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A marine heatwave (MHW) — a prolonged period of unusually warm sea surface temperatures — has been intensifying around the UK’s coastline since late 2024. As of 30th April 2025, most UK waters are experiencing a Category II MHW), with sea surface temperatures up to 3°C above the seasonal average.
As the current MHW is occurring before the height of summer, it is unlikely to reach temperatures that would cause major harm or mortality to marine species, but what are the potential impacts? Dr. Zoe Jacobs, a scientist at the National Oceanography Centre (NOC), is closely monitoring. In this article, she provides regular updates on how the MHW is developing, what the latest satellite data shows, and why it matters for UK marine ecosystems, fisheries, and communities.
What does the current marine heatwave look like?
The marine heatwave began around parts of the UK’s coastline in late 2024. By the end of March 2025, much of the UK was experiencing a MHW, with more intense conditions in northern regions. With recent higher than average temperatures around the UK, this has since spread south with the majority of UK waters now facing a category II MHW.
While this is classed as a “strong” category II MHW in some areas, it is important to note that some of the most impactful events documented globally cover larger areas, last for many months and are subject to greater intensities.
30/04/25: MHW classification from www.marineheatwaves.org/tracker
Why does this matter?
Globally, MHWs have caused wide-ranging impacts on marine ecosystems. This includes the mortality of key species such as seagrass and kelp, the mortality or reorganisation of commercial fisheries and the initiation of harmful algal blooms (HABs).
As the current UK MHW is occurring before the height of summer, it is unlikely to reach temperatures that would cause major harm or mortality to marine species.
However, the MHW could still have impacts, including affecting the timing and size of phytoplankton blooms (base of the marine food chain), potentially reducing or boosting their productivity. It’s possible that HABs could be triggered, which can produce toxins harmful to marine life and the environment.
It may also disrupt the life cycles of marine species, impacting their growth or reproduction. All of this may impact the regional marine ecosystem, fisheries and the economy in different ways.
Dr Caroline Rowland, the Met Office Scientific Strategic Head for Ocean, Cryosphere and Climate, said: “There is also evidence that the intensity and frequency of MHWs can contribute to impactful weather events around the UK, with obvious impacts for human health”.
With climate change continuing to raise global temperatures, it’s vital that these occurrences are monitored, and that we have more targeted research to understand these impacts to improve UK resilience to MHWs.
Ongoing monitoring
Read our updates below on the developing MHW around the UK using satellite maps of sea surface temperature (SST) and chlorophyll-a concentrations – an indicator of how much phytoplankton is present in the water.
18th May 2025
On Sunday 18th May, the Sea Surface Temperature (SST) anomalies continued to weaken in the western North Sea & English Channel with these regions now <1oC warmer than usual. However, the SSTs around Ireland have intensified with widespread anomalies of +3oC apparent, reaching almost +4oC off the west Irish coast (note the increased limits of the colour bar below). This region is now experiencing a category II-IV MHW. Waters around the southwest English coast, Scottish & Welsh coasts continue to experience a category II-III event with anomalies of 1-3oC more than normal for this time of year.
The phytoplankton blooms in the open ocean west of Ireland continue to expand and intensify, with widespread positive chlorophyll-a anomalies apparent across the region. Intense blooms in the southern North Sea remain (now peaking at up to 50mg/m3) but their extent appears to be reducing slightly. Weaker blooms are also still apparent in the northern North Sea but the intensity is also weakening here. In contrast the positive chlorophyll-a anomalies (>4mg/m3) are expanding in the Irish Sea and English Channel, with large blooms developing.
What do we know about the impacts on fisheries or harmful algal blooms around the UK? Cefas Scientists Dr Bryony Townhill and Dr Georg Engelhard have given their insights:
“Marine heatwaves are known to cause stress and mortality for marine species, with the impacts of heatwaves on fish, seabirds and marine mammals in the Pacific Ocean some of the most well-known. While the impacts of heatwaves on marine life in the UK are less understood, mass mortalities of whelks were reported following a marine heatwave in 2022, and previous Cefas and NOC research has found links between heatwaves and catches of sole, lobster, seabass and red mullet, suggesting the heatwaves affect the recruitment/breeding of certain species. Marine heatwaves can also lead to harmful algal blooms, which can deplete oxygen levels in the water, with consequences for marine biodiversity, fisheries and human health. As scientists, a key challenge is to understand the extent to which these phenomena are driven primarily by marine heatwaves or other factors, such as pollution or disease. Research is ongoing to find further evidence on the impacts and causes, and we are watching closely with partners, such as the Met Office and NOC, to monitor the impacts of the current UK heatwave on marine life and fisheries.”
