August 16, 2023

The effects of climate change: from rainforest to savanna

I’m standing on a tall tower, higher than the highest tree, looking out over vast stretches of rainforest…and suddenly lots of things become clear to me. First, I never knew that a fresh breeze could bring such relief from the humid, hot forest. Second, wasps love to build their nests on high wooden or metal structures and getting stung in the face is unpleasant, but thank god for that cool breeze. Third, seeing the forest from above in the early morning totally explains the term ‘hydrological cycle’, because I can practically see the trees evaporate water droplets into the air, form clouds and fall down as rain drops further away onto that same forest after being carried by that amazing breeze. 

The Amazon rainforest makes its own rain. And less forest means less precipitation. As forest destruction affects climate and vice versa, the concern is that the Amazon will be caught up in a set of feedback loops that could dramatically speed up the pace of forest loss and degradation and bring the Amazon to a point of no return. This ‘tipping point’ may occur when a certain percentage of Amazonian habitat dies, after which it will all turn into a savanna-like ecosystem. 

While it’s unsure how long we have until the Amazon reaches this tipping point, significant changes are already happening to weather patterns, the forest and the animals. Are we currently balancing on the brink of this tipping point?

Fires and droughts

Some ecosystems need fire to stay alive. For example, in parts of Europe and Northern America, grassland seeds can only sprout after a good ol’ fire. Lightning can create a much-needed spark and rejuvenate the ecosystem. However, this is not the case for a tropical rainforest. Fires aren’t natural and they are very destructive. 

In tropical rainforests, fires aren’t natural and they are very destructive. In 2022, 983 major fires were detected across the Amazon, impacting nearly 1 million hectares (2.5 million acres).

In 2022, 983 major fires were detected across the Amazon, impacting nearly 1 million hectares (2.5 million acres). These fires burned a considerable amount of biomass, often on recently deforested land. The fires are caused by humans, preparing their land for agriculture, logging, but mostly cattle ranching. A survey released by the Amazon Environmental Research Institute (IPAM) in October 2021 showed that cattle pastures occupied 75% of the deforested area on public lands in the Amazon. 

All those fires, and subsequent barren land, have an effect on the local climate. Evapotranspiration from the forest contributes up to 41% of the mean rainfall over the Amazon.  

And now, for the first time, researchers have proven a clear correlation between local deforestation and regional precipitation.

The study found that the more rainforests are cleared in tropical countries, the less local farmers will be able to depend on rain for their crops and pastures. Annual precipitation changes could cause crop yields to decline by 1.25% for each 10-percentage-point loss of forest cover, potentially intensifying future climate change and drought events. The hope is that these results may encourage agricultural companies and governments in the Amazon, Congo basin regions and south-east Asia to invest more in protecting trees and other vegetation.

A rainforest without rain isn’t right, and extreme drought is already a reality in certain parts of the Amazon. Current data show that the dry season has expanded by about 1 month in the southern Amazon since the mid 1970’s. Meanwhile, the western Amazon has been hit by multiple “once-in-a-century droughts”—one in 2005, 2010 and again in 2015/2016. This could be caused by a pressing combination of a warming climate and human population growth that the region has never faced before. Prospects are that dry spells may become more frequent as temperatures in the tropical North Atlantic Ocean rise and as humans continue to burn thousands of square kilometers of forest for farming.

These occurrences and corresponding studies add to fears that the Amazon is very rapidly approaching a tipping point after which the rainforest will no longer be able to generate its own rainfall and the vegetation will dry up. 

Carbon sink to carbon source

Basic biology teaches us that trees and plants take up carbon dioxide in their tissues and provide oxygen in return. Thanks to that, animals are able to breathe and survive. (Thank you trees!) This process also shows that forests can be an important ally to halt climate change, but…only under the right circumstances. 

While healthy tropical forests are a sink of (capture) atmospheric carbon dioxide, degraded forests are a source of (release) carbon dioxide. When trees die or get burned, they emit the carbon dioxide that was stored in their tissues. Whether a forest is a greenhouse gas source or sink depends on local losses including deforestation, biomass burning, and tree mortality. 

From Carbon Sink to Carbon Source. Whether a forest is a greenhouse gas source or sink depends on local losses including deforestation, biomass burning, and tree mortality. 

