The Earth's climate system is highly complex and its components, which include the ocean, atmosphere, and vegetation, are closely interlinked. Changes in individual parameters can have far-reaching effects on the entire system. To a certain extent, the individual components of the system are resilient and can absorb changes. Climate and Earth-system research, however, assume that there are various tipping points. If these are exceeded, the climate system can change its state within a short period of time. It is also presumed that tipping points in the climate system influence each other and can trigger chain reactions, or cascades.
Among the global tipping points are the Amazon rainforest and the large-scale Atlantic Meridional Overturning Circulation (AMOC). Further warming of the planet can lead to a significant weakening of the AMOC. This would slow down the conveyor belt that transports warm water to the northern regions, drastically changing the temperature distribution in the Atlantic. This would also have consequences for the Amazon region because the altered temperatures in the Atlantic would affect the atmospheric water cycle, and thus also the patterns and amounts of precipitation.
Analysis of Residual Pollen and Carbon
Exactly how the AMOC and the Amazon are interconnected as systems, and how marine circulation affects the Amazon region, have not yet been extensively researched. A group of researchers led by Dr. Thomas Akabane and Prof. Dr. Christiano Chiessi from the University of São Paulo has now analyzed changes in the vegetation of the Amazon region. With their international team, they have analyzed pollen and carbon residues representing the past 25,000 years from a marine sediment core taken from the mouth of the Amazon River.
This analysis provides the team with a detailed glimpse into the past of one of the most species-rich ecosystems on Earth. The data show how the vegetation, along with wet and dry periods, has changed during the climate events of the last ice age, called Heinrich Events, when the AMOC was drastically weakened. The researchers found, in particular, a dramatic decline in rainforest vegetation in the northern part of the Amazon region.
Close Connection Between Atlantic Circulation and the Amazon Ecosystem
"The study is the result of a long-term German-Brazilian cooperative project, which began in 2012 with a joint expedition of the Research Vessel MARIA S. MERIAN in the estuarine area of the Amazon. Our data show that the Amazon ecosystem was able to adapt in the past to changes in the patterns of precipitation that resulted from weakened Atlantic circulation. But a weakening of the AMOC in the future occurring simultaneously with an increase in deforestation could threaten the stability of this important global system," says Dr. Stefan Mulitza of MARUM.
Further studies employing climate and vegetation models indicate that a weakening of the AMOC under present-day conditions would have an effect on Amazon vegetation similar to that which it had during the past ice age. "The models have shown us that the AMOC need not completely collapse in order to have an effect on the rainforest. The northern areas of the Amazon region are massively impacted under mere moderate changes in the AMOC," explains Dr. Matthias Prange of MARUM.
The results illustrate how complex the global system is. "Driving processes at high latitudes, like the melting of Greenland ice, can have a substantial effect on the tropics. Such long-distance influences often have severe regional effects, very often for people who are only minimally responsible for causing climate change," adds Prof. Dr. Gerrit Lohmann of AWI.
Original publication:
Thomas Akabane, Cristiano Chiessi, Marina Hirota, Ilham Bouimetarhan, Matthias Prange, Stefan Mulitza, Dailson Bertassoli Jr., Christoph Häggi, Arie Staal, Gerrit Lohmann, Niklas Boers, Anne-Laure Daniau, Rafael Oliveira, Marília Campos, Xiaoxu Shi, Paulo De Oliveira: Weaker Atlantic overturning circulation increases the vulnerability of northern Amazon forests. Nature Geoscience 2024. DOI: 10.1038/s41561-024-01578-z
