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Break in Atlantic circulation would increase risk of Amazon collapse, study warns
Scientists have analyzed what happened to the rainforest in the past and concluded that the same could happen now, with even more serious consequences, due to deforestation
Graphical representation of the great Atlantic ocean current system (Amoc), which transports warm waters to the north and cold waters to the south – Source: NOAA
The possible collapse of the Atlantic’s ocean current system is one of the tipping points that most threaten the Earth’s climate balance, with drastic consequences for the transportation of heat and the distribution of rainfall over a large part of the planet. One of these consequences, according to scientists, would be a change in precipitation patterns in the Amazon, with reduced rainfall in the north and increased rainfall in the south of the biome. But what impact would this have on the vegetation cover of the forest?
This is the question that a study led by scientists from USP and published this Friday (11/1) in the journalNature Geoscience set out to investigate. The answer, in short, is that climate change triggered by the weakening of the Atlantic Meridional Overturning Cell (also known as Amoc), coupled with changes in soil use related to deforestation, could cause an irreversible collapse of the Amazon’s forest cover, according to the authors.
This is because the areas that would theoretically benefit from an increase in precipitation are precisely those where the rainforest has already been deforested most (in the south and east of the biome), while the currently preserved areas in the north of the biome would be strongly impacted by the decreased rainfall. “The negative effects of an Amoc slowdown on the vulnerability of the northern Amazon, combined with the current state of anthropogenic pressure on the eastern and southeastern sectors of the basin, could systematically impact the Amazon ecosystem,” the researchers write.
“For the first time, we know what is the response of the rainforest to a marked weakening of Amoc. This is very important,” says Professor Cristiano Chiessi, a specialist in Paleoclimatology at USP’s School of Arts, Sciences and Humanities (EACH), who is one of the authors of the paper. The study was conducted by geologist Thomas Akabane, as part of his doctorate research at USP’s Institute of Geosciences (IGc), under the advisory of Chiessi and professor Paulo Eduardo de Oliveira, also from IGc.
Amazon: the rainforest's health depends heavily on climatic factors that are controlled
by the ocean – Photo: Neil Palmer/CIAT/Flickr via Wikipedia
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The clues as to what might happen on the surface of the forest, oddly enough, came from the bottom of the sea. The researchers analyzed traces of pollen and micro-coal preserved in marine sediments that represent what was flowing through the mouth of the Amazon River thousands of years ago – including a period when the great circulation of the Atlantic was temporarily interrupted by climatic and oceanographic processes associated with the end of the ice age. This organic matter, which flowed through the fluvial arteries of the biome and ended up deposited on the sea floor, serves as evidence of what was happening to the rainforest within the Amazon Basin as a whole. It’s as if scientists were analyzing a “blood sample” from the Amazon of the past, preserved on the ocean floor.
In this case, the blood bag is a vertical cylinder of compacted sediment (which scientists call a core) more than 7 meters long, extracted from the seabed north of French Guiana – where the plume of sediment from the mouth of the Amazon passes – at a depth of more than 2,500 meters. The sample was collected in 2012 during an expedition with the German research vessel RV Maria S. Merian, which Chiessi led in partnership with colleagues from Germany, who also signed the study of Nature Geoscience. According to Chiessi, practically all the sediment that covers the seabed in this region is of Amazonian origin and there are many chemicals and molecular “signatures” that scientists use to prove the identity and age of this material.
Three segments of core GeoB16224-1, extracted from the marine subsoil north of French Guiana. The compacted sediments bear traces of what the Amazon was like thousands of years ago – Photo: Cristiano Chiessi/Publicity for Jornal da USP
The researchers used this historical record to understand what happened to the Amazon rainforest at two key moments: the Last Glacial Maximum, between 23,000 and 19,000 years ago (when around 8% of the planet’s surface became covered in ice), and a period of warming that followed (between 18,000 and 14,800 years ago), known as Heinrich Stadial 1 (HS1), in which the melting of glaciers in the Northern Hemisphere injected an immense amount of freshwater into the Atlantic Ocean, causing a temporary collapse of Amoc. (In a nutshell, Amoc is a large system of ocean currents that transport warm surface waters to the North Atlantic and cold deep waters to the South Atlantic, with a major influence on the planet’s climate and environmental patterns. See illustration below).
