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‘We don’t
know where the tipping point is’: climate expert on potential collapse of
Atlantic circulation
Oceanographer
Stefan Rahmstorf explains why Amoc breakdown could be catastrophic for both
humans and marine life
Jonathan
Watts
Wed 23
Oct 2024 14.00 BST
https://www.theguardian.com/environment/2024/oct/23/we-dont-know-where-the-tipping-point-is-climate-expert-on-potential-collapse-of-atlantic-circulation
The
dangers of a collapse of the main Atlantic Ocean circulation, known as Amoc,
have been “greatly underestimated” and would have devastating and irreversible
impacts, according to an open letter released at the weekend by 44 experts from
15 countries. One of the signatories, Stefan Rahmstorf, an oceanographer and
climatologist who heads the Earth system analysis department at the Potsdam
Institute for Climate Impact Research in Germany, explains here why he has
recently upgraded his risk assessment of an Amoc breakdown as a result of
global heating – and what that means for Britain, Europe and the wider world.
What is
Amoc?
Amoc, or
the Atlantic meridional overturning circulation, is a system of ocean currents
that brings heat into the northern Atlantic. Warm surface water from the
tropics flows north and releases its heat in the subpolar Atlantic, south of
Greenland and west of Britain and Ireland. Then it cools and sinks to a depth
of between 2,000m to 3,000 metres before returning south as a cold current.
Amoc is one of our planet’s largest heat transport systems, moving the
equivalent of 50 times the human energy use, and it has a particularly strong
impact on the climate in Europe, affects the ocean’s CO2 uptake and oxygen
supply, as well as rainfall patterns in the tropics.
How is
Amoc different to the Gulf Stream?
They are
connected because the northwards flow of Amoc goes via the Gulf Stream, which
is a warm and swift Atlantic Ocean current that originates in the Gulf of
Mexico, then flows through the Florida straits, up the coast of the US and then
across towards Europe. Amoc contributes just 20% to the Gulf Stream water flow
but most of the heat transport, since Amoc’s deep return flow is very cold. It
works like a central heating system.
What is
happening to Amoc?
There are
indications that Amoc has been slowing down for the last 60 or 70 years due to
global heating. The most ominous sign is the cold blob over the northern
Atlantic. The region is the only place in the world that has cooled in the past
20 years or so, while everywhere else on the planet has warmed – a sign of
reduced heat transport into that region, exactly what climate computer models
have predicted in response to Amoc slowing as a result of greenhouse gas
emissions.
Are there
other indications that Amoc is weakening?
Yes.
There is a region of excessive heating along the east coast of North America,
which is predicted by climate models and oceanographic theory as a result of a
slowing Amoc, which pushes the Gulf Stream closer to the shore.
Another
indicator is a reduction in the salt content of seawater. In the cold blob
region, salinity is at its lowest level since measurements began 120 years ago.
This is probably linked to Amoc slowing down and bringing less salty water and
heat from the subtropics.
Why is
the salt content significant?
When the
water is less salty, it is less dense, which makes it harder to sink down. That
is important because the sinking process is what drives Amoc. The fresher the
water, the slower it gets.
What is
driving the change in salinity?
Firstly,
salinity is directly affected by global heating, which enhances the water cycle
so there is more evaporation in the subtropics and more precipitation in the
subpolar oceans. This leads to a freshening of the subpolar ocean. Then there
are additional contributions from the melting of sea ice and the loss of
continental ice from the Greenland ice sheet, which is freshwater that flows
into the ocean.
It is an
amplifying feedback: as Amoc gets weaker, the subpolar oceans gets less salty,
and as the oceans gets less salty then Amoc gets weaker. At a certain point
this becomes a vicious circle which continues by itself until Amoc has died,
even if we stop pushing the system with further emissions.
When
might Amoc weakening reach a point of no return?
The big
unknown here – the billion-dollar question – is how far away this tipping point
is. It is very difficult to answer because the process is non-linear and would
be triggered by subtle differences in salinity, which in turn depend on amounts
of rainfall and cloud cover over the ocean as well as Greenland melting rates.
These are hard to model accurately in computers so there is a big uncertainty
relating to when the tipping point will be reached.
What is
the range of forecasts?
Until a
few years ago, the general thinking in the Intergovernmental Panel on Climate
Change (IPCC) was that the probability of crossing the tipping point this
century was less than 10%. Since then, there have been a number of studies
suggesting a collapse would probably be triggered this century, possibly in the
next few decades. So my risk assessment has really changed. I am now very
concerned that we may push Amoc over this tipping point in the next decades. If
you ask me my gut feeling, I would say the risk that we cross the tipping point
this century is about 50/50.
Is there
any possibility it has already happened?
I
wouldn’t rule it out completely, because it would be very hard to tell from
observations. Nothing dramatic happens at the tipping point. That just means
Amoc is then doomed and it will slowly die, but that process could take 50 to
100 years. Because the Amoc is already weakening we can’t be entirely sure
whether we already passed a tipping point, but I would say this is most likely
not the case, so it is not too late to prevent this.
What
would be the warning signs of Amoc collapse?
