NAS equals all materials added to stocks (i.e., with an expected commodity life span of more than a year) minus wastes and emissions from stocks. NAS represents the net growth rates of the material stocks of a society. NAS is highly relevant to sustainable development for the following three reasons:
• Material stocks comprise mainly built-up infrastructure such as roads. Increasing NAS therefore indicates an increase in land sealing.
• All material stocks bind future material inputs because the stocks have to be maintained and reproduced.
• In the future stocks will turn into wastes and emissions when they are disposed of. Therefore stocks have to be regarded as future waste flows and burdens to future generations.
Unfortunately, the present level of methodological sophistication in MFA hardly allows us to generate reliable indicators for NAS. To calculate NAS other than as a simple statistical difference between inputs and outputs, we need estimates for existing stocks and their life span so that annual wastes and emissions from stocks can be calculated, precise estimates for balancing items between inputs and outputs (additional air and water vapor; see Figure 12.1) so as to not statistically lump them together with stock changes, and above-average-quality data for the input of construction minerals.4 Some efforts have been made to improve this situation (see Barbiero et al. 2003), and in light of the importance of this indicator for sustainability, further efforts seem well warranted.
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