One of the most severe marine heatwaves on the planet has developed off the coast of Ireland and the UK. Water temperatures reported off Scotland have been as high as 4-5°C above normal.   

It's been so anomalously warm in our famously chill waters that the National Oceanic and Atmospheric Administration’s (NOAA)  Marine Heatwave Watch has categorised this as a Category 4 (extreme) marine heatwave.  

Meanwhile, NOAA has forecast that 50% of the world’s ocean may experience marine heatwave conditions by September.

But what does this mean for the life in our seas, and humans who are connected to marine ecosystems and rely on them for food? Is the marine heatwave behind jellyfish sightings along our coastlines?

And might warm events like this even have an impact on our coastal industries? Here are the answers to some of the key questions behind the extreme marine heatwave. 

How warm does the sea have to be for it to be defined as a heatwave? 

Professor Michael Burrows, a marine ecologist based at the Scottish Association for Marine Science (SAMS) was part of the team that in 2016 helped create the definition of a marine heatwave. Essentially it is, he said, “when local temperatures have been consistently in the top 10% of temperatures for this time of year, and that lasts for more than five days”. 

What’s remarkable about this heatwave is not just that temperature rise of around 4C, but quite how long it has lasted – for this is a heatwave that started in March and is still ongoing now.  What's more, the NOAA has forecast that marine heatwave conditions to persist west of Ireland and in the Irish/Celtic Seas through August 2023. 

Prof Burrows observed, “This marine heatwave is still really strong. It has probably faded a little from its peak, but it’s definitely still there. It’s ongoing. It’s impossible to tell how long it’s going to be going on for.” 

He tests temperatures in the sea off Oban and has noted the same pattern locally. “Since March when I last put a temperature logger in the sea, up to last week when I went to get my temperature log back out again, temperatures have been in the top 10% of temperatures for the time of year with a couple of exceptions. It’s a long period of abnormally hot temperatures.” 

The Herald: June 2023 Ocean Surface Condition map from the NOAA Physical Sciences Laboratory

Is the marine heatwave responsible for the wave of recent jellyfish sightings around Scotland? 

Up and down the country, people have been posting images of blooms at sea, or jellyfish washed up on beaches – from a bay full of moon jellyfish at Tobermory to foot-wide barrel jellyfish at Fleet Bay, Dumfries and Galloway.

But, don't our coastlines always have their summer jellyfish blooms? Are they any more prevalent than usual? 

The Herald: A barrel jellyfish at Fleet Bay. So many were floating around that swimmers left the water


A barrel jellyfish in Fleet Bay, photographed by a local swimmer.

The answer is that it’s hard to say just yet. “Every summer about this time,” said Burrows, “we get calls from people saying why are there so many jellyfish. I’ve been at SAMs for thirty years and it’s been every summer since 1991, we get a call about jellyfish. What we need is more monitoring of these sorts of things.  I think we need to be looking at the natural world more closely as it changes in response to these conditions.” 

Among those attempting to chart jellyfish populations is the Marine Conservation Society’s Wildife Sightings programme, which is encouraging the UK public to submit reports of sightings of jellyfish and leatherback turtles. 

The data it has collected has shown that reports of certain jellyfish have increased over recent years. Last week, launching their 20th year of the project, MCS said: “The charity’s data shows an increasing trend in some species being spotted on our shores over the last 20 years, such as Portuguese man o’War. Research has suggested that an increase in some jellyfish numbers around UK could be related to climate change, however, currently there isn’t enough evidence to make this link.”   

What else is a marine heatwave likely to bring? 

Among the natural phenomena made more likely by marine heatwaves are algal blooms,  some of which produce toxins harmful to fish.  

Are marine heatwaves an increasing problem and is that because of climate change? 

Research has shown, Prof Burrows said, “that they’re becoming more frequent and more intense” and also that “climate change is likely to make everything much worse, till the point that at some point in the next five or six decades, we will be in a permanent heatwave”.  

Essentially, temperatures will have shifted up so much that “relative to the historical baseline we will be in the top ten percent the whole time”.  

This would be a new climate – climate change, as it were, at work.  

So, we’ll get a warmer sea? What’s the big deal about that?  

Only the fact that, as Burrows put it, “the biology and ecology of everything we know in the oceans around Scotland and the UK will change as a consequence”.  

Are we already seeing the impact of marine heatwaves on Scotland’s marine ecology? 

Not just yet. However, Burrows has studied previous marine heatwaves around the world that have had a dramatic impact, though this has depended on whether there are species living close to their warm limit and “at the very edge of what they can tolerate”.  

He said: “Arctic species which found themselves in Scotland but not in southern England or France – are the first things to go when there is a heatwave.” 

An example of this is that in 2010, there was a big marine heatwave of similar magnitude to the one being experienced around the UK now, off Western Australia, which was then home to giant kelp forests.  

“Those kelp forests,” he said, “disappeared over hundreds of kilometers, up the west coast, around Perth, northwards. After the heatwave there was no kelp forest for several years and the life that you associate with kelp forests was lost – juvenile fishes, sea urchins and birds that like to feed there.” 

So could we potentially lose our kelp forests? 

Not in Scotland, since kelp is not at its warm limit. However, the kelp forests of southern England and northern France could be threatened. “You may well see,” said Burrows, “at some future marine heatwve event in the not-too distant future, loss of kelp forests around mid to southern Europe. They’re already disappearing completely from Spain.”  

Are there other species we might lose? 

Professor Burrows observed that most likely to be vulnerable are any Arctic species living in Scotland’s waters. One example he gives is of an Arctic limpet. But more relevant, perhaps, to most of us humans is the possibility that a marine heatwave might put additional stress on cold water species that we rely on for fishing.  

