What might happen to our buildings if the AMOC shuts down?
Climate scientists are increasingly concerned that a key Atlantic Ocean circulation may be on the cusp of shutting down – with a range of catastrophic impacts including much colder winters in the UK. What are the impacts for our buildings, asks Greengauge Director Toby Cambray.
The Atlantic Meridional Overturning Circulation (AMOC) is the main reason that those of us in north-western Europe enjoy a remarkably benign climate, considering the latitude which we inhabit. The British Isles span roughly from 50 to 60 degrees north. Startlingly, this is further north than most of North America’s major cities; Quebec and Vancouver are south of Land’s End.
Several papers published in 2024 have indicated that the probability of AMOC collapse in the relatively near future is much higher than previously thought. If we lose the source of heat that maintains our temperate climate, the implications are enormous.
It is interesting to pause and reflect on the role of models and modelling, and if like me you tend to overthink the philosophy of PHPP, I would recommend ‘Escape from Model Land’, written by a recovering climate scientist. The recent increases in perceived risk of AMOC collapse stem from the way widely accepted models – which inform the IPCC reports – tend to overestimate the stability of the AMOC. It’s like using SAP to design a passive house – it looks fine until you realise the thermal bridges are under-represented.
The most recent work on this topic, which is not universally accepted, suggests that there’s a 10% probability shut down will occur by 2034, equal odds by 2050 and a 90% chance by 2064. Clearly more work must be done to verify this, but it is alarming that a group of serious scientists are saying that this important system will probably fail within my lifetime and that of many of the people reading this. On the other hand, while the process is effectively instantaneous on geological timescales, on human timescales it’s more pedestrian, taking maybe 50 to 300 years according to Michael Oppenheimer, although my understanding is that these estimates are based on the IPCC models that tend to overestimate the stability of the AMOC.
This has obvious implications on energy demand, which I’ll get into, but that is only part of the picture. As a starter for ten we can expect infrastructure failure (frozen pipes, roads, snow loads on roofs not designed for that), food security (much more difficult farming conditions and reduced yield), health impacts (hypothermia, increased mould), increased social inequality, pressure from internal immigration and emigration, and biodiversity loss.
What are the implications in building physics terms?
Quebec is about twice as cold as most of the British Isles. It doesn’t make sense to say double the temperature unless you’re doing something in a very expensive lab, but building physicists have one simple trick for measuring the severity of a winter in terms of heat loss from buildings called degree-days. This number goes into PHPP to represent how cold and how long the winter is, and is used to calculate the annual heat losses. The number for Quebec 124 kilo-Kelvin hours (kKh) versus 60 to 80 for most of the UK. It’s a little tenuous to claim AMOC collapse will double our heating demand, because the exact, granular impacts on our climate are not clear, and there’s a load of technical and social factors that influence actual usage. Vancouver is significantly milder, presumably benefitting from the moderating influence of the north Pacific. But to paint a picture, it’s not a terrible place to start.
On one hand, it appears we need to start pulling all the levers available to mitigate against the effects of sudden and dramatic climate effects; on the other hand, keeping warm could be one of a plethora of issues our society will face if we are abruptly plunged into latitude-appropriate winters.
Notwithstanding the questionable method of transplanting Quebec onto a post-AMOC British Isles, I plugged some numbers into the PHPP I built for my personal retrofit.
| Wiltshire (Exeter climate) | Quebec Climate | ||
| Pre-retrofit | SSHD | 222 kWh/m2P.a | 415 kWh/m2P.a |
| Heat load | 114 W/m2, 11kW | 225 W/m2, 22kW | |
| Post-retrofit | SSHD | 55 kWh/m2P.a | 114 kWh/m2P.a |
| Heat load | 44 W/m2, 4.2 kW | 83 W/m2, 8 kW |
The house is an unremarkable 97m2 1950s cavity semi to which we added external wall insulation, airtightness measures (a slightly disappointing 3.5 ACH@50Pa afterwards), MEV, triple glazing and loft insulation. The PHPP indicates we reduced our demand by about three quarters (not withstanding comfort-taking), and the peak load came down by about two thirds; I’d need a 4.2 kW heat pump post retrofit (we’re on gas for now, sorry). If we hadn’t done the retrofit and we were in a Quebec climate, the demand and peak would have almost doubled. The gross peak load would be almost 22 kW, not a problem for the typically grossly oversized domestic boilers (though my radiators might have to run pretty hot), but this is a pretty chunky air source heat pump compared to what is typically being installed right now.
Needless to say, post retrofit, these impacts are still present but are much less dramatic; the retrofit in a Quebec climate performs much better than the original in the current Exeter climate, having half the heating demand, but is still double what it would be after the retrofit in a pre-AMOC collapse scenario.
René van Westen, one of the authors of the new work that estimates the much higher risk, also highlights that we can’t even bank on AMOC collapse for relief from summer overheating; I have not picked out the numbers for that.
I have written in the last couple of years about the supply decarbonisation versus demand reduction debate, highlighting that those of us who advocate for fabric measures dismiss the benefits of a renewable-dominated grid and heat pumps combined with demand flexibility at our peril. But the very real possibility of having to double our demand in the face of AMOC collapse puts yet another complexion on this. On one hand, it appears we need to start pulling all the levers available to mitigate against the effects of sudden and dramatic climate effects; on the other hand, keeping warm could be one of a plethora of issues our society will face if we are abruptly plunged into latitude-appropriate winters.
Originally published in Passive House Plus magazine (issue 49)
