Material flow studies investigate the movement of physical materials into and out of socio-economic systems. However, some of these materials will remain inside this socio-economic system for an extended period of time. This applies in particular to materials used in the built environment, machinery and equipment, and durable consumer goods (e.g. vehicles, electronics or furniture). These materials are referred to as the material stock, and the method to locate and quantify this stock is called Material Stock Analysis (MSA).

Material flows and stocks are intimately connected and for this reason this method is included in the Flows accounting methods. Most material flow methods define the changes in the material stock, either directly or indirectly, because this is equal to the difference between inflows and outflows. In the Eurostat (2001) EW-MFA method, the material accumulation (or Net Addition to Stock) is a clearly defined indicator, for instance, and there are a number of methodological rules defined around calculating the material stock. Because all material stocks ultimately leave the socio-economic system and end up being recycled or disposed of, there has to be a definition as to when to classify a material flow as a stock. Most studies use one year of permanency to be classified as a material stock, but exceptions can be made (Müller et al. 2014).

In order for a case study to be classified within this MSA method, it is important that not just the change in material stock is quantified, but instead the focus should be on the quantification of the total available stock within a city, either for a single year or for a longer period of time. MSA could focus on a single material like copper (Beers and Graedel 2003), or a particular sector like residential buildings (Condeixa, Haddad, and Boer 2017).

Information on the material stock is generally obtained using one of two approaches: bottom-up or top-down. In an in-depth review of construction material flows and stocks literature, Augiseau and Barles (2016) define these approaches as follows:

“The bottom-up approach is based on a division of the stock into categories (housing, business premises, etc.), and then by the application of material ratios or intensities (in tonnes/m 2 for example). (...) The top-down approach is to quantify stock as the sum of annual net additions to stock over a long period. Stocks are thus derived from the difference between inflows and outflows, calculated from year-to-year. These flows are known from statistical data (construction and demolition), or are estimated, based on average lifetimes or survival functions.”

Data availability often varies significantly between countries and cities, and most studies will select an approach depending on the available data. There is also no standard approach around forecasting (which is often an important component of the study). This makes it difficult to make comparisons between studies (Augiseau and Barles 2016).

MSA can, however, provide insights that MFA studies lack. This type of study can be very spatially explicit, especially when studying the built infrastructure. Furthermore, this method allows for the exploration of the potential for cities to serve as “urban mines”, in which the recovery of materials from existing stock can replace import of new materials. Such a study was undertaken recently on the city of Amsterdam, for instance (van der Voet et al. 2017). Another innovative approach is to study the available energy stock and to unpack what this means for the city’s resilience in light of possible energy supply shocks (Bristow and Kennedy 2013).