“Cod,” said Prof Burrows, "in the southern north sea have declined significantly. Cod in the North Sea are again one of those examples of a fish that is close to its southern, equator-wards range edge. If the heatwave coincided with a time when there was a critical period for that fish, like its spawning then it could have an impact.” 

The Herald:

But don’t worry about the impact of this year’s heat on the cod just yet.

It turns out, the current heatwave is unlikely to have a significant impact on cod stocks because the heat anomaly is very much restricted to the surface waters.  Cod, meanwhile, is a demersal fish that lives at the bottom.  

Might some species actually do better in warmer waters? 

Yes. According to Prof Burrows there are examples where species have benefited from a temperature rise.  For instance, in Western Australia, following the 2010/11 heatwave, tropical fish started to appear where the kelp forest once had been.  

“That,” said Prof Burrows, “was because the water was sufficiently warm enough for them to survive. The heatwave event was like a kickstart of a new ecosystem. People talk about ecological tipping points and marine heatwaves can tip ecosystems to move through new states and bring new species with it sometimes.”  

Nature, he observes, has enormous power to regenerate. For instance in Western Australia where kelp forests disappeared, astonishingly coral reefs started to form. 

Long term, this could eventually mean an increase in biodiversity in Scotland.

"There are more species," said Prof Burrows, "living in the tropics and sub-tropics than there are here now. If they move here we’ll get an increase in biodiversity. But we will lose a lot of the things we love at the moment. It will be a different world.” 

So, in that case, maybe it is not all that worrying?  

Though the natural world does indeed have an extraordinary capacity to regenerate, Burrows is nevertheless concerned about the coming impact of climate change, and the way the natural world around us is likely to transform "beyond recognition".  

“Climate change," he said, "is fundamentally worrying because it will bring change and we don’t know what that change will look like. Fundamentally we rely on the living world to sustain human populations and we mess with that link at our peril. The oceans absorb carbon dioxide. They provide food, they provide oxygen, and human populations rely on all of these things, and when we start to change them in ways that we cannot control, then the future looks uncertain and that generates a huge amount of worry.”   

But if life can regenerate – won’t it all be fine? 

Prof Burrows said: “I do think there will be thriving marine systems out there – but it might not be the life that we currently enjoy eating. It’s just whether we can manage the transition from where we are now to there and limit the damaging effects.” 

READ MORE: Marine heatwaves ‘catastrophic’ for ocean life, say scientists

READ MORE: How can famously wet Scotland be on water scarcity alert?

What will all this mean for our fishing industry? 

"A reason.” Professor Burrows said, “people in Scotland should be concerned about marine heatwaves is the potential threat they may bring to their coastal industries.”  

Heat, in other words, may impact the wild fish that we catch and also the salmon that we farm and which is a major export for Scotland.  

Prof Burrows reflected: “Might this particular marine heatwave cause mass mortalities in the salmon industry or affect the natural populations of fishes that we rely on to keep our fishery going – lobsters, langoustines? At the moment there’s no evidence that’s going to happen with this heatwave – but that’s where the immediate threats would be.” 

So could marine heat cause a problem at our salmon farms? 

Salmon deaths on Scottish fish farms nearly doubled last year and the salmon farming industry attributed many of these to swarms of microjellyfish that afflicted some of the farms.  

The aquaculture industry itself often says that the rise in these swarms is possibly linked to climate change. Hence, a key question concerning the salmon sector this year is whether this heatwave may either directly impact the health of the fish, or do so indirectly through microjellyfish blooms.  

Salmon Scotland recently released a statement from Dr Iain Berrill, head of technical. He said: “While some oceans and seas are reporting anomalous temperature spikes, temperatures in the west coast of Scotland and the northern isles are still within normal ranges for salmon. Our salmon farmers are carefully monitoring conditions on their farms and oxygenating the water where appropriate to provide optimal fish welfare.” 

READ MORE: Guillemots. Mystery of the 1000 dead Scottish seabirds

What about seabirds?

Scientists monitoring seabird health are also worried about the possible impact of the marine heatwave. 

We are very worried about marine heatwaves,” Dr Francis Daunt, a seabird ecologist at the UK Centre for Ecology and Hydrology (UKCEH) said. “Could this marine heatwave have serious consequences on the fish and then knock-on effects on the birds? We may start to see that in the weeks ahead but it’s too early to say.”  

When, last week, it was revealed that a small number of guillemots were found dead off Angelsey, the UKCEH's seabird Twitter account noted that it was worth looking at the marine heatwave as a possible factor in the deaths. 

Dr Daunt explained: “We know that in the past some marine heatwaves have had quite negative consequences for the seabirds. Probably the most well-known recent heatwave was the one that took place in 2014-16 off Alaska which was known as the Blob. Tens of thousands of birds died.” 

Those deaths, he explained, were the result of a failure in the food web which affected fish, whose metabolic rates were raised because of the temperatures, and therefore the birds that feed on them. 

Recent temperatures around the UK, he said, were high enough to raise concern over marine heatwave impacts on birds– though the actual cause of deaths of those guillemots is as yet unknown, and the mortalities are still in small numbers.  

“When we see deaths like these,” said Dr Daunt, “there may not be one explanation. It may be that there are at least two things happening at once. And when wild birds, or any wild species are faced with multiple threats it can make matters even worse than adding the two together." 

"So it may be that if there’s a marine heatwave and the quality of your food is going down, then your condition goes down and you are more susceptible to avian flu.  And, similarly, if you‘ve had a disease and perhaps you’ve recovered from it, you may not be back to your tip-top self and may be more susceptible to a drop in food quality as a result of the marine heatwave.”