The potentially catastrophic slowdown of a critical ocean current due to climate change may have one upside: it could help prop up the Amazon rainforest in the face of rising temperatures.
The collapse of the Atlantic meridional overturning circulation (AMOC) and the transformation of the Amazon rainforest into savannah are two of the most serious tipping points anticipated to occur if global warming continues – defined as abrupt shifts in Earth’s systems that can cause global, irreversible impacts. Some studies suggest that the AMOC has already started to slow down, but this is still uncertain because it has only been continuously monitored for about 20 years.
If the AMOC comes to a halt, it could lead to rapid sea level rise in North America, a sudden and severe drop in temperatures across northern Europe and serious disruption to monsoons across Asia. Meanwhile, rising temperatures are drying the Amazon rainforest, pushing it towards a tipping point where the forest shifts from a jungle to a savannah. This transition would endanger a huge portion of global biodiversity, undermine the planet’s ability to absorb ongoing emissions and disrupt the climate across South America.
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How tipping points interact with one another isn’t well understood, although suggests many of them will have a destabilising impact on others. For example, the melting of the Greenland ice sheet is expected to accelerate the collapse of the AMOC.
at the International Institute for Applied Systems Analysis in Austria and her colleagues set out to investigate how the slowdown of the AMOC might affect the Amazon rainforest. “It’s important to understand interactions between tipping points better, because we want to be able to assess the risk of tipping cascades that could be quite dangerous,” she says.
The AMOC carries warm water from the equator to the poles at the sea surface and cold water in the opposite direction in the deep ocean, so sea surface temperatures can provide some indication about the strength of the current. Using sea surface temperatures as a proxy for the strength of the AMOC, and average rainfall as a measure of Amazon dryness, Högner and her colleagues detected a link between the two phenomena over the period from 1982 to 2022. They suggest that colder sea surface temperatures in the North Atlantic Ocean would lead to the strengthening of an air current called the Caribbean low-level jet, increasing rainfall over the Amazon during the region’s May-September dry season.
“We find that a weakening AMOC leads to increased precipitation and vegetation greenness in the southern Amazon rainforest during dry season,” says Högner. “Dry season is a critical period of the year in which water stress is the highest and droughts can occur, and it plays a big role in the stability of the rainforest. This is why we can interpret this interaction that we see as a stabilising interaction between two tipping elements.”
The team found that this effect has offset around 17 per cent of the drying trend observed in the Amazon since 1982. The finding is good news, says Högner. “It’s better to find a stabilising interaction than a destabilising interaction.”
But she points out that the Amazon is already getting drier, even with the stabilising effect of a weakening AMOC. It also isn’t clear how long this influence will last, she says. “It is concerning every time we identify a mechanism in which the Earth system helps us buffer the adverse impacts of human-made climate change,” she says. “We shouldn’t rely on the Earth system doing that for us forever.”
at the Potsdam Institute for Climate Impact Research in Germany says the findings don’t demonstrate a connection between the AMOC and the Amazon. Sea surface temperatures can’t be used as a proxy for the strength of the AMOC, especially not over short time periods, she says. “I think the results are a teleconnection [or a] causality between North Atlantic sea surface temperatures and South American rainfall,” says Ben-Yami. “But it’s not a connection between the AMOC and the Amazon.”
at the University of Sussex in the UK says the findings chime with the results of recent climate modelling, which suggests AMOC weakening might make the southern Amazon wetter in the future, countering the recent drying trend.
But he also raises concerns that the proxies used for measuring the AMOC and the Amazon might not be accurate reflections of the current state of those tipping points. Other important variables were left out of the study, he points out, including the impact of deforestation in the Amazon.
“Whether the AMOC is currently slowing or not, what we do know is that it will slow down and potentially collapse with further warming, and that, in the past, this has tended to push tropical monsoons southwards, disrupting ecosystems and the global carbon cycle,” he says. “With the wild card of tipping points and their uncertain interactions thrown in too, it’s crucial to reduce the chance of Amazon dieback by halting deforestation and greenhouse gas emissions as soon as possible.”
Arxiv
