1.5C is going. Your overheating model already assumed that. Your carbon targets still have not caught up.
Two things landed within a day of each other last week and industry will likely file both under grim climate news and move on. But they do raise some interesting questions about how we design buildings in this country.
On 2 September, UNEP published Limiting Overshoot. It is the first time the UN has said so clearly that global temperature rise will cross 1.5C (likely within the next few years) and it reframes the goal entirely. Instead of prevention we now get “overshoot, peak and decline”: exceed the threshold, minimise how high it goes and how long we stay there, then bring temperatures back down through sustained net-negative emissions. And the most optimistic scenario peaks at 1.8C.
The following day the WMO said El Niño is now firmly established and heading for very strong intensity, with a near 100% likelihood of persisting through to February 2027. In places, subsurface Pacific water has been running more than 8C above average. The WMO notes this is the first time one of its updates has been so unequivocal.
El Niño is a natural cycle and is not caused by climate change. What it does is sit on top of a warmer baseline, and its effect on global temperature is usually most pronounced in the year after it develops. So 2027 is the year to watch.
What’s so interesting
If you are running an overheating assessment on a scheme, you are modelling that building against future weather files for the 2050s under a high emissions scenario (not a 1.5C world). So MEP engineers have been quietly designing for overshoot for years, because the alternative was handing over flats that literally cook.
Switch your attention to carbon on the same project. The UK Net Zero Carbon Buildings Standard sets its limits by apportioning the UK’s remaining 1.5C-aligned carbon budget and handing the built environment its share. That’s the mechanics sitting underneath the numbers people are now being verified against.
So… same building, two very different assumptions about the future. The MEP engineers assume we’ll fail. The carbon targets assume we’ll succeed.
To be fair to the Standard’s authors, they saw this coming. Question three of their own FAQ is “Can the Standard ever be 1.5°C-aligned if the world is set to pass that point?” Their answer is that even if other sectors exceed their budget, the built environment should still aim to stay within its own, and that this will need continual review and agreement between sectors. That was a reasonable holding position, but it seems to be a much more live question.
What to do
I suppose the lazy conclusion is that if carbon targets are now pointless and we may as well relax them. That is exactly wrong, and the report says so.
In the new “overshoot, peak and decline” world, how fast emissions are cut is exactly what determines how high the peak goes and how long we sit above it. Beyond the widely talked about impacts of climate change, any extra time up there in overshoot territory raises the risk of “tipping points” (critical threshold beyond which a system reorganises, often abruptly and/or irreversibly), such as ice sheet destabilisation or disruption of the Atlantic overturning circulation.
Final thoughts
Adaptation stops being a nice-to-have and becomes the survival case for the asset. We are never going to be able to demolish and rebuild the UK stock to cope with 2C, so very nearly all of that work is retrofit.
I would put money on nobody having asked project teams what climate scenario each consultant has assumed. That could be something to take to your next design meeting or think about for your project brief.