Animation by NASA shows how the Earth's great ocean circulation system works. Red represents warm waters traveling across the surface, while blue represents cold waters traveling through the depths of the ocean. The Atlantic Meridional Overturning Cell (Amoc) is the part of this system that runs along the Atlantic Ocean, between Antarctica and Greenland – Source: Nasa/Goddard Space Flight Center Scientific Visualization Studio
Analyses of the material in the core indicate that Amazon’s forest cover underwent important transformations during these two periods. During the Last Glacial Maximum, according to the researchers, there was an expansion of species from montane ecosystems (better adapted to the low temperatures of the period) from the Andes and the Guianas to the lowlands of the Amazon, where species from warm and humid climates had previously predominated. “There has been a fairly significant change in the composition of the rainforest, but it has never stopped being a rainforest,” Akabane pointed out in an interview with Jornal da USP.
On HS1, things got more complicated. With the end of the ice age, the temperature rose again, pushing the montane vegetation back to the top of the mountains and making way for tropical vegetation to take over the lowlands of the Amazon. At the same time, however, the collapse of Amoc caused a shift in the “tropical belt” of rainfall to the south of the Amazon, leaving the northern part of the biome dehydrated. In addition to evergreen forests, the region was occupied by a mosaic of seasonal forests and stretches of savanna. It wasn’t a complete “savannization”, but the rainforest became much drier. It wasn’t until around 14,800 years ago, at the end of HS1, that the Amazon’s forest cover began to take on its current form.
Future projections
Having seen what happened in the past, the next step was to simulate what might happen in the future if the environmental conditions present in HS1 manifested themselves again – this time as a result of man-made climate change. To do this, the researchers made computer projections using programs that simulate how the planet’s climate works in different configurations. In this case, the scientists simulated what would happen to Amazon’s climate in a scenario of a severe weakening of Amoc by 50%.
Projections indicate that the “tropical rain belt” will shift to the south of the biome, just as it did in HS1. In other words, the north of the Amazon will become drier and less conducive to the survival of the tropical rainforests that currently exist there, while the southern and eastern portions of the biome will become wetter and more conducive to forest growth. The problem is that these are precisely the most deforested and occupied regions of the Amazon today, “where it is difficult to imagine that the forest will have free areas to expand,” Akabane punctuates.
The first map shows the ecoregions of the Amazon, the boundaries of the Amazon Basin, and the point from which sedimentary core GeoB16224-1, used in the study, was extracted (Source: Akabane et al.). The middle map shows a simulation of how a 50% weakening of Amoc would affect precipitation and the probability of tropical rainforests (FTU) in the Amazon (Source: Akabane et al.). The third map shows the distribution of forests and deforestation in the Brazilian Amazon today (Source: Inpe/Deter) – Preparation: Herton Escobar/Jornal da USP
“That’s the big risk: the combination of climate change and changes in soil use,” summarizes Chiessi. He fears that the weakening of Amoc will be “the last straw” that will break Amazon sustainability’s back for good. “What’s the point of the decrease in precipitation in the north being balanced by an increase in the south, if it’s in this region that deforestation reigns?” the professor asks. “That sum could bring the forest to its tipping point.”
No one can say for sure when this Amoc break-up might occur, but recent studies indicate that the system is already losing strength and that there is a real risk of it collapsing still in this century, due to the melting of the Greenland ice cap and other factors associated with global warming. This means that the transport of water masses between the north and south of the Atlantic would cease to occur, or would occur much more slowly than it does today. Making an analogy, it’s like turning off or slowing down a conveyor belt system that transports luggage at an airport, for example – in this case, the conveyor belts are ocean currents and the luggage is thermal energy (heat).
According to the researchers, it is essential that future studies on this subject consider the “combined impacts of potential future scenarios of Amoc weakening under real conditions of anthropogenic forest degradation and global warming”.
In addition to Brazil and Germany, the Nature Geoscience study is signed by researchers from Switzerland, France, China, the Netherlands, and Morocco.
For more information with Thomas Akabane ( thomask.akabane@gmail.com), Cristiano Chiessi (chiessi@usp.br) or Paulo Eduardo de Oliveira (paulo.deoliveira@usp.br).
*Intern under the supervision of Moisés Dorado
English version: Nexus Traduções
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A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.
A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.