468174 Evolution and Robustness of the Global Agricultural-Phosphorus Trade Network

Tuesday, November 15, 2016: 1:45 PM
Union Square 14 (Hilton San Francisco Union Square)
Andrew Beck, Carla Ng and Vikas Khanna, Civil & Environmental Engineering, University of Pittsburgh, Pittsburgh, PA

Containing one of the three essential nutrients for plant growth, phosphatic fertilizers are an integral input to modern agriculture, and will always have a global demand. As the world population continues to grow through mid-century, a significant increase in food production must follow, potentially furthering our current dependence on synthetic fertilizers. While the global resource-criticality of phosphate rock reserves – from which the majority of these fertilizers are derived – may be uncertain, the trade network upon which many countries depend for access to these fertilizers (and their precursors) is a well-documented complex system, subject to an array of physical, economic, and geopolitical constraints. If we are to collectively achieve global food security by 2030, as per the U.N. Sustainable Development Goals, the global security of agricultural nutrients must also be ensured. To formalize questions of access to these nutrients, network analysis provides an attractive quantitative framework with which to examine features of individual relationships and of the underlying structures of complex systems.  

The present work focuses on characterizing the structure and understanding the systemic vulnerabilities of the global agricultural phosphorus trade network (GAPTN). We accomplish this by modeling a directed, weighted global network of annual phosphorus-containing commodity exchanges, including all synthetic phosphatic fertilizers and their chemical precursors. All commodities are further normalized to their weight content of elemental phosphorus. As such, nodes are taken to be countries participating in phosphorus trade, and edges are taken to be the aggregate of the directed trade flows of elemental phosphorus between each pair of countries, resulting in the construction of a global agricultural phosphorus trade network (GAPTN). The primary objectives of this study are to (1) characterize the network topology and evolutionary trends of the GAPTN. (2) Elucidate the sets of nodes and edges that are critical to the overall function of the network. (3) Identify those countries most dependent on the network’s functioning, and outline engineering and policy measures such that they may ensure their consistent access to this essential resource.  

Edges within the GAPTN display mild heterophily, preferentially connecting nodes of dissimilar degree. Based on statistical testing, the distribution of GAPTN node degrees follows a heavy-tail distribution, as does the distribution of in-, out-, and total-strength for 2014. Such distributions indicate that a small subset of all of the participating countries exchange most of the resources that comprise the system, leaving the network robust to random disruptions and highly vulnerable to inopportune ones. The potential for inopportune disruptions stems from both the price volatility of phosphatic fertilizers with respect to energy prices, and the geopolitical tension that surrounds Western Sahara & Morocco, which owns some 75% of the estimated global phosphate rock reserves. Further confirming the implications of the degree distributions, a robustness analysis of randomized versus targeted removal of nodes from the network indicates that some 80% of the network’s overall activity halts when some 5% of the nodes do not participate. Results from node-level analyses reveal a small subset of countries to be particularly vulnerable to supply disruptions, even as the overall network has become more redundant and dense from 1990 to 2014. In light of these findings, both international cooperation on phosphorus management and country-level efforts to close local nutrient cycles can aid in improving food security while also minimizing the ecological impacts associated with fertilizer run-off, especially eutrophication.

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