19 The global cargo ship network FIG. 2: Degrees and weights in the global cargo ship network ∗ (insets: subnetworks for container ships □ , bulk dry carriers ◦ , and oil tankers △ ). (a) The degree distributions P ( k ) are right-skewed, but not power laws, neither for the GCSN nor its subnetworks. The degree k is defined here as the sum of in- and out-degree, thus k = 1 is rather rare. (b) The link weight distributions P ( w ) reveal clear power law relationships for the GCSN and the three subnetworks, with exponents µ characteristic for the movement patterns of the different ship types. (c) The node strength distributions P ( s ) are also heavy-tailed, showing power law relationships. The stated exponents are calculated by linear regression with 95% confidence intervals (similar results are obtained with maximum likelihood estimates, see Electronic Supplementary Material). (d) The average strength of a node 〈 s ( k ) 〉 scales superlinearly with its degree, 〈 s ( k ) 〉 ∝ k 1 . 46 ± 0 . $^{1}$, indicating that highly connected ports have, on average, links of higher weight.