Ship-induced methane emissions from shallow seabed sediments have recently emerged as a previously overlooked greenhouse gas pathway. Recent research published in high-impact factor journal Communications Earth & Environment has provided substantial new data on ship-induced methane emissions. While my initial math on oceanic cargo shipping suggest that such emissions remain negligible compared to the broader maritime carbon footprint in the class, analysis of large coastal vessels, particularly cruise ships and large roro and ropax ferries, indicates a potentially more meaningful climate impact.

Methane emissions have increasingly come into focus in global climate policy due to methane’s pronounced warming effect in the short term. Over a 20-year period, methane has a global warming potential 82.5 times greater than carbon dioxide. As policy makers increasingly prioritize rapid emission reductions in the coming decades, accurately identifying and mitigating methane sources becomes strategically important.

The mechanism by which ships trigger methane release is straightforward. Large vessels with draughts over 9 meters traveling at speeds exceeding 12 knots in shallow coastal waters generate significant underwater turbulence and pressure reductions beneath their hulls. These rapid changes in pressure and the resulting disturbance of seabed sediments can liberate methane stored within organic-rich sediment layers, allowing it to escape into the atmosphere. The recent data from field measurements at Neva Bay near St. Petersburg, Russia, demonstrated clearly measurable methane release from seabed sediments disturbed by ships with draughts exceeding nine meters.

A quick examination of the oceanic cargo shipping sector showed why these emissions remain marginal for that segment. Oceanic cargo vessels primarily travel through deep waters, limiting their exposure to the shallow sediment-rich areas necessary for significant methane emissions. My napkin math using the Neva Bay data for major global ports such as Rotterdam, Antwerp, Hamburg, Houston, and Shanghai suggested methane-related carbon dioxide-equivalent emissions of roughly 7,300 tons per port annually even at methane’s high GWP20. Given the enormous scale of international shipping emissions, approximately one billion tons of carbon dioxide per year, these port-area methane emissions represent a minuscule fraction of the shipping sector’s total climate impact.

Dredging activities in major ports represent another source of methane emissions due to sediment disturbance. Routine dredging involves mechanical removal of organic-rich sediments, which often contain substantial reservoirs of methane produced through anaerobic decomposition. Such operations can trigger short-term releases of methane into the atmosphere, with single dredging events typically producing emissions in the tens to hundreds of tons of CO₂-equivalent, depending on sediment type and the scale of disturbance. However, dredging events generally occur only periodically — annually or biennially at most major ports — and therefore generate emissions that  also remain negligible when compared to the billion tons of CO₂ emitted annually by oceanic cargo shipping worldwide.

As the rest of shipping decarbonizes, it will become important to clean up edge case emissions, but it’s not the first order of business for shipping. The International Maritime Organization’s fuel carbon price is the first order of business, as are operational efficiencies like slow-steaming, hull interventions for lower drag, biofuels and of course batteries.

However, shifting focus toward large coastal vessels provides a different picture. Cruise ships and large roro (roll on, roll off) and ropax (roro + passengers) ferries consistently operate in shallow, sediment-rich coastal zones worldwide. Regions such as Scandinavia, the Mediterranean, Southeast Asia, the Caribbean, coastal North America, and Alaska feature substantial fleets of such vessels operating in estuarine and near-coastal waters. These ships commonly transit shallow waters multiple times daily, continuously disturbing methane-rich seabed sediments.

A conservative estimate demonstrates why these coastal vessels likely have a more significant impact. Suppose globally there are roughly 1,000 large vessels between cruise ships and major coastal ferries combined that regularly pass through sediment-rich shallow coastal routes. Each vessel might disturb approximately 2.5 square kilometers of seabed per day, considering typical wake width and transit distances. Using the Neva Bay methane emission data, this scenario would produce roughly 3,600 tons of carbon dioxide-equivalent emissions per vessel annually over a typical 200-day operating season.

Extrapolating this figure across a global fleet of approximately 1,000 coastal vessels yields around 3.6 million tons of carbon dioxide-equivalent emissions each year. This emission volume far exceeds estimates calculated from major port areas associated with oceanic cargo ships, illustrating that while oceanic cargo shipping’s seabed methane emissions remain negligible, coastal passenger and ferry services could represent a more substantial methane emission source globally.

The global distribution of shallow coastal waters further amplifies this concern. Shallow estuarine and coastal environments rich in organic sediments occur extensively along heavily trafficked ferry and cruise ship routes. Furthermore, methane production peaks seasonally, driven largely by warmer water temperatures during summer months, which closely aligns with peak tourism and passenger ferry seasons. This seasonal overlap exacerbates the potential methane emissions from coastal passenger ships precisely when maritime activity is highest.

Fortunately, addressing ship-wake methane emissions from these large coastal vessels appears straightforward and economically viable. Operational measures such as reducing vessel speed within critical shallow methane-sensitive zones can significantly curtail emissions. For instance, implementing modest speed limits of approximately 6 to 8 knots within these shallow coastal areas during high-risk summer months could meaningfully reduce methane emissions. Such speed reductions might add only minutes or at most a few hours to typical ferry or cruise ship voyages, making these interventions both economically feasible and operationally practical.

Yet, current estimates remain preliminary and speculative, based mainly on a single comprehensive dataset from Neva Bay. To effectively guide policy and regulatory decisions, a more robust dataset is necessary. Replicating the Neva Bay study in multiple regions globally, especially in high-traffic coastal ferry and cruise ship routes, would provide a far clearer understanding of emission magnitudes, regional variability, and practical mitigation potential.

Although ship-induced seabed methane emissions initially appeared negligible within the vast context of global oceanic shipping emissions, closer scrutiny reveals coastal passenger vessels as a potentially notable methane source. Given methane’s potent short-term climate forcing effects, swiftly addressing these emissions through targeted operational strategies offers practical, easily implementable solutions.

Policy makers and regulatory bodies should prioritize further research and measurement campaigns in these methane-sensitive coastal areas. Accurate, location-specific measurements would allow for the establishment of well-defined speed limit zones during high-emission periods. Incorporating these targeted operational measures into maritime sustainability policies represents a simple, low-cost pathway for rapidly reducing short-term greenhouse gas emissions.