We need
to keep monitoring the flow of water in the Atlantic, which is being done with
the Rapid project. We should also monitor deep winter mixing in the northern
Atlantic and Nordic seas. If the deep mixing starts to decline a lot, that
could be an early indicator that we are approaching a tipping point. There are
some signs of this, but we don’t have enough data yet to be sure.
What
would be the consequences of Amoc breakdown?
This has
happened repeatedly in Earth’s history, most recently during the last ice age,
when big ice masses slid into the ocean – so-called Heinrich events – adding
meltwater that diluted the salinity of the north Atlantic. These are among the
most massive upheavals of climate conditions in Earth’s history.
The
effects include a cooling of the northern hemisphere, particularly northwestern
Europe. There would also be a shift of the tropical rainfall belt to the south,
which is bad because the rains will move away from the rainforests to regions
that are not used to so much rainfall. So this will mean droughts in some
regions and floods in others.
Amoc
collapse would also have a major impact on the northern Atlantic sea level,
which would rise by half a metre or so, in addition to the rise caused by
global heating. It would also reduce the CO2 uptake of the ocean because Amoc
sinking in the northern Atlantic takes a lot of CO2 down into the deep oceans
where it is safely locked away from the atmosphere.
Amoc
collapse would also change the nutrient supply and reduce the oxygen of the
deep oceans. This would have a massive effect on marine biology and the entire
ecosystem of the northern Atlantic.
Many of
these things are happening already, aren’t they?
Yes, to
some extent. This is partly because Amoc is weakening and so is its counterpart
in the southern hemisphere, the Antarctic bottom water formation, according to
research by Australian colleagues.
Could the
cooling effect of Amoc collapse offset the heating caused by human emissions?
I can’t
think of anywhere that will be better off. If it were just a case of averages,
then somewhere like Germany might see a balance. But weather is not a climate
average; it is seasonal and highly variable. Within the average you can get
warm air from the south or cold polar air outbreaks from the north. These
contrasts will be more pronounced if Scandinavia and Britain cool while Spain
and Italy warm. This will drive much greater variability in the weather, which
is bad for agriculture, and it will cause more storms. I would expect major
extreme weather events that we have not seen in the past.
The key
thing about climate change is that both the ecosystem of the Earth as well as
human settlements and infrastructures are highly adapted to what the climate
was like in previous centuries. So any change – whether global heating or
global cooling – will always be bad because it will lead to maladaptation.
Think of the tremendous flooding we have seen somewhere in the world almost
every week in the last months. If it had been like that for centuries, then
river and sewer systems would be adapted to take up that water. But because we
are not used to that, there are disasters. That is the problem of climate
change.
How
certain is the science about Amoc collapse?
It is
well established that Amoc is weakening and that a tipping point exists. The
uncertainty is about when we will cross that threshold. We also have very few
studies about what the combined effect of Amoc collapse and global heating
would exactly look like.
It is a
question of risk assessment. I compare it to being told that there is a 10%
chance of an airplane crashing. Would you get on that plane? I wouldn’t. The
disastrous consequences are unacceptable.
Why
haven’t the IPCC made more of Amoc risks?
They have
not done enough risk assessment because they tend to focus on the most probable
scenarios for future climate change. Some colleagues say we shouldn’t talk
about extreme possibilities like an Amoc collapse because it sounds alarmist
and might distract people from more certain impacts of global heating, which
are bad enough. But I think those extreme risks are part of the whole picture
that we need to consider, to make responsible and rational decisions.
How long
would an Amoc collapse last and how survivable would it be?
The last
time, it took about 1,000 years to recover, though the past is not a direct
analogue because there is also massive CO2 forcing this time – CO2 is already
higher than any time in 15m years. There are physical reasons why some form of
deep overturning circulation will eventually come back.
One thing
is for sure: humanity will not die out. But for some countries that will be in
the midst of this, like Norway, and Scotland, the risks will be existential and
raise the question whether people can continue to live there or whether most of
them would rather move.
How does
the Amoc threat compare to other climate tipping points?
That is
hard to tell. It is a trade-off between more distant futures and things that
are already happening.
We have
already crossed the tipping point of many coral reefs, which are now in middle
of global die-off. This is very depressing because it is already too late to do
anything about it, though marine biologists have warned about the risks for a
long time. The Amazon rainforest is also dangerously close to a tipping point.
As we speak, it is going through the worst drought on record.
Then in
the very long run, we have the ice sheet tipping points in Greenland and west
Antarctica. From Greenland alone, this will lead to a seven-metre global sea
level rise that will wipe all major coastal cities off the map. But that will
occur over many centuries because ice sheet melt is a slow process.
Amoc is
on an intermediate timescale because it unfolds over decades to 100 years.
I am
worried about all of these things to be honest. And the conclusion for all of
them is the same: this is all driven mainly by fossil fuel emissions and also
deforestation, so both must be stopped. We must stick to the Paris agreement
and limit global heating as close to 1.5C as possible. I don’t think it is my
job to talk about my feelings, but I do have two children and I am very worried
about what future they will live in. I sometimes joke that physicists don’t
have feelings. But even physicists care about their kids.
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