18/05/25: MHW classification from the UK Met Office’s MHW Monitoring Tool.
18/05/25: Sea Surface Temperature and respective anomalies (top), chlorophyll-a concentrations and respective anomalies (bottom). NOTE the extended colour bar on the SST anomalies.
14th May 2025
On Wednesday 14th May, Sea Surface Temperatures (SST) remained 1-3oC warmer than usual for this time of year across the entire region. While the majority of the North Sea and English Channel are experiencing a category I MHW, parts of the eastern and southern regions are no longer classified as a MHW, with SSTs around 1oC warmer than usual. The anomalies around the southwest UK coast and much of the Irish coast have intensified to +3oC with these warmer waters also extending further offshore to the west. These regions are undergoing a category II-III MHW. A large portion of Scottish waters are also classified as category II-III with anomalies of 2-3oC expanding northwards.
Intense phytoplankton blooms remain in the southern North Sea and off the coast of East Anglia with chlorophyll-a concentrations of up to 30mg/m3 recorded. While of lower magnitude (8-10 mg/m3), the anomalous blooms around the Shetland Islands and generally north of Scotland are expanding to cover a larger region. Some of the blooms in the open ocean to the southwest of Ireland are also intensifying and expanding. In the Irish Sea and parts of the English Channel, smaller blooms persist with some signs of intensification in the Irish Sea.
An update from Dr Ségolène Berthou from the Met Office on the drivers of this event: “The high-pressure system bringing settled conditions is likely to continue to warm the ocean around the UK and Ireland well into next week. The long-range forecast doesn't indicate any likelihood of weather regimes which could disrupt the MHW until Friday 23rd.”
14/05/25: MHW classification from the UK Met Office’s MHW Monitoring Tool.
14/05/25: Sea Surface Temperature and respective anomalies (top), chlorophyll-a concentrations and respective anomalies (bottom).
11th May 2025
On Sunday 11th May, Sea Surface Temperatures (SST) remained warmer than usual for this time of year across the entire region. The anomalies in the English Channel and Southern North Sea remained lower than the rest of UK waters (+0-1oC) while the rest of the North Sea has warmed up with widespread anomalies of +1-2oC, and up to +3oC in the eastern North Sea, apparent. The SSTs west of Ireland are also intensifying with the whole region now experiencing +2-3oC anomalies, peaking off the southwest Irish coast. These waters are now classed as experiencing a category II-III MHW. With anticyclonic conditions providing high levels of sunshine and weak winds, these anomalies are anticipated to persist for the rest of the week.
Phytoplankton blooms remain in the open ocean to the southwest of Ireland and in the northern North Sea, off the east coast of Scotland, although their intensity is declining to +2-3mg/m3. However, anomalous blooms are now emerging around the Shetland Islands. Additionally, the chlorophyll-a concentrations in the English Channel and particularly in the Southern North Sea around the East Anglian coast and Thames estuary are intensifying with blooms exceeding 10mg/m3 visible across the entire region. This contrasts with waters off the northeast English coast, which are experiencing weaker phytoplankton blooms than usual for this time of year.
What do we know about the impacts of marine heatwaves on phytoplankton? Dr Elena García-Martín, a plankton ecologist from the NOC, has given some insights:
“There is still a lot of debate on the impact of marine heatwaves on phytoplankton. We don’t know if the increase in temperature has promoted an increase in the phytoplankton blooms and whether it will collapse quickly or persist for a while. The speed of their decline will depend on the environmental conditions over the next few days. The intensity and duration of the bloom are important as they could have consequences on coastal productivity, as well as carbon export to deeper waters.”
11/05/25: MHW classification from the UK Met Office’s MHW Monitoring Tool.
11/05/25: Sea Surface Temperature and respective anomalies (top), chlorophyll-a concentrations and respective anomalies (bottom).
