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Analyse en réseaux complexes des flux physiques internationaux de produits agricoles : structures et évolutions de 1986 à 2016

Abstract : The issue of climate and energy transitions is crucial for agri-food systems, which are highly sensitive to environmental conditions, and rely for over 95% on fossil fuels (oil, coal, gas) as well as they are subject to intense international trade. The multiplication of maritime, air, rail and road transport infrastructures throughout the world since several decades has made it possible to strengthen international trade and interconnect agricultural systems at the global scale. Thus, the issue of food security goes beyond domestic frontiers and requires taking into consideration the globalization of food and feed supply chains. The understanding of the mechanisms underlying the evolution of the global trade network is addressed here through and forward-looking analysis of the recent trajectory of global agri-food flows. The methodology is based on the theory of complex networks composed of nodes, which are the countries, and links, which are the trade flows in mass. The FAOstat database provides the origin-destination matrices of agricultural products flows per country since 1986. We study the world trade network and its evolution from 1986 to 2016 by aggregating the traded commodities into six product categories: cereals, meat, oilseeds and fruits & vegetables which compose the basis of diets and cocoa and coffee which are luxury products. Agri-food networks have become significantly denser over the period and their structure has evolved similarly for the six product categories. In 1986, the six networks present the same characteristics of a scale-free network with the hubs being the largest Western exporting countries. A transition in the network topology is observed between 1995 and 1997 and is characterised by a shift towards a network with an exponential distribution of degrees. This transition means that the number of countries with a high degree of connectivity is increasing. The change in the network structure coincides with the implementation in 1995 of the Agreement on Agriculture targeting the liberalization of markets. The temporal structure and distribution of trades is similar for the six product categories and the trades are differentiated according to their temporal stability. We identify three subnetwork structures with distinct and time-invariant topologies. The backbone subnetwork is composed by uninterrupted trades throughout the study period and represents about 20% of total links and 70% of total mass and is scale-free. Trades with durations included between 2 and 29 years present an exponential degree distribution and compose the intermediate subnetwork, which has built on commercial agreements with emerging countries and grows rapidly. Finally, labile trades with durations included between 1 and 2 years form the transient subnetwork which gather the majority of total trades (60 to 70% depending on the years) and shows random characteristics. This last subnetwork represents commercial noise of extremely unstable and possibly opportunistic trades. To address the issue of food security, we build the global food network by integrating product transformation processes and by converting traded mass into calories. For each country, we calculate the net balances of calories also accounting for the unavoidable metabolic losses of livestock. The network of calories highlights large deficits and, therefore, food dependency on global trade of several countries in Africa, Eastern and Northern Europe and Central Asia. The structure and evolution of the network of calories is similar to that of the six product categories and its analysis allows studying the specific structures between surplus and deficient countries. The subdivision of trades according to their temporal stability reveals the same three invariant subnetworks structures. This thesis provides a solid basis for constructing prospective network modelling approaches for agriculture and food security in a context of increasing globalization.
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Submitted on : Wednesday, August 25, 2021 - 10:00:13 AM
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Marie-Cécile Dupas. Analyse en réseaux complexes des flux physiques internationaux de produits agricoles : structures et évolutions de 1986 à 2016. Physique [physics]. Université de Paris, 2020. Français. ⟨NNT : 2020UNIP7190⟩. ⟨tel-03325616⟩

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