There is much being written about how buildings can be a big part of the solutions to truly sustainable cities and a big part of this claim hinges on their perceived ability to deliver not only net carbon zero or positive outcomes but other essential ecosystem services as well.

But how much is pie-in-the-sky and how much is grounded in reality?

There is no doubt that on-site and off-site renewable energy sources, energy efficiency and innovative carbon sequestering structural materials will have a major role to play in moving buildings toward a more sustainable future. There is also no doubt that vegetated cities and interiors create better human health outcomes and ‘green infrastructure’ such as on-building vegetation in all its forms is assumed to be an essential component of the future sustainable city. But does it really deliver carbon sequestration benefits?

As one of the original proponents of earth covered buildings, green roofs and walls since the early 1980s, I have to say that I have been an avid proponent of the potential for landscape on and in buildings as key to the future of sustainable cities.

But as things have moved forward and the deep earth covering or ‘intensive green roofs’ of 0.5 to 1.5 metres of soil covering with major shrubbery and even small and large trees has given way to ‘extensive roofs’ and green walls as little as 50 to 150 millimetres of lightweight soil ‘media’ with small succulent and decorative plants, I have begun to wonder just how much carbon absorption such little soil and such small plants without woody stems can actually provide.

After all, for trees to trap and sequester carbon, they grow woody trunks and branches that largely stay on the tree for the life of the tree, increasing in size as they absorb and store carbon. They also develop and foster the continued growth of soil carbon as a result of the interaction of micro-flora and fauna with leaf detritus and further carbon from the air.

These are both, long term growing carbon stores that increase in size with the age of the trees. Furthermore, these woody stemmed plants live a long time and in combination with deep, healthy soils laid over sand and filter fabric, the water cleansing, air cleansing and biodiversity encouraging dense foliage provided without doubt provides meaningful ecological services.

To save structure and embodied impacts and costs, these ‘deep and meaningful’ soil and plant associations have been pared back, but the perceived attributed benefits have not. It’s time to re-assess and quantify the benefits of greenery in all its different forms on and in buildings if we are to continue to depend on them to reduce atmospheric carbon as part of project net carbon zero or positive commitments.

There have recently been a number of illuminating studies on this subject that can shed some light on the metrics and performance of a range of plants and soil depths.

Urban trees help sequester carbon in new growth every year. Given that about 50 per cent of wood by dry weight is comprised of carbon, trunks and roots can store carbon for decades or even centuries with several tons of atmospheric carbon dioxide being absorbed for an average sized tree over its life (though this varies to some extent by species).

In fact it has been estimated that the 100,000 public trees in Melbourne would sequester about one million tonnes of carbon, while Brisbane’s residential tree cover was estimated to be absorbing the equivalent amount of CO2 emitted by 30,000 cars per year, and cooling surface temperatures in the relatively mild month of October 1999 by up to five degrees Celsius.

Trees also play an important role in protecting soils, which is one of the largest terrestrial sinks of carbon. Soils are an extremely important reservoir in the carbon cycle because they contain more carbon than the atmosphere and plants combined.

Finishing in 2006, the University of Michigan studies 32 extensive lightweight green roofs in North and North Eastern USA. They were composed primarily of sedum species, substrate depths ranged from 2.5 to 12.7 centimetres and over the two-year test, were found to sequester 0.375 kilograms of carbon per square metre in above- and below-ground biomass and substrate (note the studies don’t call it soil!) organic matter.

This raises a few questions in my mind: ‘is this sequestration rate going to continue annually or without management of the soil and biomass will it decrease, and if so how far?’ and ‘if management is needed to keep the sequestration rate level, what does that management look like?’ and ‘should such short term carbon sequestration even be counted in carbon models?’

While typically used on extensive roofs even in Australia – sedums are particularly suited to the harsh conditions of extensive roofs – they aren’t prolific growers, are comparatively short lived and they don’t have woody stems so will store less carbon than trees, shrubs and woody vines. Other species used on green walls are typically also not woody stemmed or long lived, so we get to a point where we have to ask ourselves, is it even valid to count extensive roofs and green walls in long term carbon sequestration calculations that typically have a timeframe of 30 to 100 years?

While there is no doubt that green roofs, green walls and landscaped podiums have substantial health and environmental benefits, for me the jury is still out on whether thin soil greenery elements can provide enduring carbon sequestration. Based on the research I have found to date, there is still much we need to know before we can be definitive, and while extensive green roofs can only absorb around 0.2 kilograms of carbon per square metre per year, it’s unlikely that they will ever be any more than just a very small part of any net carbon or net positive carbon cycle.

To even suggest that such elements on and in buildings can be ‘regenerative’ is to misunderstand the concept.