The sea is unusually warm off Irish coasts. We should be worried

For most people who swim occasionally in the Atlantic off the western seaboard of Ireland, more often than not it’s a bracing experience because of the cold; an initial shock to the body but ultimately uplifting.

For regulars, the usual pattern is the sea is discernibly warmer in late August – during a reasonable summer – and a joy in September and even into October as sea takes longer to warm up compared with land. But this usual pattern does not pertain in 2023.

So what is happening?

An unheard-of marine heatwave off the coasts of Ireland and the UK in recent weeks has meant sea temperatures are four to five degrees warmer than normal. Globally in April, May and June, sea temperatures reached all-time highs.

The marine heatwave in our waters is classified as “extreme” by the US National Oceanic and Atmospheric Administration.

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The highest temperatures are concentrated in two pockets offshore from Co Galway up to north Co Donegal, and off east Scotland in the North Sea.

Increased frequency of marine heatwaves is a consequence of global warming. It can be directly linked to carbon emissions derived from burning fossil fuels.

But what is worrying climatologists is that this sea trend is likely to persist – and may even get worse in the short term. This is because it is arising from a combination of human-induced warming and natural climate variability caused by an El Niño – “a natural oscillation” driven by ocean temperatures and winds in the Pacific that in turn warms the Atlantic.

Experts believe cleaning up sulphur from marine shipping fuels is probably adding to the greenhouse gas-driven warming, as is less Saharan dust over the ocean this year.

[ Mayo sea swimmers fear long-term effects of climate change on marine life ]

Should we be worried?

The water may be “heavenly” – as described by one swimmer at Old Head, Louisburgh, Co Mayo, this week – but an excessively warm sea in what are normally cool ecosystems are a threat to marine plants and mammals.

Such heatwaves elsewhere have caused huge mortalities besides being the biggest cause of polar ice loss and bleaching of coral reefs.

More locally, this heatwave, if it carries on through summer, could result in widespread mortality of kelp, seagrass, fish and oysters, according to Dr Dan Smale of the Marine Biological Association.

“This is unprecedented and possibly devastating. Current temperatures are way too high but not yet lethal for the majority of species, although stressful for many,” he told the Guardian.

Does this have other consequences for humanity?

The bigger picture is even more concerning. The damage caused in these hot spots is also harmful to humanity, which relies on the oceans for oxygen, food, storm protection and removal of climate-heating carbon dioxide in the atmosphere.

Warmer seas intensify hurricanes and storms. There are particular changes to Atlantic currents due to warming that could alter normal patterns profoundly.

Is this another indication we are living in a here-and-now climate crisis?

This is yet another manifestation we are hurtling towards planetary destabilisation; where we quickly lose the ability to avoid permanent impacts of human-induced global warming, not to mention unanticipated impacts – the consequence of climate tipping points. If this is not a call to immediate and radical intervention on slashing emissions, nothing is.

Marine heatwaves have caused billions of losses in fisheries and tourism income. They have impaired the carbon storage ability of nature, undermined cultural values and triggered mass habitat loss and extinctions.

Prof Daniela Schmidt of the University of Bristol put marine inaction best: “As long as we are not dramatically cutting emissions, these heatwaves will continue to destroy our ecosystems. But as this is happening below the surface of the ocean, it will go unnoticed.”