Demolition and rebuild is often justified through predictions of carbon savings in the future, but such calculations are often misleading, writes Liam Bryant
As designers we’re increasingly monitoring the climate impact of our projects, with a surge in efforts to assess and quantify carbon emissions; however, the methods we use are still relatively rudimentary and agreement on even relatively “simple” issues such as how to record sequestered carbon is still far off.
Without making the perfect the enemy of the good, there are numerous flaws in our current approach that can lead to perverse decisions, leading to schemes proposing demolition and rebuilding on the basis of operational carbon savings that won’t manifest until years in the future.
We’ve all seen schemes selling the virtue of proposals that demolish existing buildings on the basis of a slightly reduced Whole Life Carbon (WLC). But is this really sustainable?
In Whole Life Carbon assessments, the total construction (now) and operational and maintenance (future) carbon is summed up and used to calculate the environmental performance of a proposal; treating carbon emitted today the same as carbon in sixty years’ time.
In reality any attempt to predict the future is almost guaranteed to be wildly inaccurate, with even careful prognostication highly uncertain and these calculations should be treated with caution. Only construction carbon is guaranteed, whereas operational carbon is a guess stretching into the future.
Carbon emitted today has an immediate global warming impact, while carbon emitted in 10 years will only start heating from then. That’s a decade of additional warming and by the time we’re 30-40 years in the future as per the WLC assessments, there’s a reasonable chance that the building has been demolished / caught fire / become outdated. By relying blindly on the output of WLC calculations to define “sustainable” we’re at risk of making worse decisions.
Maybe the construction industry can be green now and in the future
We often hear talk of a “carbon budget”, the theoretical amount of carbon that humanity can emit without causing critical climate damage. Now there are a lot of issues with relying on economic terms for such a complex system, but let’s take this analogy a step further and consider this as if it were our own finances.
If your accountant told you that you could spend £1,000,000 over the next decade and you immediately spent £900,000, but promised you’d be very careful for the next 9 years, they would likely have some comments.
Yet somehow this giant prayer across the construction industry, to make ourselves virtuous and green, but not yet, goes relatively unremarked. We’re borrowing carbon from the future, with the promise that we’ll be good for it and our deposit is the health of the planet; allowing ourselves options that future generations won’t have.
Now, this is clearly a simplification, but it is a real problem. So, what can we do?
Well, let’s go back to economics, where they use a system called Net-Present Value, discounting future cash to a value today, working out the value of promises of payments far into the future. This helps understand the cost of mortgages, bonds and more.
Similarly, we need a net-present value of carbon. To minimise our climate impact we should increase the importance of carbon emitted today over that of potential future emissions, accounting for the reduced probability (maybe we don’t emit it at all?) and the reduced impact (at any given point in the future it will have been in the atmosphere for a shorter time / had a lesser impact on the climate).
So, what does this mean? Well, 1kg of carbon emitted tomorrow is still pretty bad, but 1kg of carbon in 50 years, is, let’s say, only a 50/50 bet, so maybe it’s worth 0.5kg Net Present Carbon. Between those two points there’s a sliding scale. Technically there’s some exponents if we do this properly, but that’s a follow up and rather dry article… This adjustment could simply then be slotted into a WLC calculation to give a more accurate representation of the climate impact of a scheme.
This isn’t intended to replace WLC, but to supplement it, and enable a more nuanced look at our schemes. I suspect that we’ll find that the climate impacts of demolition and re-build will be highlighted, and the benefits of deep retrofit projects are more clearly marked.
With this more nuanced tool we could identify the real “sustainable” options and stop the absurdity of bulldozers promising a greener future. Maybe the construction industry can be green now and in the future.
Liam Bryant is an Associate Structural Engineer at Webb Yates