossil fuels. These greenhouse gases keep the sun’s energy in the atmosphere and oceans, with warming driving drought conditions and wildfires.

The rainforest ecosystem is built around water. If it loses the core area needed to maintain the moisture it will revert to savannah, changing wind and weather patterns worldwide and interacting with climate change in unexpected ways (some good and some bad). That is the concern from shrinking the size of the Amazon, that it will upset the carbon and water cycles.

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I have previously written about the climate impact from feedback loops involving the albedo effect, where melted ice causes more sunlight to be absorbed, and permafrost melting, where frozen methane is released as the tundra warms. Other concerns have included slowing ocean currents and drought fed wildfires. Another complex ecosystem with global warming implications are tropical rainforests. Each of these trends is subject to a tipping point that is hard to anticipate in advance.

Tropical rainforests are fragile. The Sahara region in Africa transitions between dry and wet due mainly to changes in the earth’s tilt about every 20,000 years but agriculture, mainly grazing, may have accelerated the most recent drying process. The Sahara was last a lush green, fed by monsoon rains, about 11,000 years ago.

The Amazon in particular is at risk of becoming a savanna within a generation due to deforestation. Forests are being cut or burned, warming the atmosphere and reducing fresh water availability as a direct consequence of this feedback loop. Other places, like North America, will see indirect impacts as weather patterns change to reinstate a stable balance.

Scientists extrapolate the tipping point from deforestation in Amazonia to happen within 15 years. Emerging risk scenarios being built today should consider a collapsing Amazon ecosystem as a stress test. How will changes to weather patterns impact agriculture and economic outcomes? This is not a storyline that we are comfortable addressing today. We haven’t been trained to do that. We will need to expand our knowledge about changes to climate and its ramifications to food supplies, mortality and morbidity, economic growth and finally interest rates. Narrative scenarios will become more important in years to come. Interactions between risks are becoming more important, including feedback loops and changing trends about assumptions. Even if the analysis is qualitative, it is important to expand our thought process to include environmental risks along with other factors driving regime change.