Four storeys high and made almost entirely of wood, the ZEB Lab building in Trondheim, Norway, had, even before it existed, sucked as much carbon from the atmosphere as it would probably produce in construction. Now, thanks to its arboreal origins, as well as to the sleek expanse of solar panels on its roof and to other energy efficiency measures, it is a carbon-negative building. In other words, from birth to demise, it will have drawn down more carbon than it emitted.