‘Green’ ferry emits more carbon than old diesel ship

The carbon footprint of a long-delayed “green” ferry, MV Glen Sannox, will significantly exceed that of the 31-year-old diesel-powered MV Caledonian Isles, which it is set to replace on the route between Scotland’s mainland and the Isle of Arran.

According to an emissions analysis by CalMac, green ferry Glen Sannox is estimated to release 10,391 tonnes of CO2 annually, compared to 7,732 tonnes from Caledonian Isles. While the new ferry boasts greater car capacity, its larger engines emit methane—a greenhouse gas far more potent than CO2.

The ferry’s owner, ferries procurement agency CMAL, contested the comparison, citing green ferry Glen Sannox’s larger size. However, the increased carbon footprint also results from its use of liquified natural gas (LNG), which is less climate-friendly than previously claimed.

Transport emissions expert Prof. Tristan Smith from UCL highlighted that including “upstream” emissions, such as the environmental cost of importing LNG from Qatar, might make diesel a more sustainable option. “At best, LNG offers negligible benefits, and at worst, it worsens emissions,” he said.

Green ferry Glen Sannox, launched in 2017 and finally delivered last month after years of delays and cost overruns, is the UK’s first ferry capable of running on both LNG and marine gas oil (MGO). While its launch was hailed as a step toward Scotland’s climate goals, the challenges of LNG technology, coupled with its larger size, have complicated the ship’s eco-credentials.

With a capacity for 127 cars compared to Caledonian Isles’ 90, the ship also cannot dock at Ardrossan until harbor upgrades are complete. Although it promises fewer weather-related cancellations and reduced air pollutants (NOx and SOx), its greenhouse gas emissions remain a concern. Methane slip—unburned methane released during engine use—adds over 1,800 tonnes of CO2 equivalent annually. Additionally, transporting LNG from a terminal in Kent to Scotland further increases emissions compared to locally sourced MGO.

While CMAL defends LNG as a “transition fuel,” critics argue for a more comprehensive assessment of its climate impact, including upstream emissions from gas extraction and transportation. Prof. Smith likened ignoring these emissions to “rearranging deck chairs on the Titanic,” emphasizing the urgency of adopting zero-emission alternatives.

CalMac’s analysis, conducted for the Scottish government, acknowledges that the real-world emissions of Glen Sannox could be higher than its estimates, which assume ideal operating conditions. Methane blowdowns during fuel transitions and other operational inefficiencies were not fully accounted for.

The debate over Glen Sannox reflects broader challenges in balancing increased demand for ferry services with climate goals, as environmentalists push for more ambitious emissions reductions.