Identifying strategies for reconciling human development and climate change mitigation requires an adequate understanding of how infrastructures contribute to well-being and greenhouse gas emissions. While direct emissions from infrastructure use are well-known, information about indirect emissions from their construction is highly fragmented. Here, we estimated the carbon footprint of the existing global infrastructure stock in 2008, assuming current technologies, to be 122 (-20/+15) Gt CO2. The average per-capita carbon footprint of infrastructures in industrialized countries (53 (± 6) t CO2) was approximately 5 times larger that that of developing countries (10 (± 1) t CO2). A globalization of Western infrastructure stocks using current technologies would cause approximately 350 Gt CO2 from materials production, which corresponds to about 35-60% of the remaining carbon budget available until 2050 if the average temperature increase is to be limited to 2 °C, and could thus compromise the 2 °C target. A promising but poorly explored mitigation option is to build new settlements using less emissions-intensive materials, for example by urban design; however, this strategy is constrained by a lack of bottom-up data on material stocks in infrastructures. Infrastructure development must be considered in post-Kyoto climate change agreements if developing countries are to participate on a fair basis.

Light-weighting of passenger cars using high-strength steel or aluminum is a common emissions mitigation strategy. We provide a first estimate of the global impact of light-weighting by material substitution on GHG emissions from passenger cars and the steel and aluminum industries until 2050. We develop a dynamic stock model of the global car fleet and combine it with a dynamic MFA of the associated steel, aluminum, and energy supply industries. We propose four scenarios for substitution of conventional steel with high-strength steel and aluminum at different rates over the period 2010-2050. We show that light-weighting of passenger cars can become a "gigaton solution": Between 2010 and 2050, persistent light-weighting of passenger cars can, under optimal conditions, lead to cumulative GHG emissions savings of 9-18 gigatons CO2-eq compared to development business-as-usual. Annual savings can be up to 1 gigaton per year. After 2030, enhanced material recycling can lead to further reductions: closed-loop metal recycling in the automotive sector may reduce cumulative emissions by another 4-6 gigatons CO2-eq. The effectiveness of emissions mitigation by material substitution significantly depends on how the recycling system evolves. At present, policies focusing on tailpipe emissions and life cycle assessments of individual cars do not consider this important effect.

To reach required product qualities with lowest costs, aluminum postconsumer scrap is currently recycled using strategies of downgrading and dilution, due to difficulties in refining. These strategies depend on a continuous and fast growth of the bottom reservoir of the aluminum downgrading cascade, which is formed by secondary castings, mainly used in automotive applications. A dynamic material flow model for the global vehicle system was developed to assess the likelihood, timing, and extent of a potential scrap surplus. The results demonstrate that a continuation of the above-mentioned strategies will lead to a nonrecyclable scrap surplus by around 2018 ± 5 if no additional measures are taken. The surplus could grow to reach a level of 0.4-2 kg/cap/yr in 2050, corresponding to a loss of energy saving potential of 43-240 TWh/yr electricity. Various intervention options for avoiding scrap surplus are discussed. Effective strategies need to include an immediate and rapid penetration of dramatically improved scrap sorting technologies for end-of-life vehicles and other aluminum applications.

Aluminum recycling currently occurs in a cascading fashion, where some alloys, used in a limited number of applications, absorb most of the end-of-life scrap. An expected increase in scrap supply in coming decades necessitates restructuring of the aluminum cycle to open up new recycling paths for alloys and avoid a potential scrap surplus. This paper explores various interventions in end-of-life management and recycling of automotive aluminum, using a dynamic substance flow analysis model of aluminum and its alloying elements with resolution on component and alloy level (vehicle-component-alloy-element model). It was found that increased component dismantling before vehicle shredding can be an effective, so far underestimated, intervention in the medium term, especially if combined with development of safety-relevant components such as wheels from secondary material. In the long term, automatic alloy sorting technologies are most likely required, but could at the same time reduce the need for magnesium removal in refining. Cooperation between the primary and secondary aluminum industries, the automotive industry, and end-of-life vehicle dismantlers is therefore essential to ensure continued recycling of automotive aluminum and its alloying elements.

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