11 August 2021
Ireland is located at
a latitude equivalent to parts of Canada and Siberia. These places can see snow
all winter and temperatures dip into the negatives. So why is Ireland so much
warmer?
A big part of it has
to do with an ocean current. Last week, a study warned that this ocean current – the AMOC – is seeing signs that could
lead to its collapse.
But what could happen
if it does collapse? Why is this happening? And what exactly is the AMOC?
We called a couple of scientists to dive into this oceanography.
Let’s start with
the basics – what is AMOC?
Atlantic Meridional
Overturning Circulation (AMOC) – I know, it’s a mouthful – is a large system of
ocean currents.
Warm, shallow water
moves north from the tropics, and then is returned by a system of colder,
deeper currents. Think of it like a conveyer belt that transfers heat across
the Atlantic Ocean.
Maynooth University
ICARUS Oceanographer Dr Gerard McCarthy describes it as “a river of warm water
coming across the Atlantic, kind of cuddling up next to Europe.”
To dig a bit deeper
into the science of this, the density of the water affects the depths to which
it travels. Dense water will sink if it’s heavier than the water below, while
less dense water will rise.
So water from the
tropics will sink if its saltier than the rest of the ocean. Currently, the
AMOC sees this water travelling near the surface, though this could change. (Keep
this in mind when we talk about salinity later on).
Sometimes ‘AMOC’ is
used as a synonym for the Gulf Stream. But that’s not exactly right. Maynooth
University ICARUS Postdoctoral Researcher Dr Levke Caesar said “it’s part of
the AMOC, but it’s more than part of the AMOC.”
The Gulf Stream has a
horizontal circulation, a wind drift, as well. It flows beginning from the
North part of Florida, along the east coast of the United States, then into the
Mid-Atlantic, and finally into the North Atlantic current. Again, bringing heat
across the ocean from the south, near the Caribbean and transferring the heat
to Europe.
This system is related to the AMOC, but the Gulf Stream is more than just ocean currents. But understanding the concept of the Gulf Stream helps in understanding what AMOC does. Basically, if you can grasp the concept of the Gulf Stream, you already have a basic understanding of the AMOC.
Okay, so this
current transfers heat across the Atlantic. What is happening with it now?
The AMOC is at the weakest it’s been in the last millennium.
Both Dr Caesar and Dr
McCarthy determined the stability of AMOC for the past 1,600 years. They analysed
everything from sea sediment to coral reefs to figure this out. After a long
and relatively stable period, there was an initial weakening, then a more rapid
one, and now we are seeing the AMOC in its weakest recorded state.
Even after this
finding, the models widely predict more of a slowdown. But there isn’t a point
that it is determined to collapse yet. Even the sixth IPCC report reported with
“medium confidence” that the
AMOC would not collapse before the end of the century.
Rather, it’s more
likely that we will see a slow-down. The study released last week looked at
this and at how close the AMOC is to a tipping point. It warned that there are
early signals to its collapse.
“Once we cross this tipping
point, it will be probably close to impossible to stop or reverse it and then
it’s really hard to get it back,” Dr Caesar told The Green News.
Do we know why the AMOC
might be at its tipping point?
It’s not entirely
clear. Ocean systems are complicated.
Part of it has to do
with natural variability. But another part is because – you guessed it – of
human-caused climate change.
For example, melting
ice caps and sea level rise is related to the currents’ weakening. It has to do
with the salinity, or the saltiness, of the current. As more freshwater from
glaciers melts into the sea, it changes the density of the water, and then depth
of the current.
Remember that deeper
dive into the science that we talked about earlier on? If the salinity of the
ocean changes, it could change the way the warm water travels across the ocean,
changing the entire heat transfer of the current.
But Dr McCarthy says
this hasn’t started happening yet, as he told The Green News, “We don’t know when it will start happening, So this
is kind of a technology crunch point.”
Models can only go so
far to predict what will happen in reality.
But why is the AMOC
important? And what could happen if we pass the tipping point?
With less warm water
transferred across the Atlantic, Ireland and Europe would experience some
cooling. But the temperature could be counteracted with global warming. It is
isn’t very clear what the net effect would be.
“The whole of the
world is warming up. So this is just a relative cooling around it,” Dr McCarthy
said.
What is more
unpredictable are the potential precipitation changes. In Ireland, we are likely
to see drier summers. While it’s unclear what the effects could be in the winter
there could perhaps be more intense storms.
Dr McCarthy warned of
these potential shifts in precipitation, noting that “rising populations,
creaking water infrastructure, drier summers coming out in the future is a
serious threat.”
Around the world,
there are other “dramatic
dangers” of a shifting AMOC. The rainfall and monsoons around the world would
likely be affected, especially in areas around the equator. This could lead to catastrophic consequences.
Along with presenting
research and warnings, the scientists we talked to urged action.
“Acknowledging that
when there’s a danger that it could happen, this might be enough to actually
start trying to avoid it,” Dr Caesar urged. “Information is power.”
“It’s about taking the
science seriously, and seriously dealing with adaptation and mitigation,” said
Dr McCarthy.
“Climate change
effects can be avoided. You know, it’s not too late.”
By Sam Starkey
The post An ocean current keeps Ireland warm. What could happen if it collapses? appeared first on Green News Ireland.
