In every city around the world, processes are underway to remove carbon from the atmosphere. While some people may imagine industrial carbon capture and sequestration projects like the Boundary Dam project in Saskatchewan, there are a number of other ways that we can remove carbon from the atmosphere (you can probably some of these at this very moment...). In this research, my doctoral advisor Chris Kennedy and I wanted to quantify these carbon sinks, and determine if they make a substantial impact in "offsetting" the emissions from cities. We used the case study of the Greater Toronto Area (GTA) as it's a large area (~7,100 square kilometres), with lots of green areas and farmland; if anyplace is able to store a substantial quantity of carbon annually, the GTA seemed to be a likely candidate.

Direct Carbon Sinks

We differentiated sinks into two types. The first of these is "Direct Sinks", so named because the sequestration of carbon occurs directly within the politcal boundary of the GTA. These include carbon sinks such as forests and soils. Of course there is some overlap there, as forest soils also sequester carbon, but estimates weren't available as to how much soil might be sequestered (we used IPCC 2006 inventory guidelines). We looked at the the carbon stored in:

• Urban forests (trees within the built-up areas));
• Regional forests (large stands of trees outside these developed areas);
• Perrenial crops (trees grown on Christmas tree plantations, fruit tree farms, and nursuries), and;
• Agricultural soils (soils on farmland transferred from crops)

we get a figure of 317 kilotons of carbon being stored in the GTA in 2005! If the land uses that host these sinks aren't changed to a great degree, we can expect roughly this amount of carbon to be stored every year.

Embodied Carbon Sinks

The second category of carbon sink that we used is less intuitive than the first. As it happens, carbon sinks that have the potential to store substantial amounts of carbon enters the GTA's boundaries each year. We identified three that we thought would be pretty large:

• Concrete (stores carbon as the calcium hydroxide in the cement reacts with CO2 in the air);
• Landfills (store carbon though the biogenic materials that are stored within them - food scraps, lumber, and paper), and;
• Harvested Wood Products (store carbon that is imported in lumber, furniture, etc).

From these 3 types of carbon sinks, we calculated that roughly 234 kilotons of carbon are stored annually. You might be wondering "Isn't more carbon released in the manufacture of these materials/goods than would be stored in cities?" Well, you're certainly right with the case of concrete and you're probably right in the latter two cases. As well, landfills emit a potent greenhouse gas in the process of storing these biogenic materials. So it's important to note at this point that we've quantified "gross" carbon sinks - we didn't include emissions associated with the production either direct or indirect sinks. As well, there's some uncertainty in the calculations made about the quantities that are stored.

Okay, let us assume that there is some degree of certainty in the figures we've calculated for direct sinks (the carbon stored that is a little less dubious). How does that compare to the carbon emitted from the GTA in 2005 (just considering emissions from natural gas, diesel, gasoline, and electricity generation). Well, judging from the figure below, it's not very promsing. Taking a daily average of 2005's emissions, the carbon stored in these direct sinks all year long amounts to the carbon emitted by about January 7th. In short, it'll take a long, long time before the GTA can claim to be carbon neutral within its own boundaries.

Certain carbon sinks have value outside of the direct sequestration of carbon, which should highlighted in light of our carbon calculations. For example, regional forests provide nearby recreational opportunities, improve watershed quality, reduce flood risks, and provide habitat for wildlife. Urban forests can do the same, with the additional benefit of reducing local air pollution and lowering energy bills by blocking the sun/wind, as well as evaporative cooling. Conversely, urban forests can also increase GHG emissions through their maintenance and the maintenance of infrastructure that they can damange when not planted carefully. Pursuing carbon sinks requires a very careful calculation of net benefits, so that there is a clear understanding of what benefits will be achieved.

The Danger of Double Counting

When publishing this article, it occurred to us that a danger existed in quantifying embodied carbon sinks; for example, what if you grow trees that are used in harvested wood products consumed in the urban boundary? Wouldn't those get counted as sinks twice? It is a concern, especially in cities that produce such resources within their borders. As a result, if one were to use our approach, they would have to also provide an estimate of the quantity of the HWP consumption that is locally sourced and subtract this figure.

In the review of our paper, one reviewer came up with an alternative approach to accounting that can reduce the error from these boundary concerns (Wiedmann, 2012). He proposed a system that is similar to what is done with carbon emissions footprint - you quantify the sinks that occur directly in your boundary (no matter the type), as well as those which exist through your consumption. While this is certainly a more precise approach from an accounting perspective, we found that the data necessary to begin to quantify these wasn't available at the urban scale. This was ultimately why we chose the direct vs embodied approach. Hopefully as data becomes available at a finer scale, the approach suggested by Wiedmann will become possible for all cities.

You can download the preprint version of the paper here or if you have a subscription to the Journal of Industrial Ecology, you can download the published version here.