To make correct predictions about the level of CO2 capture and release, we have to look at the landscape more closely. The Amazon includes not only intact forests, but also:

• degraded and logged forests
• natural non-forests
• agricultural and urban areas
• aquatic systems including wetlands

All of which contribute to regional carbon cycling. A recent study used 12 years of satellite data and found that parts of the Amazon have become a net carbon source, due to deforestation and reductions in carbon density within the local standing forest. In some regions, the output of CO2 has become larger than the uptake of CO2 by forest growth, which can be considered as already having ‘tipped over’ into an unsustainable situation.  

On a global scale, some forests have become clear carbon sources, while others remain carbon sinks. It has been shown that over the past 20 years, forests across Southeast Asia have collectively become a net source of carbon emissions due to massive clearing for plantations, uncontrolled fires and drainage of peat soils. The Amazon rainforest is a source in some places, and a sink in others. Of the world’s three largest tropical rainforests, only the Congo has enough standing forest left to remain a strong net carbon sink. The Congo’s tropical rainforest sequesters 600 million metric tons more carbon dioxide per year than it emits, equivalent to about one-third of the CO2 emissions from all U.S. transportation.

In total, all global forests combined, forest are currently a net carbon sink (−7.6 ± 49 GtCO2e yr−1), reflecting a balance between gross carbon uptake (−15.6 ± 49 GtCO2e yr−1) and carbon emissions from deforestation and other disturbances (8.1 ± 2.5 GtCO2e yr−1). The authors stress that protecting the remaining forests in all three regions is critical to mitigating climate change. They predict that in the near future, tropical forests are likely to become a carbon source, due to continued forest loss and the effect of climate change on the ability of the remaining forests to capture excess atmospheric carbon dioxide. This will make it harder to limit global warming to below 2 °C. 

To see these carbon gas fluxes projected on a detailed world map, check out this Global Forest Watch ‘greenhouse gas net flux’ data map.
Check it out

Two of the scientists that are studying these atmospheric gas fluxes in the Peruvian Amazon are prof. Eric Cosio and engineer Fabian Limonchi, from the Pontificia Universidad Católica del Perú. They are pro’s in dangling from observation towers at dizzying heights – at least 196 ft / 60m, way above the canopy – connecting nuts with bolts from their scientific equipment in order to measure the air above the trees. The installed gas flux meters measure carbon, methane, and water vapor in the atmosphere. Their study helps to understand when forests act as sinks or sources of these gasses, and also show how they differ in different ecosystems – primary forest, secondary forest, wetlands and Andean highlands. 

Fabian: ‘The project started in 2011, with the establishment of the first tower in the Tambopata region in southeastern Peru, as part of a global project initiated by SAGES.’ SAGES stands for the Scottish Alliance for Geoscience, Environment and Society.  ‘They already had a lot of data from the Brazilian Amazon, but not from Peru. This part of the Amazon has different soils and different rain patterns, and it is close to the Andes which creates an overall different environment and therefore accurate local measurements were needed.’ The global effort of measuring atmospheric gas requires every party to use the same methodology. The technique that is used is called ‘Eddy covariance’ – a micro-meteorological method to pick up minute changes in gas fluxes by taking measurements every minute, every hour, every day, month, and season. 

So, when are tropical forests carbon sinks and when are they sources? Besides the obvious results that forests are carbon sources when they are on fire, Fabian explains that the patterns indicate that when the local climate is dry, the forest becomes a source. “We see that when the evapotranspiration stops, the trees also stop taking up CO2. This can be a temporary change, because when the rainy season starts, the forest should directly switch back to being a sink.” 

Dr. Eric Cosio specifies: “In terms of net CO2 exchange, the forests in Tambopata represent a yearly net source of CO2 to the atmosphere [~5 MgC/ha/yr]. These emissions were likely related to an increase in mortality of big trees as shown by the analysis of data from permanent forest plots. Mechanisms for this increase in tree mortality include, among others, extreme weather events such as prolonged droughts.” He visited the American Geophysical Union Conference last year and fellow scientists pointed out that forest resilience to extreme weather events is being affected, with especially the larger trees dying. “This may trigger a successional cycle in these forests with uncertain outcomes.”

How can climate science help protect the forest, according to climate scientists? Fabian thinks that publishing more of the same scientific results doesn’t really help much, he says: “Simplify the message and educate people! We need politicians to take action, for example by creating alternative living opportunities that do not negatively affect the forest. But not only Peruvian politicians – this is a global issue which needs global efforts.”

Sensitive species

A polar bear on melting ice is an obvious image that comes to mind when thinking of climate change affecting biodiversity. Species in the tropics are already used to the heat, so would it matter if it gets a little hotter? The reality is that species in the tropics may not be adapted to climatic extremes at all, since the tropics have a relatively stable climate year round. Most tropical species are sensitive to outliers in temperature and drought. 

