>The developer claims the wall – made up of a mix of plants – will generate seven tonnes of oxygen in a year and extract nine tonnes of C02 annually among other benefits
So the plants in the wall get bigger and heavier each year? Presumably there's a limit, structurally as well as visually. I'm no expert, but I believe 9 tonnes of CO2 means an extra 2.5 tonnes of carbon every year.
The wall is close to 200m long and 40m tall. If it were made of 4000 cubic meters of concrete, it might weigh 10,000 tons, and the whole building might weigh 4 times that amount. 2.5 tons is a rounding error!
Unfortunately, the chemistry and energy use to make a cubic meter of concrete releases about a third of a ton of CO2. So this building, implemented in concrete, would extract 9 tons of CO2 annually and take more than 1,000 tons of CO2 to pour.
Edit: I now see that the images (especially [1]) describe a more expensive steel-and-glass building. It will probably still include lots of concrete, but 2.5 tons across the whole building is not a big concern. (Not to mention they'll likely have to do some gardening occasionally).
Keep in mind that the 2.5 tonnes is carbon only, the mass of additional plant matter is likely more than double that given the water content and other elements. Additionally, it's not evenly distributed over the building, but cantilevered out.
That said, I agree with you that it's likely to be trimmed for aesthetic reasons well before it becomes a structural issue; which is why the annual carbon offset claimed is misleading - it assumes the biomass increases annually without intervention.
I suppose they could be trimming and composting/bio-charring excess plant material, but given the lack of information I kind of doubt that a serious CO2 life-cycle and residence analysis has been done.
Yeah, this is most likely just a gross multiplication of CO2 absorption with plant mass that completely ignore the fact they are hiring some plant care company that's just gonna bin any overgrowth and completely neglect the absorption.
But hey, it lets guests make themselves look good on twitter and isn't that all that really matters?
So the plants in the wall get bigger and heavier each year? Presumably there's a limit, structurally as well as visually. I'm no expert, but I believe 9 tonnes of CO2 means an extra 2.5 tonnes of carbon every year.