In an increasingly interconnected world, supply and demand are largely spatially segregated while reconnected through distant commodity trade and ecosystem service flows. Many of these transboundary flows can generate both positive and negative impacts and lead to trade-offs in achieving different sustainability goals. However, these impacts are largely neglected due to the inherent challenge in identifying and quantifying those transboundary flows. To better understand how the flows connect and impact different systems environmentally and socioeconomically, this dissertation aimed to provide a methodology synthesis for mapping different transboundary flows, and then exemplify the applications in coupled land-water systems (chapter 2), food-environment systems (chapter 3), and intercoupled global socio-environmental systems (chapter 4). This dissertation research applied the integrated framework of metacoupling (human-nature interactions within a system, between adjacent systems, and between distant systems) to systematically uncover the effects of transboundary flows on sustainability across multiple scales. Chapter 2 developed a typology of transboundary flows using six flow attributes (i.e., type, magnitude, direction, distance, time, and mode). Besides, this chapter provided a portfolio of multidisciplinary methods for characterizing transboundary flows, including trade analysis, big data analysis, and various modeling approaches. This synthesis will facilitate quantitative metacoupling research and inform flow-based governance. Chapter 3 revealed the spatiotemporal dynamics of coastal hypoxia (or dead zone) in the Gulf of Mexico across 20 years (2000-2019), and found chlor-a concentrations and sea surface temperatures are the top predictors of hypoxia, indicating both excess nutrient inflows and climate warming fueled the consequence. This chapter laid a foundation for further linking nutrient hotspots in the watershed with the eutrophicated coasts to predict the timing and amount of nutrient flows for reduction strategies. Chapter 4 tracked the flow of virtual nitrogen (N) embedded in food trade among 132 U.S. regions, and found that 17.1% of the N surplus was induced by food consumption elsewhere but not local. The U.S. Midwest bore most of the N surplus burdens for the nation, with nearly one-third contributed by the lower Mississippi River basin and states other than the basin. This study revealed that downstream food consumers suffering from nutrient pollution are also part of contributors. Chapter 5 comprehensively evaluated a range of transnational flows and their effects on the performance of 17 SDGs for 189 countries globally. It found transnational connections and interactions (e.g., trade) are important for advancing countries' overall SDG performance (improving roughly 20%). Despite the overall benefit, high-income countries generally benefited more, while low-income countries benefited less and were occasionally disadvantaged. The analysis also found that transnational interactions more frequently occurred among distant countries with unequal economic levels. This study offered an integrated picture of the often-ignored transnational impacts on achieving sustainability. This dissertation advanced both theoretical and methodological understanding of transboundary flows in the metacoupled Planet. The cross-scale analysis of measuring multiple transboundary flows and their various socio-environmental impacts on achieving the SDGs will help develop proactive strategies for addressing transboundary challenges, mitigating SDG tradeoffs, and achieving co-benefits toward global sustainability.
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