This paper
addresses a property of the AMOC as a “warming hole” or “cold blob” in the
North Atlantic Ocean. The cold blob has dropped in temperature by an estimated
1 degree C since 1900. It is currently the only place in the world to have
cooled significantly since then. It is acknowledged that both ocean heat
transport and surface fluxes contribute to the cooling, but there is debate
about which is the dominant cause. Both temperature and salinity drive ocean
circulation via density changes, which move water from surface to depth. The
cold blob is the result of the slowing of the AMOC currents. Helen Coffey notes
in an article for The Independent.
“This is a complex and nuanced area of science.
Oceanographers and climate scientists have speculated that two different
factors could be causing this level of cooling: a combination of changing ocean
currents and changes in surface heat fluxes. However, they are “debating the
relative importance of the two mechanisms”, says Flavio Lehner, a climate
scientist and assistant professor at Cornell University. Consensus has yet to
be reached.”
A major concern about the
AMOC is that as climate change increases, Greenland ice melt and very cold
fresh water is added to the ocean near the AMOC. This cold, fresh water floats,
and this disrupts and slows the sinking effect at the AMOC. It is thought that
cooling in the subpolar North Atlantic region affects European heatwaves by
altering the path of the jet stream and how far south or north it moves across
the continent. The AMOC could be a major reason why Europe enjoys milder
temperatures at higher latitudes. If it weakens, it could cool the continent
more than desired, according to models.
Dr Lee de Mora of Plymouth
Marine Laboratory noted:
“Because the cold blob is so big, it also affects the
air above it. The jet stream, a fast-flowing current of air flowing from west
to east, hits the cold blob and is forced to go around it. “That’s when you get
these heat dumps and cold snaps, where it hits the bottom and then it creates a
wave in the jet stream that passes over Europe,” says Dr De Mora. “That’s
what’s so scary about the cold blob – it has this huge impact on everything
around it.”
Interestingly, Coffey notes:
“The very question of whether the Amoc is “weakening” is
controversial. That’s because there simply isn’t enough data to say for sure
yet – direct observations of the ocean have only been recorded for the past 25
to 30 years, and experts say we need at least 60 years’ worth of data to come
to a definitive conclusion.”
That is in contrast to my
previous post, where researchers seemed confident that the AMOC was indeed
weakening, and more than previously estimated. Without enough data, they should
not be as confident, one would think, especially with the methods used in the
current paper, which, as noted below, come with “sizeable uncertainties.”
The researchers used indirect
observations. or “reanalysis data” from a computer model fed with real-world
measurements wherever they were available.
“It’s in many ways the best we can do if we want to look
into the time before satellite measurements and ocean moorings, so before about
1980 – but it comes with sizable uncertainties,” Lehner points out.
“Dr Evans says that scientists who look at direct
observations of the ocean would be more “cautious” when it comes to making
sweeping statements about whether or not the Amoc is significantly weakening.”
“However, all of the big climate models, such as those
from the IPCC reports, are universally predicting that the Amoc will weaken
this century. “There is that conflict between: this is what the models tell us;
this is what we expect from the science; and this is what the observations are
doing,” says Dr de Mora. “It is still an open question, but we are almost
certain that the Amoc will weaken this century.”
“Within that assumption rumbles another debate. There
are essentially two fiercely opposing “camps” of scientists, according to
Lehner, “one that is increasingly concerned that the Amoc might collapse before
long and one that thinks it is more stable.”
While I think this is a
reasonable scientific debate, its implications could be severe if the weakening
is definite and such a possibility can be exploited by those who favor
catastrophism to use as more evidence, real or potential, to advocate for
climate change policies that restrict fossil fuel use.
The most recent IPPC report
says the weakening is more likely to be a gradual decline than a rapid
“catastrophic” collapse. It is noted that scientists agree that the effects of
a strong weakening or a collapse would be dire, but there is disagreement about
the severity of the threat, and whether or by how much it will weaken. Lehner
seems to favor a precautionary approach:
“Reducing greenhouse gas emissions is the only known way
to avoid the collapse, so from a risk reduction perspective, we have enough
information to take this scenario seriously.”
In other words, we can fix
it, but only if we act now. That sounds like the familiar pressure sales pitch.
The paper was published in Geophysical Research Letters. In the paper’s abstract
below, I think the most important conclusion is:
“…multidecadal heat content variations are generally
larger and more tightly correlated with ocean heat transport than with surface
heat flux variability.”
As can be seen in Figure 5
from the paper below, the ocean heat content rate of change matches closely
with the implied ocean heat transport anomaly, much better than it matches the
surface heat flux anomaly.
The paper’s conclusion notes:
“Surface heat loss appears to respond as a negative
feedback to heat content changes: periods of increasing heat content coincide
with periods of large surface heat loss. Thus from observational data, we reach
the same conclusion as (K. Y. Li and Liu, 2025) did based on the analysis of
model results.”
They don’t rule out that
surface heat flux contributes. They state that the presence of the blob
indicates a slowdown or weakening of the AMOC. I think there is widespread
acknowledgement of an AMOC weakening, but little agreement on the magnitude of
the weakening and how much it will weaken in the future.
“On shorter time scales this includes a robust observed
weakening of the Gulf Stream over the past 4 decades (Piecuch & Beal,
2023), consistent in magnitude with the 15% AMOC weakening inferred from the
subpolar SST data (Caesar et al., 2018), and an ocean density reduction in the
subpolar gyre since 1950 which “is suggestive of a long-term AMOC weakening of
2.2 Sv or 13%” (Chafik et al., 2022).”
The fundamental argument is
about the stability of the AMOC and whether it will remain stable. Time will
tell us more.
References:
Multidecadal
Atlantic “Warming Hole” Heat Content Variations Are Caused by Ocean Heat
Transport, Not by Surface Fluxes. Stefan Rahmstorf, Jan Jendrkowiak, Ruijian
Gou, Lijing Cheng, Angel Ruiz-Angulo, and Halldór Björnsson. Geophysical
Research Letters. 28 May 2026. Vol. 53, Issue 11. Multidecadal
Atlantic “Warming Hole” Heat Content Variations Are Caused by Ocean Heat
Transport, Not by Surface Fluxes - Rahmstorf - 2026 - Geophysical Research
Letters - Wiley Online Library
We’re
in a heatwave – so why is a ‘cold blob’ causing so much concern? Helen Coffey. The
Independent. July 7, 2026. We’re
in a heatwave – so why is a ‘cold blob’ causing so much concern?
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