Studies show that animals are affected by climate change in various direct and indirect ways:

• A changing climate can cause habitat destruction by fire, floods, drought, ocean acidification etc., demanding individuals and entire species to move to new areas or perish. Likewise, a changing climate causes species to survive in altitudes and latitudes that they previously couldn’t withstand, expanding their home range and increasing competition with the species that were already there.
• Species have to adapt to shifts in seasons and concurring natural phenomena, for example by reproducing later or hibernating longer.
• Climate shifts can cause is a change within the species itself, in its physiology, to cope with extreme heat or cold. Summarized: species can respond to climate change challenges by adapting in space (e.g., range), time (e.g., phenology), and self (e.g., physiology). 

Within tropical birds, the effects of climate change are already visible. A study by Dr. Vitek Jirinec et al. (2021) showed that all 77 studied species of nonmigratory understory birds within Amazonian primary rainforest showed lower body weights since the early 1980s. Additionally, a third of species had increased their wing length as a response to increased temperatures. 

Dr. Jirinec explains that his multiple-year field study wasn’t a piece of cake: “I did this for my PhD dissertation, the field site was close to Manaus. It was very difficult to do climate-related research in Brazil, as there was lots of bureaucracy and this was even before Bolsonaro was installed as Brazil’s president.”

“We wanted to figure out what was happening to the bird communities in the forest. The overall decline in abundance was obvious, but we wanted to know if there were also changes in body condition. We were suspecting a decrease in body mass over time as a response to higher temperatures. In extreme heat, it pays off to have a larger surface area [skin area] to body mass [size] in order to get rid of excess body heat. If you are trying to conserve heat, it matters to have a small surface to body mass ratio in order to not lose body heat. Vice versa, you lose heat better with a large surface area, for example, think of small desert foxes with large ears.” 

Dr. Jirinecs hypothesis proved to be correct: they measured an increase in surface area to volume ratio, smaller birds with larger wings, across the whole community, even for the species that were increasing in abundance.

Bergmann’s rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. It seems that climate change is extrapolating this rule, making the smaller species even smaller. But can evolution move fast enough for these species to cope with yearly increasing extreme temperatures? And how small can the birds eventually get? Jirinec: “This evolutionary adaptation makes them survive climate change. But it has limitations, these species cannot shrink forever. We think that these understory birds are potentially already hitting that threshold.”

Jirinecs study also showed that climate sensitivity depends on where in the forest these birds are living: “There is a gradient in climate effects from the forest floor to the canopy. The climate fluctuations aren’t that profound at the ground since the trees act as a temperature buffer, but at the canopy level temperature fluctuates a lot. Canopy birds can move down towards the forest floor, but forest floor birds cannot move to even lower grounds. That’s why we see that understory birds are declining the most.” 

Jirinec agrees that this climate change effect on tropical birds isn’t easy to prevent: “It’s tough, tackling something like habitat fragmentation is much easier to solve. This study is yet another reason to combat climate-change on both a local and a large scale. Something that could be done fairly quickly is to create microclimate refugia, where birds are buffered from large-scale temperature fluctuations.”

While climate change affects biodiversity, it also works the other way around – biodiversity contributes to the ecological and climatic stability of the Amazon Basin. However, it is increasingly threatened by deforestation and fire. In Brazil, forest policies that were initiated in the mid-2000s corresponded to reduced rates of burning. However, relaxed enforcement of these policies in 2019 by president Bolsonaro has seemingly begun to reverse this trend: approximately 4,253–10,343 km2 of forest has been impacted by fire, leading to some of the most severe potential impacts on biodiversity since 2009.

Did we already reach the ‘tipping point’?

With this ecosystem maintaining its own rain cycle, vital for its survival, what would happen if this feedback-loop gets disrupted by extreme drought? In a 2009 opinion paper in the journal Environmental Sustainability, scientists Nobre & Borma first spoke of ‘Tipping Points’ for the Amazon forest. They warned that the forest-climate equilibrium could be disturbed by a number of human-caused changes, which would quickly and uncontrollably transition the rainforest into a savanna-like ecosystem. At that time, they assessed that the ‘tipping point’ will be reached when the total deforested area reaches more than 40% and for global temperatures would increase by 3–4°C. 

