The present and future contribution of ships to the underwater soundscape

Since the industrial revolution the ocean has become noisier. The global increasein shipping is one of the main contributors to this. In some regions, shippingcontributed to an increase in ambient noise of several decibels, especially at lowfrequencies (10 to 100 Hz). Such an increase can have a substantial negativeimpact on fish, invertebrates, marine mammals and birds interfering with key lifefunctions (e.g. foraging, mating, resting, etc.). Consequently, engineers areinvestigating ways to reduce the noise emitted by vessels when designing newships. At the same time, since the industrial revolution (starting around 1760)greenhouse gas emissions have increased the atmospheric carbon dioxidefraction x(CO2) by more than 100 mmol mol-1. The ocean uptake ofapproximately one third of the emitted CO2 decreased the average globalsurface ocean pH from 8.21 to 8.10. This decrease is modifying soundpropagation, especially sound absorption at the frequencies affected byshipping noise lower than 10 kHz, making the future ocean potentially noisier.There are also other climate change effects that may influence soundpropagation. Sea surface warming might alter the depth of the deep soundspeed channel, ice melting could locally decrease salinity and more frequentstorms and higher wind speed alter the depth of the thermocline. In particular,modification of the sound speed profile can lead to the appearance of new ductsmaking specific depths noisier. In addition, ice melting and the increase inseawater temperature will open new shipping routes at the poles increasinganthropogenic noise in these regions. This review aims to discuss parametersthat might change in the coming decades, focusing on the contribution ofshipping, climate change and economic and technical developments to thefuture underwater soundscape in the ocean. Examples are given, contrasting theopen ocean and the shallow seas. Apart from the changes in sound propagation,this review will also discuss the effects of water