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.

  • Let's be honest About a year after these green walls get installed no-one will maintain them and they'll die because of pollution, lack of water, weeding or a really hot spell. Better off investing in a green project away from the concrete jungle, which only covers about 2% of the earth's surface and is a poor environment to grow plants

  • Hi David, You are right. Green roofs are little more than window dressing in the total energy demand of a building. But we are building a zero net energy medium rise office block as a proof of concept, solar thermal powered building in Adelaide Australia; the building is heated cooled and ventilated by a combination of solar thermal and solar PV power.
    Cheers Roger Davies

  • I share the concerns raised here, and while I support the principle and value of Green Roofs for, say, local food production, and for their cooling benefits, and understand the costs of the weight involved with deeper soils, I have always believed in the value of good soils in achieving the best results for plant growth. Despite the value of green roofs, we can't grow forests on them.

  • Would you allow "off site" sequestration benefit, or othe GHG savings in your calculations? If we make buildings that are net producers of energy and thereby save on the use of fossil fuels, would this meet your criteria for being "regenerative?"
    Most of the green roofs that I know of are not designed to be carbon sinks directly, but are intended to slow down runoff, and avoid having to dimension stormwater drains for a fewer number of more intense weather events.
    On the other hand, encouraging true rooftop parks with (small) trees, vines, and shrubs, or encourang people to have (productive) woody plants on their balconies, is more directly a form of sequestration, even if it doesn't necessarily occur on a vast scale.

  • Well said, David, and a timely reality check. The difference between 'green' and 'greenwash' is the difference that business-as-usual is happy to ignore, and I fear that the advance of real greening and sequestering is being done no favours by the current fashion for, in effect, spraying a thin film of vegetation over otherwise conventional construction. As your article intimates, more rigour is needed, and more commitment to measurable outcomes.
    Meanwhile, as the roof garden on the Christie Walk project in Adelaide demonstrates, a real intensive green roof can perform in a comparable manner to a garden whilst providing water management and thermal benefits to the built environment.

  • May be off-topic, but I always thought that "old growth forest" was a net GHG emitter (CH4, CO2, water vapour).

  • Good article, as we continue to improve buildings we should sequester the carbon associated with using the building elsewhere such as higher in the water catchment by offsetting the construction and operational emissions through Greenfleet' carbon forestry program.

  • I don't see how solutions like deep earth covering and intensive green roofs are even remotely viable in Australian cities given their rising population density and the attendant increase in the number of high rise buildings.

  • Thanks David – as you say, there are sufficient benefits from even quite shallow green roofs and walls to justify their installation, but seeing them as being a serious element in carbon sequestration is naïve and little better than green wash.

    I'd love to see all councils and governments adopting a 'three tree fee' system which simply says that any time you cut down a tree (whether through building activity or re-landscaping) you are obliged to replace it with three new ones: either suitable on-site plantings that stand a good chance of long-term survival, or a sensibly-estimated fee to the local council/government to plant and manage three trees in a suitable place on your behalf. Now that WOULD be regenerative!

  • Sounds like we'd be better off harvesting mature street (and park) trees and replanting; to keep storing carbon.