arXiv:1001.2172v1 [physics.soc-ph] 13 Jan 2010 The complex network of global cargo ship movements Pablo Kaluza, Andrea K��olzsch, Michael T. Gastner, and Bernd Blasius* Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky Universit��at, Carl-von-Ossietzky-Str. 9-11, 26111 Oldenburg, Germany Abstract Transportation networks play a crucial role in human mobility, the exchange of goods, and the spread of invasive species. With 90% of world trade carried by sea, the global network of merchant ships provides one of the most important modes of transportation. Here we use information about the itineraries of 16,363 cargo ships during the year 2007 to construct a network of links between ports. We show that the network has several features which set it apart from other transportation networks. In particular, most ships can be classified in three categories: bulk dry carriers, container ships and oil tankers. These three categories do not only differ in the ships��� physical characteristics, but also in their mobility patterns and networks. Container ships follow regularly repeating paths whereas bulk dry carriers and oil tankers move less predictably between ports. The network of all ship movements possesses a heavy-tailed distribution for the connectivity of ports and for the loads transported on the links with systematic differences between ship types. The data analyzed in this paper improve current assumptions based on gravity models of ship movements, an important step towards understanding patterns of global trade and bioinvasion. Keywords: complex network ��� cargo ships ��� bioinvasion ��� transportation *Electronic address: blasius@icbm.de 1

The global cargo ship network 2 I. INTRODUCTION The ability to travel, trade commodities, and share information around the world with unprecedented efficiency is a defining feature of the modern globalized economy. Among the different means of transport, ocean shipping stands out as the most energy efficient mode of long-distance transport for large quantities of goods (Rodrigue et al. 2006). According to estimates, as much as 90% of world trade is hauled by ships (International Maritime Organization 2006). In 2006, 7.4 billion tons of goods were loaded at the world���s ports. The trade volume currently exceeds 30 trillion ton-miles and is growing at a rate faster than the global economy (United Nations conference on trade and development 2007). The worldwide maritime network also plays a crucial role in today���s spread of invasive species. Two major pathways for marine bioinvasion are discharged water from ships��� ballast tanks (Ruiz et al. 2000) and hull fouling (Drake & Lodge 2007). Even terrestrial species such as insects are sometimes inadvertently transported in shipping containers (Lounibos 2002). In several parts of the world, invasive species have caused dramatic levels of species extinction and landscape alteration, thus damaging ecosystems and creating hazards for human livelihoods, health, and local economies (Mack et al. 2000). The financial loss due to bioinvasion is estimated to be $120 billion per year in the United States alone (Pimentel et al. 2005). Despite affecting everybody���s daily lives, the shipping industry is far less in the public eye than other sectors of the global transport infrastructure. Accordingly, it has also re- ceived little attention in the recent literature on complex networks (Wei et al. 2007, Hu & Zhu 2009). This neglect is surprising considering the current interest in networks (Al- bert & Barabasi 2002, Newman 2003a, Gross & Blasius 2008), especially airport (Barrat et al. 2004, Guimer`a & Amaral 2004, Hufnagel et al. 2004, Guimer`a et al. 2005), road (Buhl et al. 2006, Barthelemy & Flammini 2008) and train networks (Latora & Marchiori 2002, Sen et al. 2003). In the spirit of current network research, we take here a large-scale perspective on the global cargo ship network (GCSN) as a complex system defined as the network of ports that are connected by links if ship traffic passes between them. Similar research in the past had to make strong assumptions about flows on hypothetical networks with connections between all pairs of ports in order to approximate ship move- ments (Drake & Lodge 2004, Tatem et al. 2006). By contrast, our analysis is based on

The global cargo ship network 3 comprehensive data of real ship journeys allowing us to construct the actual network. We show that it has a small-world topology where the combined cargo capacity of ships calling at a given port (measured in gross tonnage) follows a heavy-tailed distribution. This capac- ity scales superlinearly with the number of directly connected ports. We identify the most central ports in the network and find several groups of highly interconnected ports showing the importance of regional geopolitical and trading blocks. A high-level description of the complete network, however, does not yet fully capture the network���s complexity. Unlike previously studied transportation networks, the GCSN has a multi-layered structure. There are, broadly speaking, three classes of cargo ships ��� container ships, bulk dry carriers, and oil tankers ��� that span distinct subnetworks. Ships in different categories tend to call at different ports and travel in distinct patterns. We analyze the trajectories of individual ships in the GCSN and develop techniques to extract quantitative information about characteristic movement types. With these methods we can quantify that container ships sail along more predictable, frequently repeating routes than oil tankers or bulk dry carriers. We compare the empirical data with theoretical traffic flows calculated by the gravity model. Simulation results, based on the full GCSN data or the gravity model differ significantly in a population-dynamic model for the spread of invasive species between the world���s ports. Predictions based on the real network are thus more informative for international policy decisions concerning the stability of worldwide trade and for reducing the risks of bioinvasion. II. DATA An analysis of global ship movements requires detailed knowledge of ships��� arrival and departure times at their ports of call. Such data have become available in recent years. Starting in 2001, ships and ports have begun installing Automatic Identification System (AIS) equipment. AIS transmitters on board of the ships automatically report the arrival and departure times to the port authorities. This technology is primarily used to avoid collisions and increase port security, but arrival and departure records are also made available by Lloyd���s Register Fairplay for commercial purposes as part of its Sea-web data base (www.sea- web.com). AIS devices have not been installed in all ships and ports yet, and therefore there are some gaps in the data. Still, all major ports and the largest ships are included, thus the