Environmental considerations for the decommissioning of subsea cables.

Since the first trans-oceanic telegraph cables were laid in the nineteenth century, a subsea network of cables has grown across the global ocean; becoming upgraded with co-axial, and more recently, fibre-optic cable systems. Demands for digital connectivity mean that this network continues to expand, with an estimated >3.5 million km of all types of subsea cable installed to date, and a growing amount of now out-of-service infrastructure requiring decommissioning. While the environmental impacts have been well-documented for installation and operational phases, to date no study has addressed the potential impacts associated with the decommissioning of out-of-service subsea cable systems, which typically have a 20-25-year design life. Here, we address this evidence gap and present a first synthesis of the drivers for, and environmental considerations relevant to, the decommissioning of subsea telecommunications cables. We show that recovery of subsea cables has potential to return commercially valuable materials to the economy with a high degree (>95 %) of recyclability; however, various dynamic environmental conditions and logistical challenges mean that it is not always viable to recover the entire length of a cable system. While cable recovery activities are found to have a localised and short-lived environmental impact, it is important to ensure that any impacts are minimised, particularly in sensitive habitats, or exceptional cases where intense biological colonisation of a cable has occurred. Options for decommissioning out-of-service systems, other than recovery, include in-situ repurposing to make scientific measurements or re-deploying sections of recovered cables elsewhere. Many future opportunities exist for further enhancing the evidence base concerning cable decommissioning and with regards to the gathering of environmental data during cable recovery operations to inform future decision-making and to enhance the wider scientific understanding of shallow to deep sea environments.