It isn't so much that the overall temperatures will change too much when averaged out, that would relate more to the slow down/stopping of the AMOC.

First order effect, this would mean that you get less predictable weather events. Essentially, heat domes and artic blasts become much more persistent, sticking around for weeks or even months (worst case) rather than just brief periods.

Second order, A month long heat wave can kill a lot of crops followed by a month low wet spell that can completely flood the same area.

Third order effect, In metropolitan areas this is an inconvenience. For the food system, this is deadly.

Forth order effect, a fragile food system that feeds the metropolitan areas - things get dicey fast. Tough times, tough leaders, rise of authoritarianism etc. May you live in interesting times.

The fourth order effect is overlooked and it's my biggest short-term concern. In particular the destabilizing of low income countries near the equator in the face of precarious wet bulb temperatures, indirectly leading to the destabilizing of the global north in response to climate refugees fleeing the equator.

Short term that is the biggest issue that I fear.

I am in Australia and I have said it for many years, if it goes badly, we will welcome refugees with open arms. Those arms being machine guns and explosives.

That we will move into the realm of Lifeboat ethics.

https://en.wikipedia.org/wiki/Lifeboat_ethics

I do hope that do not take this path as it would become the greatest moment of shame for our whole species.

The Gulf Stream is responsible for the temperate climate of Europe which enables the swathe of farmland that it has.

A slowdown is expected to lead to a temperature drop from artic air being able to circulate further down, which would shorten growing seasons and probably change farmland distribution.

The effect of this happening over individual human generations in an environment where established farmland is hundreds to thousands of years old would be considerably expensive to adapt to.

How did humanity respond the last time this happened?

There is no history of this happening before, at least not on the time frames we are talking about here. This is thousands of years of change in decades. It isn't so much just the change but the pace that is the issue. Like how jumping off a building and taking the elevator achieve the same amount of change but on a very different time frame and thus very different results.

Do we know how the Gulf Stream worked during the last ice age maximum, when the Rhine river still flowed over land-locked British Channel (1)?

1. https://www.arcgis.com/home/item.html?id=f1e7378b962d42168fd...

That is a good question but I suspect we wouldn't know. Just educated guesses and I cannot find those at the moment.

I think the other big issue is that back then we didn't have industrial civilization and high level agriculture dependent on specific environmental factors.

We weren't agrarian back then, we were still hunting stuff with sticks and simple stone tools (12,000 years ago/ 10,000 BCE).

From the paper:

While the entire globe has warmed, the subpolar North Atlantic has resisted and even cooled. This is exactly the region where the AMOC delivers much of its heat, and exactly the region where climate models have long predicted cooling as a result of the AMOC slowing down.

[A] cold subpolar North Atlantic correlates with summer heat in Europe (Duchez et al., 2016).

For example, in summer 2015, the subpolar Atlantic was the coldest since records began in the nineteenth century— while Europe suffered a strong heatwave. Subsequent study has shown that heatwaves are increasing three to four times faster in Europe than in other regions of the Northern Hemisphere [...]

Western Europe is a misnomer.

Most of it should be all called Northern Europe and North of Northern Europe.

Paris sits between Vancouver and Seattle.

London is the same latitude as Calgary.

Hamburg is at the same latitude as Edmonton.

If the golf stream slows downs or gets diverted north towards Greenland much of Europe will freeze.

Indeed, this was the intent of my inquiry: what will climate look like for the next 100 years between certain latitudes in Western Europe (Porto vs Lisbon, Madrid and Bilbao vs Valencia, Malaga and Sevilla, Bordeaux vs Toulouse, etc).

Portugal is quite up north too, Porto is the same latitude as Chicago, Lisbon is the same latitude as DC.

That's not how geography works...

Do you consider Sao Polo to be in north America because Johannesburg is more southern?

No, and it’s clear that it wasn’t my point so don’t be silly or obtuse.

People don’t realize how far north most of Europe is because of its very temperate climate powered by the golf stream.

Everything from the English Channel up to the Baltics should be much much colder than it is.

The daily mean temperature in London is around 6c in January vs lower than -8c in Calgary.

The record low temperatures in London are higher than the mean daily minimum in Calgary.

Besides the impact on things like farming the infrastructure of many European cities isn’t built to withstand the winters it may have in a few decades.

> The daily mean temperature in London is around 6c in January vs lower than -8c in Calgary.

You can't compare a coastal city with an average elevation of like 10 meters and an inland city perched more than 1000m above sea...

Fine let’s compare it to Charlottetown it’s 5 degrees further south than London and its daily mean in January is -8c as well…

One way to get rough idea is to compare regions that are similarly far north but without a Gulf Stream. E.g. UK and Canada.