Urban Geoengineering part 2 - Urban Afforestation

After discussions of last week and CDR our preferred option, how can we expect to practically achieve negative emissions by urban geoengineering? With CDR fields at varying degrees of maturity, it is essential to remember this is not the CO2 silver bullet. Authors such as Christopher Field and Katherine Mach highlight drawbacks in solely relying on CDR, necessitating critical thinking when exploring what urban CDR can really offer.

Lets remember, we need to remove several billion tonnes of Carbon(GtC) annually to limit warming to 2 degrees by 2100 and we release another 10 billion tonnes a year. So, when considering urban CDR, we have to integrate mitigation with our removal strategy.

Melbourne City Urban Forest Strategy Precinct Plan Source: AWOL, 2015

But first, a side note on sinks

To evaluate urban CDR, we first need to consider carbon sinks. Globally, terrestrial and ocean sinks sequester 3.1 GtC into the biosphere, which is far from parity with the 10 GtC we emit annually. However, effective sink management can add 2.5 GtC in capacity, which makes a none-too-shabby 5.6Gt C/year. Urbanite activities influence non-urban sink management, but focusing directly on urban sinks will help to maximize urban CDR's impact. Cities need to be closed loop to be sustainable after all, and this suggests streamlining urban sink management.

Mohareb and Kennedy(2012) outline these sinks in figure 1(below):

Mohareb and Kennedy outline these carbon sinks in their 2012 paper, splitting them into:
1) Direct sinks(a) and 2) Embodied sinks(b, c and d)

Clearly, urban environments possess a diversity of novel sinks, but how can we be sure this method of CDR is really worth investment of resources? Lets break down Urban CDR options.


Our first CDR candidate, urban afforestation

Globally, afforestation and reforestation promise declines of CO2 equivalent to 40-70ppm by 2100, making urban afforestation attractive. While critics have pointed out the susceptibility of biosphere carbon to re-release, cities such as London and and New York are investing £3.8 million and $400 billion respectively into enormous urban afforestation projects, placing enormous trust its potential. Municipal governments have also began mobilizing carbon credits to fund urban forest maintenance and encourage corporate environmental responsibility. This video demonstrates some of the benefits to this strategy:

At risk of being too easily sucked in by these greening perspectives, what concentrations of carbon are being sequestered? The 2015 London tree core report found that Greater London's urban forests sequester 77,200 tonnes per year(see figure 2), with a value of £4.36 million, while American cities are projected to sequester 22.8 million tonnes per year, worth $460 million. With co-benefits of mitigating urban heat island(UHI) through evapotranspiration and energy efficiency savings mitigating a further 300 million tonnes in some models. Afforestation seems to be full of promise, but are we being lulled into a false, tree-hugging, sense of security? Clearly, the numbers being sequestered are a relatively small fraction of anthropogenic emissions and sequestration targets, but certainly contribute to the effective sink management strategy the IPCC have proposed when considering global scale applications.

Figure 2: A tree graph that I made, demonstrating the carbon sequestration (Tonnes per year) in different urban forests in a number of American and European cities: I used data from: London Tree Core Report 2015 to construct this graph.

Vertical Greenery Systems(VGS) 

Urban afforestation need not be limited to the ground. This video provides insight into urban VGS application and some difficulties to expect.

Indeed, in line with Juli Capella, VGSs provide a host of benefits in an urban context including reduced UHI effect, reduced cooling energy load and direct carbon sequestration. However, in order to make this a viable option for effectively sequestering carbon, we need to assure that we properly manage residual biomass and choose the right species for the job. A model of a 25 year Italian project for one facade of 98 square meters captured up to 97kg CO2 a year, with an average long lasting CO2 accumulation of up to 1640KgCO2eq a year. This, along with other studies, asserts VGS as a strong afforestation option in urban contexts. However, much like general afforestation previously mentioned, this sequesters a small share unless deployed at massive scale.

Green roofs (GR) 

GRs are another branch of urban afforestation. They bear huge potential in cooling buildings, increasing evapotranspiration, reducing cooling energy load and mitigating UHI effect. However, we should take caution in treating this as a significant CDR option. While limited studies demonstrate reductions in ambient CO2 concentrations by GR, with sequestrations of 7kg of Carbon a year in a Chengdu study for one roof, this is significantly lower than ground sequestration afforestation systems, which are already relatively low. However, when considering that roofs constitute 25% of the urban surface, it becomes clear that widespread deployment will certainly contribute significantly to sequestration along with added mitigation benefits and adaptation, as seen in New York(video below).

  

                                                

To green or not to green?

While afforestation is the most mature CDR field, authors such as Emily Oldfield highlight issues around effective urban afforestation, with establishing a self sustaining urban forest being poorly understood and requiring extensive management. Further criticism is drawn around the increased susceptibility of the biosphere stocks to release carbon with climatic changes. The sequestration numbers alone highlight these limitations.

And yet, I'm 100% for urban afforestation. Yes, urban afforestation cannot sequester all the carbon we need to but currently, not one solution can achieve this. Diversification and integration with mitigation strategies and other CDR techniques is crucial to maximize impact, and refining urban biomass carbon sinks is equally part of a global drive to streamline the terrestrial sinks I mentioned earlier to add a 2.5GtC capacity. Urban Afforestation is promising, from direct carbon sequestration to reducing energy demands in cooling and UHI, but needs to form part of an integrated plan using a number of diverse technologies and applied across both urban and non-urban contexts, and is not a standalone strategy.