Urbanisation increases the need to include city surfaces in assessments of the world’s carbon cycle. Already 4 percent of our planet’s land surface is urbanised and that percentage is likely to increase over decades to come.

But although new research shows the British city of Leicester is presently absorbing 231,000 tonnes of carbon, this is not a sink we would trust, expanding the scale in space and time – not without focused biomass management at least.

A group of British scientists have in that central English city of 300,000 inhabitants measured the carbon-absorbing capacity of the parks, gardens, industrial land and other forgotten green zones, like road verges, playgrounds and river banks. Earlier this week the results were published in the Journal of Applied Ecology.

Photosynthesis in your backyard

They find the urban area sequesters some 3-16 kilograms of carbon per square kilometer. But before we conclude from this that cities are a net carbon sink, we should know what causes the absorption of CO2. It’s vegetation, the researchers say – and not just any, but trees in particular: some 97 percent of the sequestered carbon.

First of all trees are not an exclusive landmark to cities. Chop down a forest to build housing blocks and -of course- you’ll lose carbon. It would be down to the architects and planners to include some green zones to compensate part of the damage. People in Leicester may appreciate gardens and golf courses – people on continents where true urbanisation currently takes place may hold far more efficient views regarding the use of precious city space – concrete views.

Growing towards equilibrium state

Secondly any net CO2 absorption by trees is always a temporary phenomenon*. For Leicester it may be the phase of garden and park development. It may even be that high latitude negative ‘forest’ feedback. But it will stop as soon as the trees reach their maximum height and die-off will equal growth.

Of course even in such an equilibrium phase any carbon store is valuable. This means parks and domestic gardens can still have some benefit to the climate – though highly marginal on the global scale.

Including urban biomass management

Things could be different though. As long as you allow the living plants to keep growing, absorbing CO2 from the air – it depends on what you do with the dead biomass. Burry it, turn it into biochar or burn it in your local power plant (so you no longer need to add fossil carbon to the atmospheric cycle) – anything is better than just having it rot away, slowly releasing all that CO2 back to the atmosphere, indeed exactly these 231,000 tonnes of carbon we thought we were gaining on the atmosphere today.

You need some bookkeeping talent to fully understand the Earth’s surface carbon cycle. And if you have that, you could indeed do great work as a green city architect. We shouldn’t be cheering before that challenge is met.

[*) Except for places where climate and geography allow the formation of thick layers of peat, like the taiga pine forests or the rainforests of Borneo. Here though we get to soil biochemistry. Yesterday we were reminded once again this only complicates the story – because then you’d suddenly need to include other greenhouse gases to the equation.]

© Rolf Schuttenhelm | www.bitsofscience.org