Bengaluru: In a first study of its kind, a group of climate scientists and the National Geographic Society teamed up to analyse the influence of the Amazon rainforest on global climate and Earth’s rising temperatures.
The study found that both natural and human-caused factors have contributed to an imbalance in the uptake or absorption of carbon by the rainforest, as well as increased emissions of other greenhouse gases.
The researchers — in the study published in the journal Frontiers in Forests and Global Change last week — concluded that the world’s largest rainforest is now a contributor to rising temperatures due to increasing greenhouse gas emissions from itself.
In the study, the researchers looked beyond just carbon dioxide and the cycling of carbon storage and release, and focussed on processes that produce other greenhouse gases — such as methane and nitrous oxide — as well as other processes that alter global temperatures like release of aerosols, black carbon, and Earth’s albedo or reflectivity. They also studied the impact of these on regional and global climate systems.
“We conclude that current warming from non-CO2 (carbon dioxide) agents (especially methane [CH4] and nitrous oxide [N2O]) in the Amazon Basin largely offsets — and most likely exceeds — the climate service provided by atmospheric CO2 uptake,” wrote the researchers in the paper.
They also stated that the majority of human-induced or anthropogenic activity increases the “radiative forcing” potential of the Amazon basin, meaning that more sunlight now gets absorbed by the region than reflected back into space, thus increasing temperatures.
When it comes to carbon dioxide itself, the study found that the largest causes for emissions is land use change and forest degradation. Land use change occurs when human activities transform the natural landscape and how land has been used, typically for economic activities.
Drought has also been a contributing factor. Emissions occur due to not just respiration by plants, but also long-term and persistent mortality of trees, their decaying process, degradation of soil, as well as fires. Recent studies have suggested that tropical and subtropical forests could be releasing much more carbon than they store.
The emission of carbon from the rainforest and into the oceans by water bodies is a major contributor to carbon transportation between land and ocean.
The aquatic environments feeding into the Atlantic Ocean is typically a carbon sink, thanks to sediment deposition from rivers into the ocean, but when combined with offshore areas, the coastline, and floodplains, where freshwater does not mix with ocean water, the researchers stated that the net sinking effect is lowered and there are higher emissions of both carbon dioxide and methane.
Well-draining soils are a good sink for methane, lowering their emission. However, waterlogged soil, rich sediments, as well as vegetation act as a source of methane emission.
In waterlogged soil, bacteria anaerobically decompose organic matter, releasing large quantities of methane in the process. And the Amazon Basin floods each year, with up to 20 per cent of the region waterlogged.
Studies have indicated that tropical rainforests can release up to 10 times more methane than the rest of the world when waterlogged or flooded.
Methane is also released from biomass burning and land use for agriculture. Livestock also account for about 6 per cent of the emissions.
Drying wetlands, biomass burning, hydroelectric reservoirs, and decaying wood are also a major source of nitrous oxide. Fertilising and tilling in agricultural processes also release a limited amount of the gas.
Increased burning, deforestation, agricultural land conversion, and urbanisation also raise the levels of ozone in the Amazon Basin.
What the rainforest holds
The Amazon rainforest, which came into being 55 million years ago, is partially a self-sustaining regional climate and water system, which is thought to be on the brink of collapse. It is not uniform — it contains mountainous forests, mangroves, savanna, and other types of vegetation that grow on a variety of soils.
It is fed by a complex network of river systems that flood the region seasonally. It is fertilised by dust blowing in from the Sahara Desert, which is rich in phosphorus that is typically washed off in the wet rainforest.
The region is the most bio-diverse and species-rich on the entire planet, with every one in 10 species in the world living here.
Studies indicate that humans first began to settle in the Amazon region, which spans nine countries and sits primarily in Brazil, about 11,200 years ago. By the year 1,250 CE, humans had also begun to alter the forest cover.
Deforestation has been steadily increasing in the region, and studies in 2018 showed that about 17 per cent of the rainforest was destroyed beyond repair.
Land has been lost to the slash-and-burn method of agriculture, which also causes farmers to move constantly as the soil that gets washed away by intense rains is fertile only for a short period of time. Land has also been cleared in large quantities for cattle grazing.
There has also been intense drilling for oil inside the rainforest.
Additionally, the rainforest is susceptible to growing forest fires. In the deadly 2019 Amazon wildfires, there were nearly 40,000 fires inside the Amazon. Forest fires, as well as razing for farming increases black soot and aerosols that augments the absorption of heat and sunlight in the forest.
Effects of human activity
Increased production of soybeans are a huge catalyst for deforestation in the southern parts of the forest. There has since been intense gold mining in the northern parts as well.
While major deforestation has occurred historically in Brazil, countries like Peru and Bolivia, also a part of the Amazon Basin, are now contributing to large-scale deforestation. The palm oil industry and gold mining have been the driver for the Peruvian side, while soybean continues to expand agriculture on the Bolivian side.
“Since 2017, the processes driving deforestation across the region are widely seen as more dynamic and uniquely sensitive to the political decisions of regional governments,” wrote the authors.
With the signing of the peace agreement in Colombia in 2016, the authors noted that even protected areas, previously occupied by FARC (Revolutionary Armed Forces of Colombia) guerrilla forces, are now used as cattle pastures.
Other anthropogenic activity contributing to rising emissions and temperatures in the region are intentional fires, use of machinery, increased grazing, growing of legumes not native to the region, reservoir constructions, mining, oil and gas drilling and extraction, mercury pollution, hunting and overfishing, and general rising of temperatures around the world.
For natural factors that are causing emissions to climb, the authors attributed severe droughts and floods, intense rainfalls and longer spells of dryness, and severe storms.
Limitations and funding
The authors pointed out several difficulties in processing accurate numbers when it comes to emissions or uptake of greenhouse gases and carbon, primarily compounded due to the inability to navigate the rainforest and take accurate measurements.
However, they also stated that their findings are based on the most conservative data and the reality is likely much worse than believed.
This study was a part of a project called ‘Perpetual Planet’, and is funded by the National Geographic Society in partnership with Rolex.
(Edited by Debalina Dey)