In 2018, Dr. Thomas Lovejoy (1941-2021) joined Dr. Carlos Nobre in using the term ‘Amazon Tipping Point’ to call for immediate action as a last chance for the Amazon. At that time, estimates for when the tipping point would be reached got a lot more conservative: ‘We believe that negative synergies between deforestation, climate change, and widespread use of fire indicate a tipping point for the Amazon system to flip to non-forest ecosystems in eastern, southern and central Amazonia at 20-25% deforestation.’ The authors pointed out that the severity of the droughts of 2005, 2010 and 2015-16 could well represent the first flickers of this ecological tipping point. These events, together with the severe floods of 2009, 2012 (and 2014 over SW Amazonia), suggest that the whole system is oscillating. 

Fast forward to the most recent insight on this soon-to-be-approached tipping point. It has been found to matter where this deforestation takes place. With the moisture cycles for rainfall coming from the Atlantic Ocean, from the east, it is most important to keep the forests in the east intact. But, as can be seen by the 2022 MAAP (Monitoring of the Andean Amazon Project) maps, it shows that already 31% of the eastern Amazon is gone. ‘This finding is critical’ the report says, as the tipping point will likely be triggered in the east. 

With the 20% deforestation limit already being exceeded in the eastern Amazon, it is hard to ignore the changes all throughout the forest. According to this recent study, for every three trees that die due to drought in the Amazon, a fourth tree, even if it’s not directly affected by drought, will also die. With fewer trees in the east to recycle moisture due to drought and deforestation, the rest of the Amazon becomes drier. Again, the authors press that nonlinear thresholds in the hydrological balance of the rainforest might be exceeded under drier future conditions, leading to self-amplified forest transitions.

If that isn’t daunting enough, it seems that the Amazon rainforest exhibits significant teleconnection with other global tipping points. This study identified a ‘strong correlation’ between temperature anomalies in the Amazon and the Tibetan Plateau, roughly 15,000 kilometers (9,300 miles) apart, over the past 40 years. Simultaneously, the study identified the same relationship with the Amazon and Antarctica.

Solution: large scale and local governance

Recently, a profile called ‘The Amazon We Want’ popped-up in my ‘suggested Instagram profiles’ feed. I didn’t think much of it at first, with less than 1k followers, and a clickbait-ish name. But I was wrong: ‘The Amazon we want’ initiative is the popular name for the Science Panel for the Amazon (SPA) of the United Nations Sustainable Development Solutions Network, founded in 2021. The SPA is composed of over 200 preeminent scientists and researchers from the eight Amazonian countries, French Guiana, and global partners. These experts came together to debate, analyze, and assemble the accumulated knowledge of the scientific community, Indigenous peoples, and other stakeholders that live and work in the Amazon. 

This is a first-of-its-kind Report which provides a comprehensive, objective, open, transparent, systematic, and rigorous scientific assessment of the state of the Amazon’s ecosystems, current trends, and their implications for the long-term well-being of the region, as well as opportunities and policy relevant options for conservation and sustainable development. 

The reports that they created are everything you’ve ever wanted and needed to know about the Amazon. They are incredible and open-access at www.theamazonwewant.org 

The solutions that they describe are based on scientific and traditional knowledge, guided by the principles and values of a ‘Living Amazon’- vision. This vision proposes a sustainable development model for the Amazon that is socially just, inclusive, and ecologically and economically flourishing. It recognizes the role of the Amazon in the 21st Century and the need for economies that can sustain ecological integrity and diversity, protect terrestrial and aquatic ecosystems, restore and remediate impacted ecosystems, empower Amazonian people, protect human rights and the rights of nature, and promote human-nature well-being. 

The solutions proposed are based on three pillars:

1. Conservation, restoration, and remediation of terrestrial and aquatic systems
2. Development of an innovative, healthy, standing forests, flowing rivers bio-economy
3. Strengthening Amazonian citizenship and governance

Some of the key actions mentioned in the report:

• Immediate prevention of deforestation and degradation, especially in the south and east of the basin where several species are Critically Endangered. 
• The focus on retaining forests and preventing degradation must be complemented by actions to protect aquatic and non-forest ecosystems. This will require multi-sectoral changes in the planning of energy and mining and the use of agrochemicals. 
• A new vision for the Amazon’s people and nature, renewed support for protected areas and Indigenous lands and investment in alternative economic strategies. 
• Conservation progress will benefit from a step change in investment in science within the Amazon to evaluate species status and distributions, and integrate Indigenous and local knowledge in this process. 

If the UN follows all recommendations of this report, there might be hope for a Living Amazon rainforest instead of a Living Amazon savanna. Meanwhile, it would be a good idea to start replanting some trees on those burned and barren cattle pastures.