This is the first of four studies I am reviewing about the weakening of the Atlantic Meridional Overturning Circulation (AMOC). The late geochemist Wally Broecker is responsible for a lot of the early science of thermohaline circulation, as I recounted in the previous book review post. This is the second of five posts about thermohaline circulation.
New research published in
Nature suggests the AMOC, which conveys warmer surface water from the tropics
that has higher salinity, and is denser due to more evaporation, to deeper
water in a region of the North Atlantic where the denser water sinks into the
deeper ocean. The research suggests that the AMOC conveyor belt could slow by
as much as 50% by 2100. That’s a sharper drop than many earlier projections and
raises new questions about how quickly impacts could unfold. This would be
considered to be a significant weakening of the thermohaline circulation.
Brocker, who died in 2019, considered that there was a risk of the AMOC
weakening or slowing down, but very little chance of it completely collapsing,
which could theoretically lead to the Earth shifting to an icehouse mode. The
AMOC is a major feature of Earth’s climate system and is considered to be
stable and not in danger of stopping, just weakening a little faster than
previously estimated. As Jennifer Gray of the Weather Channel notes, the thermohaline
circulation system distributes heat around the ocean and the earth and affects
global temperatures and rainfall patterns.
The new study claims to have
narrowed the uncertainty about the rate and degree of AMOC weakening by
combining model simulations with real-world observations, such as ocean
temperature and salinity patterns. Gray writes:
“The study also suggests the decline may not be gradual
and predictable. Instead, it could approach critical thresholds earlier than
expected, increasing the risk of abrupt changes.”
She notes that the impacts of
a significant weakening of the AMOC by 2100 are a matter of debate. Modeling
suggests that it could accelerate sea level rise along the U.S. East Coast,
change storm tracks, and affect marine ecosystems. Other possible effects are a
cooling of Northern Europe and disrupting the tropical monsoons on which people
depend. It could also alter the carbon budget of the
ocean.
Gray notes about the study
that the uncertainties are a result of a lack of detailed data coverage for
local and regional salinities and temperatures, since small changes in either
can affect the estimations considerably. This might be another good reason for
the Trump administration to abandon, which they already have fortunately,
previous plans to dismantle our ocean monitoring systems.
The paper’s abstract notes
that the AMOC’s effects on the carbon cycle and subsequently global temperature
remain seriously underexplored. As noted, predicting those impacts is fraught
with uncertainties.
The paper discusses possible
past AMOC collapses and slowdowns:
“Paleoclimate data provide strong evidence that AMOC
changes caused abrupt climate changes during glacial periods. Although the
mechanisms of glacial climate changes are different from those caused by
anthropogenic emissions, model simulations show that global warming may cause a
weakening and even a complete shutdown of the AMOC in the future. Despite no
significant signal from direct observational measurements, some studies based
on indirect proxies suggest that the AMOC is weakening, and is currently in its
weakest period in a thousand years, although the reliability of such
reconstructions is still debated. The IPCC AR6 concludes that, with medium
confidence, AMOC shutdown is unlikely this century. However, recent research
suggests this risk may be underestimated, highlighting the potential for
substantial, persistent global impacts if tipping points were crossed. Modeling
studies also show that the weakening or shutdown of the AMOC can affect the
functioning of the marine carbon cycle and cause additional changes in CO2.”
The researchers define
an AMOC collapse as:
“…a rapid weakening of the AMOC to a nearly complete
shutdown with the maximum strength below 5 Sv.”
They also note that the CO2
response to an AMOC shutdown remains highly uncertain, even though they predict
an increase of atmospheric carbon dioxide by 47–83 ppm and an increase in the
global average temperature of 0.2 deg C. Below, they give an explanation of why
they think the atmospheric CO2 increase would be much more than previously
estimated if the AMORC were to collapse, as modeled by their CLIMBER-X
simulator. They focus on the simulations showing that higher baseline CO2
levels in the atmosphere would lead to more CO2 outgassing in the Southern
Ocean near Antarctica.