7th May 2025
On Wednesday 7th May, Sea Surface Temperatures (SST) remained 0-1oC warmer than usual for this time of year across the entire region. The southern and central North Sea anomalies continue to decrease slightly with only some areas now classified as a category I MHW. However, the SST anomalies in parts of the northern North Sea, the Irish Sea, the Celtic Sea and the waters to the west of Ireland are intensifying and are now exhibiting anomalies of +2oC. The +3oC anomalies to the southwest of the UK and south of Ireland on 4th May remain visible and appear to be expanding, with +3oC anomalies now also present in the Irish Sea. These regions are all experiencing a category I-III MHW.
But what could be driving the MHW? Dr Ségolène Berthou, a scientist from the Met Office who researches the atmospheric drivers of MHWs, has given insight into what may be driving this current event:
“We've had a long period of "summer-like" weather regimes in March and April – warm, anticyclonic conditions with weak winds. Similar conditions were identified to have led to the June 2023 MHW. At the start of April, tropical air was brought to the UK because of a remote storm over the Atlantic, while an anticyclone centred over Denmark kept low-levels of winds and high levels of sunshine, both contributing to warming SSTs around the UK. Our long-range forecast doesn't show signs of weather regimes which could break this MHW, so it's likely to continue well into next week”. To read more about why spring has been so warm and dry this year, read their recent blog post here.
Phytoplankton blooms continue to develop in the open ocean to the southwest of Ireland and in the northern North Sea, off the east coast of Scotland, with chlorophyll-a anomalies exceeding 4mg/m3. Both regions are currently experiencing anomalous warming of up to 2oC. Blooms of a similar magnitude to that observed on 4th May are still apparent in the western English Channel but continue to subside in strength and coverage off the East Anglian coast and southern North Sea.
07/05/25: MHW classification from the UK Met Office’s MHW Monitoring Tool.
07/05/25: Sea Surface Temperature and respective anomalies (top), chlorophyll-a concentrations and respective anomalies (bottom).
4th May 2025
On Sunday 4th May, Sea Surface Temperatures (SST) remained 0-1oC warmer than usual for this time of year across the entire region, with anomalies of up to +2°c apparent in the central North Sea and off the coast of East Anglia.
To the southwest of the UK and south of Ireland, +3°c anomalies are visible in a few coastal areas while offshore, +2°c anomalies are widespread.
Overall, warm anomalies remain over the entire region, but these have reduced slightly in the southern North Sea since 30th April. Phytoplankton blooms have also subsided in strength and coverage. However, greater than average concentrations (>4mg/m3 ) of chlorophyll-a still exist in the English Channel, Irish Sea and around the East Anglian coast.
04/05/25: Sea Surface Temperature and respective anomalies (top), chlorophyll-a concentrations and respective anomalies (bottom).
30th April 2025
On Wednesday 30th April, we observed that SSTs were 0-1°c warmer than usual for this time of year across the entire UK region, with anomalies of up to +3°c apparent in the southern North Sea and off some of the UK’s southwest coast.
While most of the North Sea shows minimal chlorophyll-a anomalies, phytoplankton blooms are visible, and larger than usual (>4mg/m3), off the east coast of Scotland and the Shetland Islands and around East Anglia. Anomalously large blooms are also apparent in much of the English Channel and in the Irish Sea.
30/04/25: Sea Surface Temperature and respective anomalies (top), chlorophyll-a concentrations and respective anomalies (bottom).
Stay tuned for the next update.
More on marine heatwaves
What is a marine heatwave?
Heatwaves, or periods of unusually warm temperatures, occur not just on land, but also in the ocean. Unlike heatwaves on land, marine heatwaves can persist for many weeks or months. They extend over much larger areas and may warm the ocean to depths of hundreds of metres.
As climate change causes our planet to warm, marine heatwaves are becoming more frequent, more intense, and longer lasting. In fact, the number of marine heatwaves has doubled since 1982, and most recently, we have seen extreme events in the North Atlantic, Antarctica, and around the Mediterranean.
Learn more about marine heatwaves
What do we know about marine heatwaves around the UK?
A recent study led by NOC revealed where marine heatwave events are most likely to occur in UK waters, for the first time.
The study, led by NOC’s Dr Zoe Jacobs and published in Frontiers in Marine Science, shows there are regional marine heatwave hotspots in the Southern North Sea and English Channel. Events here are weaker than in other areas around the UK, but last longer.
View the data
Download near-real-time satellite remote sensing data at the following links: