April 14, 2025

expert reaction to study looking at the effect of global warming on persistence and intensity of marine heatwaves

A study published in PNAS looks at global warming and the intensity of marine heatwaves. 

Dr Caroline Rowland, Scientific Strategic Head for Ocean Cryosphere and Climate, Met Office, said:

“We should all care about marine heatwaves because societies around the world depend on the ocean; changes to ocean temperatures can have wide-ranging impacts on ecosystems and fisheries. There is growing evidence that the intensity and frequency of marine heatwaves – driven by climate change – can contribute to impactful weather events both around the UK and globally with obvious impacts for human health.

“The record-breaking UK land temperatures in June 2023 were amplified by an unprecedented marine heatwave in UK waters that occurred simultaneously with the land heatwave. The increasing intensity and frequency of marine heatwaves due to climate change can increase the severity of storms – including tropical cyclones – and heatwaves on land.

What are the key points readers should take from the study?

“Anthropogenic climate change is the major cause of observed increases in intensity, frequency and duration of marine heatwaves (MHW) both globally and regionally

“Natural year-to-year climate variability does contribute to extreme Marine temperatures, however, MHW are longer and more intense due to the long-term warming of the seas due to anthropogenic climate change.

“MHWs are shown to increase in length in the tropics and in intensity and length in the mid-latitudes. The North and Baltic Seas have seen a larger increase relative to other regions (likely because of their shallow nature and their lower salinity: their excess heat is less likely exported to the deeper ocean).

“The results of this paper are largely consistent with the assessment of MHWs in IPCC AR6 WG1 Box 9.2.

Why are marine heatwaves important for people – why should they care?

“The public should care about Marine Heatwaves because humans depend on the ocean; changes to ocean temperatures can have wide ranging impacts including to ecosystems and fisheries. There is growing evidence that MHWs contribute to weather events both around the UK and globally which can have significant impacts to human and animal health. Record-breaking UK land temperatures in June 2023 were amplified by an unprecedented Marine Heatwave in UK waters that occurred simultaneously with the land heatwave [1]. The intensity of cyclones in tropical regions has also been shown to be magnified by MHW. The increasing intensity and frequency of MHW due to climate change has the potential to increase the severity of storms and heatwaves on land.

Are there any important caveats that we should be aware of?

“Although we know that there are wide ranging impacts to ecosystems and fisheries from global MHW, there are large gaps in our knowledge of MHW impacts to ecosystems in UK waters, particularly coastal regions.

 “This paper also further strengthens the need for evidence to quantify the role of anthropogenic climate change on the frequency and duration of MHWs.”

1- https://www.nature.com/articles/s43247-024-01413-8

Dr Jeffrey Kargel, Senior Scientists, Planetary Science Institute, said:

“The work by Marta Marcos and colleagues is richly informative about global warming’s oceanic influences. The authors computed sea surface temperatures under the observed atmospheric conditions with global warming included, and then re-doing the sea surface computation by removing the long-term influence of the warming atmosphere and just leaving in the hot spells and cold spells.

“Interestingly, the authors found that most–but not all–of the observed intensity of extreme hot ocean surface episodes, and about half of the number of days of extreme ocean warmth are explained directly by global warming. This relationship is readily understood. However, when I read this, I zeroed in on the fact that there have been more days and greater intensities of extreme ocean surface warmth than global warming alone accounts for. I suspect that the excess over predictions could be related to the controversial but much-reported slow-down of the jet streams and the frequent “stuck” jet streams. Instead of continuing on their usual eastward migration, jet streams and associated high- and low-pressure systems have frequently become “stuck” in place, leaving parts of the ocean surface (land surface as well) subjected to blistering hot conditions. This is a likely indirect influence of global warming. Global climate change is not only about warming, but about the disruption of all parts of the climate system, including jet streams. That influence, in turn, affects the sea surface temperature in extreme ways, which then affects hurricanes and weather systems that extend into continental interiors.   

“The weakening and stagnating jet streams in relationship to global warming and melting sea ice was first discovered and explained by Jennifer Francis. If her model is correct, as seems increasingly likely as observations accrue, then combined with this new paper by Marcos and colleagues, it could suggest that the stuck jet streams may impose a multiplier effect on global warming’s influence on sea surface temperatures and extreme weather on land, too.”

Dr Zoe Jacobs, Ocean and Climate Modeler, UK National Oceanography Centre, said:

What are the key points readers should take from the study?

“Human-induced global warming is responsible for nearly half of global marine heatwave (MHW) events since 1940.

“Human-induced global warming has caused a 3-fold increase in the number of days per year experiencing MHW conditions. Globally, this means that, on average, we are experiencing 34 extra MHW days per year in recent years compared with the early period. This reaches 80 additional MHW days per year in some regions, including the equatorial Western Pacific and Atlantic, and parts of the tropical Indian Ocean.

“It has also led to a global mean increase in MHW maximum intensity of 1^oC, with the greatest increases observed outside the tropics.

“Overall, their work reveals the dominant contribution of anthropogenic forcing to MHW increases, especially since the year 2000.

“It is important to note that there will be high variability between individual events. For example, the MHW in the Tasman Sea in 2015-16 is found to almost entirely be due to global warming, whereas the Pacific Blob (2014-15) is found to be due to a mixture of global warming and natural climate variability.

Why are marine heatwaves important for people – frankly, why should they care?

“Wide ranging impacts have been documented for marine heatwave events worldwide. They have decimated coral reefs, seagrass meadows and kelp forests as well as fisheries and seabird colonies and even impacted larger species like seals and whales.

“Because of this they pose significant risks to society, with some individual events causing millions of dollars of losses due to impacts on the fishing, aquaculture, and tourism industries. They have also been found to exacerbate heatwaves on land and have amplified extreme weather like hurricanes and storms.”

Are there any important caveats that we should be aware of?

“Using sea surface temperature (SST) data before the satellite record (pre-1980s) will have considerable uncertainties. Using a. 0.25^o grid will have led to a lot of interpolation between actual observations due to incomplete spatial coverage.

“The study focuses on summer MHW events only but MHWs can occur year-round, with seasonal differences in persistence and intensity likely.”

‘Global warming drives a threefold increase in persistence and 1 ◦C rise in intensity of marine heatwaves’ by Marcos et al. was published in PNAS at 20:00 UK time on Monday 14th April 2025. 

DOI: 10.1073/pnas.2413505122

Declared interests

Dr Jeffrey Kargel “I have no conflicts of interest related to this article or my commentary.”

For all other experts, no reply to our request for DOIs was received.