The world's population is now over 50% urban, and cities make an important contribution to national greenhouse gas (GHG) emissions. Many cities are developing strategies to reduce their emissions. Here we ask how and why emissions differ between cities. Our study of ten global cities shows how a balance of geophysical factors (climate, access to resources, and gateway status) and technical factors (power generation, urban design, and waste processing) determine the GHGs attributable to cities. Within the overall trends, however, there are differences between cities with more or less public transit; while personal income also impacts heating and industrial fuel use. By including upstream emissions from fuels, GHG emissions attributable to cities exceed those from direct end use by up to 25%. Our findings should help foster intercity learning on reducing GHG emissions.

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Summary Human activities have significantly intensified natural phosphorus cycles, which has resulted in some serious environmental problems that modern societies face today. This article attempts to quantify the global phosphorus flows associated with present day mining, farming, animal feeding, and household consumption. Various physical characteristics of the related phosphorus fluxes as well as their environmental impacts in different economies, including the United States, European countries, and China, are examined. Particular attention is given to the global phosphorus budget in cropland and the movement and transformation of phosphorus in soil, because these phosphorus flows, in association with the farming sector, constitute major fluxes that dominate the anthropogenic phosphorus cycle. The results show that the global input of phosphorus to cropland, in both inorganic and organic forms from various sources, cannot compensate for the removal in harvests and in the losses by erosion and runoff. A net loss of phosphorus from the world's cropland is estimated at about 10.5 million metric tons (MMT) phosphorus each year, nearly one half of the phosphorus extracted yearly.

SummaryThis study provides a global substance flow analysis for gallium (Ga), germanium (Ge), and indium (In) for 2011, quantifying the amount of metal lost during extraction, beneficiation/smelting/refining, manufacturing of intermediate products, and the amount embodied in end-use products. Thus far, studies illustrating their cradle to end-use life cycle on a global scale are either missing or outdated, and thus opportunities to increase their supply remain unknown and/or not quantified. The results illustrate the losses and inefficiencies stages, thereby identifying potential additional supply by process improvement, recovery, and recycling. Results show that there are significant opportunities to meet future demand of Ga and Ge by concentrating recovery efforts in the extraction and beneficiation/smelting/refining stages. Further, 1.4% Ga, 0.7% Ge, and 54% In of the theoretical available amount in the attractor ores are extracted to meet the primary refined demand in 2011. Of the 9,065 tonnes (t) of Ga embodied in the Bayer liquor (from aluminum production), only 263 t are refined. This is owing to low capacities of Ga refining, combined with a refining efficiency of 60%. Ge presents a similar case for the same reasons, in which only 43 t of Ge of the 7,636 t of Ge available from zinc leach residue are refined. Meeting future In demand, on the other hand, will require greater efforts in increasing end-of-life recycling. Process efficiencies for Ga (46%), Ge (56%), and In (78%) demonstrate further potential. We quantify the flows into use by distinguishing among dissipative and nondissipative end uses, as well as the recyclable fraction for each metal for 2011. Keywords:criticality industrial ecology metal demand scarce metals sustainable metal management waste Supporting information is available on the JIE Web site