How can Chinese metropolises drive global carbon emissions? Based on a nested multi-regional input-output model for China

Given metropolises' participation in complex regional and global trade networks, they have huge demands for vast carbon-embodied intermediate and final goods. To clarify embodied carbon transfers of metropolises in regional and international trade, a metropolis-centered model was constructed by nesting the World's multi-regional input-output table and China's multi-regional input-output (CMRIO) table. Based on this model, we analyzed the multi-scale impact of two typical Chinese metropolises, namely Beijing and Shanghai, on global carbon emissions. Structural decomposition analysis and social network analysis (SNA) were used to explore the driving factors of consumption-based carbon (CBC) and the roles of metropolises in the carbon networks. Results showed that both Beijing and Shanghai are net embodied carbon consumers, which respectively drove 231.19 and 219.52 Mt global carbon emissions in 2017. These figures were underestimated by 12.54 % and 15.41 % when using the CMRIO. After China's economy entered a new normal, instead of technological progress, structural adjustment became the prominent factor driving the CBC reduction of metropolises. During 2012-2017, the consumption structure optimization reduced 18.87 and 32.48 Mt CBC in Beijing and Shanghai, respectively. Compared with other domestic regions, the CBC of Beijing has continued to increase, whereas that of Shanghai has declined. At the international scale, the combined net carbon emission imported by the two metropolises was 88.43 Mt in 2017, equivalent to 18.09 % of China's total carbon deficit. This indicates that metropolises have become pioneering regions for China to alleviate the carbon deficit in international trade. By using SNA, we further found that both metropolises are crucial carbon consumers in the global carbon network, with strong stability and obvious hub roles. Furthermore, various urban functions and geographical locations form the heterogeneous structural characteristics of CBC in the two metropolises, highlighting the need for different strategies for embodied carbon mitigation in these metropolises.