Spotlight on Science

Spotlight on Science

Each month, an expert from the Marine Institute will shine a light on their area of work, sharing insights on our ocean knowledge. As the national agency responsible for supporting the sustainable development of our marine resources, the Marine Institute delivers scientific knowledge, expert advice and services to government, industry and a wide range of stakeholders. 

Marine Heatwaves in Irish Waters

Dr Glenn Nolan, Section Manager of Oceanographic and Climate Service at the Marine Institute                          

Our observations extend back to the early 1980s and come from oceanographic sensors deployed at the coast, on offshore buoys and from satellites that orbit the Earth making measurements of ocean temperature beneath their track.

The number of heatwaves has increased since the 1980s. Back then, we tended to see more marine cold spells than heatwaves but in recent years we have seen an increase in the number of marine heatwaves experienced on a yearly basis.

For example, there was one marine heatwave west of Ireland in 1983, but six so far in 2025. These events can last from a few days in duration to about two months (as happened in 1995). There was an intense marine heatwave in Irish waters in 2023 where temperatures were about 4°C above the long-term average.

People may be familiar with heatwaves related to weather conditions on land, but heatwaves also occur in the ocean. When the sea temperature is higher than the top 10% of long-term measured temperature values for a particular location for 5 days or more, a marine heatwave is underway. In some cases, marine heatwaves can be several degrees warmer than the long-term temperatures. Marine heatwaves are common and are often linked to calm sunny weather conditions over an area with extra heat entering the upper ocean during these events.

The sensors I referred to at the outset allow oceanographers to systematically monitor marine heatwaves in Irish waters and beyond. 

Scientists also try to gain insights into how ocean conditions are likely to change in the future using climate models. These models mathematically try to represent the future ocean under different greenhouse gas emission scenarios. Such climate projections range from optimistic low emission scenarios where society transitions to low-carbon activities and lifestyles to pessimistic scenarios where humans continue to use fossil fuels at a similar or higher rate than today. Under the pessimistic future scenario we are likely to see an increase in marine heatwaves (both at the sea surface and seabed) during the 21st century.

While the prospect of warmer oceans may sound encouraging to those engaged in swimming and other marine leisure pursuits, marine heatwaves can have profound impacts on marine life. 

The reproduction and growth of key marine species can be negatively impacted by such heatwaves and some species may die when their temperature tolerance limits are exceeded. 

At the Marine Institute we have developed a suite of tools to visualise and understand the magnitude and extent of marine heatwaves. This allows us to see whether such events are localised to certain parts of the coast or whether they extend offshore. 

We can overlay the heatwave information on the known distribution areas of key fish and shellfish species at whatever time of year the heatwave (or cold spell) occurs.

This is not just scientific research work but has practical application, for those working in the marine sector. This research will help fishers and aquaculture operators to better manage the impacts of marine heatwaves. Our research also helps widen public appreciation and understanding of the marine sphere.

This image shows Marine heatwave conditions to the south of Ireland on July 2. The colours indicate the difference between the daily temperature compared to long-term measured temperature values.

This article first appeared in The Marine Times (August 2025). 

ENDS

For more information, please contact:

Kathleen Sweeney  e: kathleen.sweeney@marine.ie | m: +353 (0)87 094 5754

Sinéad Coyne  e: sinead.coyne@marine.ie | m: +353 (0)87 947 7090

Communications Office, Marine Institute  e: communications@marine.ie | t: +353 (0)91 387 200

Marine Institute

The Marine Institute is Ireland’s scientific agency responsible for supporting the sustainable development of the State’s maritime area and resources. The Institute conducts applied research and provides scientific knowledge, advice and services to government, industry and other stakeholders and users. www.marine.ie