IT may sound like a seafarer's version of the "wrong kind of leaves on the line", but Scotland's largest ferry operator has said extreme sea swells is hampering its ability to deliver reliable services to island communities.

CalMac said it was "increasingly aware of significant changes in prevailing weather conditions", and blame this for a rise in ferry cancellations and disruption.

Guy Dale-Smith, head of marine at CalMac, said: "In recent winters, our Masters have witnessed an increase, not only in the severity of extreme weather events but also in their duration and frequency, all of which have impacted on our fleet's ability to operate services safely.

"Such extreme weather events also have a considerable impact on the ability of the ports and slipways we operate from to safely support the delivery of our ferry services.

"The safety of our vessels, crew, passengers and port facilities is our number one priority. Of course we fully appreciate our customers' frustration that weather changes are leading to an increase in disruptions across our network. But there is no quick fix to this and it will be a factor those responsible for replacing ships and upgrading harbours will have to take into account when planning for the future."

There is genuine science behind the claim - although climatologists stress that "the jury is still out" on whether recent extreme sea swells and gusts are a permanent change driven by global warming, or natural trends.

In February 2013, wave heights of almost 47ft - the highest in the world - were recorded off the Western Isles. The country was also battered by an exceptional run of severe winter storms from December 2013 until February 2014.

Research published in 2013 warned that "any deterioration of the wave climate will result in a disproportionately large increase in ferry-service disruption", with "severe social consequences" for the isolated island communities that rely on them.

In 2013, residents on Mull faced 80 service cancellations - the highest in 15 years. Islanders on South Uist also complained that the newly launched Lochboisdale-Mallaig service had had more cancellations than sailings. Bad weather was blamed in both cases.

Dr John Coll, a statistical climatologist and lead author of the 2013 paper examining the potential impact of wave and wind climate on ferry services off Scotland, said the "clustering and persistence" of storms last winter had been "highly unusual".

He added that the "highly energetic" weather pattern resulted from an unusually warm Atlantic off the south-east United States colliding with a polar vortex bringing cold air further south than normal.

He said this was like "a conveyor belt pushing storms our way".

He added: "If that becomes the feature of a changed climate, then clearly there's implications for storm tracking, and if we get more frequent storms closer together then I think that could become an issue for the Western Isles."

However Dr Eddie Graham, a meteorologist at the University of the Highlands and Islands, said weather was an "easy scapegoat".

"CalMac have chosen something that is hard to either verify or disprove," he said. "But it would be difficult for me to counter that statement because the weather on the west coast has been persistently unsettled."

Dr Graham said that the storminess of the most recent winter was underpinned by a mid-Atlantic high pressure zone known as the Azores High which, from December until now, has been driving low pressure towards the west coast "more fiercely and more rapidly".

However, he added there was nothing to prove that recent fluctuations were down to global warming.

"There's a tentative suggestion that our climate is becoming more extreme and more variable, but whether this change will continue - it's just too early to say."