“Our results show that AMOC collapse induces much
stronger CO2 increases at higher baseline CO2 levels (compared to 280 ppm),
which is further supported by paleoclimate evidence. Recent studies show that
CLIMBER-X reproduces critical aspects of millennial-scale climate variability
during glacial times61. In particular, consistent with more complex models
(e.g., Vettoretti et al. 202262; Malmierca-Vallett et al. 202463), it produces
Dansgaard-Oeschger like events as part of internal variability of the climate
system under mid-glacial ice sheets and low CO261. It is widely accepted that
these Dansgaard-Oeschger events are associated with abrupt changes in the AMOC,
involving transitions between weak and strong AMOC states. However,
Dansgaard-Oeschger variability is not associated with a complete collapse of
the AMOC and ice core data show only minor responses of atmospheric CO2
of ~ 5 ppm during Dansgaard-Oeschger events64. This is consistent
with what was found in model simulations (e.g., see ref. 65) and also with what
CLIMBER-X produces60. As a response to a complete AMOC shutdown following
freshwater input into the North Atlantic from Heinrich events during glacial
times, CLIMBER-X produces a much larger response in atmospheric CO2
of ~ 15 ppm, which is again in good agreement with ice core data
during Heinrich Stadials60. The process behind this large CO2 increase is a
sudden onset of convection in the Southern Ocean in response to an AMOC
shutdown60. Several studies have suggested that deep Southern Ocean convection
can promote the release of stored oceanic carbon37,66,67. Because CLIMBER-X
reproduces past climate-carbon cycle variability linked to AMOC changes, we
have confidence in its response to a potential AMOC collapse under warmer
conditions. In our simulations, the comparatively larger atmospheric CO2
increase is primarily associated with the development of deep Southern Ocean
convection, which ventilates carbon-rich deep waters and drives substantial
outgassing to the atmosphere. The CO2 increase in response to an AMOC shutdown
grows strongly with higher baseline CO2 levels (Fig. 3c). Higher CO2 baselines
correspond to larger ocean carbon inventories (Supplementary Fig. 11),
contributing to substantially more carbon to be released during a collapse,
suggesting that the amplitude of past glacial CO2 responses is not a proper
analog for the expected amplitude of CO2 increase in a warmer, higher-CO2
world. Consistent with this, other models show that in warmer climates a sudden
Southern ocean convection onset can raise atmospheric CO2 by ~ 40
ppm66, much larger than observed glacial-period changes. Our results suggest
that the same Southern Ocean convection-driven mechanism could operate under
future warmer conditions, producing a substantially larger CO2 response than
during glacial times.”
Media Reactions to This Paper
Since the AMOC has long been
considered a potential tipping point for global climate and since the paper
predicts that we are moving faster than previously estimated towards that
potential tipping point, there have been some media reactions to the paper. One
of the authors, though not the lead author, is Johan Rockstrom, of the Potsdam
Institute for Climate Impact Studies. He is the originator of the planetary boundaries model of earth
systems analysis that suggests we have crossed or are close to crossing several
planetary boundaries, basically tipping points, which have various impacts.
That model is debatable and many, including me, have criticized it as being too
catastrophist and not taking into account things like resource abundance and being overly reliant on simulations and modeling. Papers in which he is an author seem to draw a lot of media attention, usually in support of catastrophism.
Sometimes sensible, sometimes
not, environmentalist (and sometimes loudmouth) George Monbiot wrote an article
about the paper in The Guardian entitled: A catastrophic climate event
is upon us. Here is why you’ve heard so little about it. That is the very
definition of a doomsday headline. His article seems to take the worst-case
scenarios and the most extreme possible ranges of predictions given in the
paper and run with them as if they were the only numbers. He says that the new
modeling shows that an AMOC collapse would not be low probability as previously
estimated, but high probability. I believe that what the paper is saying is
that the probability increases, but nowhere does it say it moves from low
probability to high probability. He acts as if this one paper that suggests an
increase in probability is the most important discovery of the year or decade.
His predisposition toward catastrophism led him to describe an AMOC collapse
as:
“…potentially leading to a “hothouse Earth” in which
very few survive.”
He spends the rest of the
article badmouthing people like William Nordhaus, who developed climate impact
economics projections, and Bjorn Lomborg, who has spoken out against climate
catastrophism, and Bill Gates. More importantly, he misrepresents the paper’s
implications and its level of uncertainty and shouts out that catastrophe is
imminent when, in reality, that does not appear to be the case at all.
References:
Scientists
warn a critical Atlantic current could weaken faster than expected, reshaping
global weather. Jennifer Gray. The Weather Channel. April 23, 2026. Scientists warn a critical Atlantic
current could weaken faster than expected, reshaping global weather
A
catastrophic climate event is upon us. Here is why you’ve heard so little about
it. George Monbiot. Opinion. The Guardian. April 23, 2026. A catastrophic climate event is upon
us. Here is why you’ve heard so little about it | George Monbiot | The Guardian
Collapse
of the Atlantic meridional overturning circulation would lead to substantial
oceanic carbon release and additional global warming. Da Nian, Matteo Willeit,
Nico Wunderling, Andrey Ganopolski & Johan Rockström. Communications Earth
& Environment volume 7, Article number: 295 (March 2026). Collapse of the Atlantic meridional
overturning circulation would lead to substantial oceanic carbon release and
additional global warming | Communications Earth & Environment
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