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The sea surface temperature are breaking records according to a dataset compiled by the Met Office.
Professor Stephen Belcher, Chief Scientist at the Met Office, said:
“May 2023 has seen the highest temperatures of any May since 1850. But it doesn’t stop here. It has also been the highest month above average compared with any individual month in the series.
“Climate scientists often look at anomaly maps which compare current measurements with the average for a given long-term period.
“Looking at the anomaly map for current sea surface temperatures in the North Atlantic, some areas really stand out. You can see that the eastern Atlantic, from Iceland down to the tropics, is much warmer than average. But areas around parts of north-western Europe, including parts of the UK, have among some of the highest sea-surface temperatures relative to average.
“May 2023 was nominally the warmest May in the sea-surface temperature around the near-coastal waters of the UK since the late nineteenth century. The value is about 1.60 °C above the average for May over the period 1961-1990.”
Dr Dan Smale, Senior Research Fellow, Marine Biological Association, said:
What is happening and why is it happening?
“An extreme marine heatwave is developing in the northeast Atlantic, leading to pockets of extremely warm water around parts of the UK. The drivers of this phenomenon are complex but likely to include weaker trade winds leading to lower evaporation and more uptake of solar radiation. The warming events occur naturally and are part of the complex ocean climate system. However, when they occur now they are superimposed onto 100 years of significant ocean warming, so the starting point is much higher (around 1 deg C on average in the N Atlantic). We are recording temperatures in excess of 18 deg C, when the average sea temperature for June around the UK is around 12-14 deg C.”
Is this unprecedented?
“Marine heatwaves do occur from time to time, but the heat content of the N Atlantic is at a record high. The amount of energy in the system means that contemporary heatwaves are likely to be amongst the highest on record.”
What does it mean for marine life?
It is very complex and hard to predict, but we do know that marine species have evolved over millions of years to occupy specific thermal windows. When temperature thresholds are exceeded, this can lead to high stress, reproductive failure and widespread mortality, with implications for foodwebs and the entire ecosystem. We’ve seen massive die-offs of kelp, seagrass, corals, fish, mammals and sea birds from marine heatwaves elsewhere, such as the Mediterranean, California and the west coast of Australia. Marine life around UK and Ireland is adapted to colder water and some species will become stressed by this heatwave if it persists for too long. If it continues through summer and actual temperatures start to exceed 20 deg C we might see mortality events and shifts in ecosystem structure with both winners and losers of the disturbance event. Currently, warm temperatures are likely to affect food supply, migration patterns and reproductive output.
Prof Yueng-Djern Lenn, Professor in Physical Oceanography, School of Ocean Sciences, Bangor University, said:
What is happening and why is it happening?
“The North Atlantic, including the seas around the UK are exceptionally warm for this time of the year and it was been on this unusual upward trajectory since mid-January. Sea surface temperatures to the east and west of the UK and Ireland are in places up to 4-5 deg. Celsius higher than normal, and have been so for more than 5 days, so much so that this event has now be classified by the USA’s National Oceanic and Atmospheric Administration as a level 4 marine heatwave which is the the ‘extreme’ category.”
“We can’t say for sure what the specific causes of this event are yet, but it will most likely be due to a combination of human and natural causes. The oceans have already absorbed 90% of the excess heat due to anthropogenic greenhouse gases, and continue to warm. At the same time, we are witnessing an El Niño develop in the tropical Pacific, which is the warm phase of the Pacific’s natural cycle. During El Nino’s the global ocean tends to store more heat than usual. We are certainly expecting warm temperature records to be broken more this year, both on land and sea across the globe. There may be other more complicated feedbacks at play as well in driving the UK’s current marine heatwave, but it is too early to say definitively.”
Is this unprecedented?
“It is certainly unprecedented for this region in the era of global ocean observations of sea surface temperature from satellites that go back several decades. However, marine heatwaves have been increasing in frequency and intensity around the globe for some time. The Mediterranean for instance has experienced many marine heatwaves.”
What does it mean for marine life?
“Marine heat waves can be devastating for marine life. There is no escape from the heat for sea creatures – they aren’t turn on the air conditioners – and this can exert heat stress on their bodies that can lead to death. In the case of coral reefs, marine heatwaves increase the risk of coral bleaching where they lose their colourful symbiotic zooxanthallae. Marine heatwaves in the Pacific in 2016-2018 were blamed for a large decline in seabirds. We don’t know yet how long this UK event will last, but we would expect our marine ecosystems to not escape unscathed. And if there isn’t enough of a break between such events, allowing the ecosystems time to recover, the only winners will be the open ocean swimmers in the short term.”
Dr John Pinnegar, Lead Advisor on Climate Change and Director of the International Marine Climate Change Centre at Cefas said:
“Cefas has been monitoring seawater temperatures at various sites around the UK since 1902, and over the past 20 years has deployed state-of-the-art autonomous technology to give a more complete and accurate understanding of how climate change is affecting our seas. What we are seeing is a continuation of yearly increases in sea temperatures across the world, including the UK. In 2022, through our network of WaveNet buoys, we recorded some the highest sustained sea temperatures ever recorded around the British Isles.”
“Our research has shown that warmer seas are already having an impact on fish stocks around the UK with an increase in warm-water species, such as anchovy, bluefin tuna, squid and red mullet and a retreat of cold-water species, such as wolf-fish and Atlantic cod. Heat waves can allow non-native species to establish themselves and spread, but can also create the conditions necessary for harmful algae blooms. Elsewhere in the world, heat waves have been widely implicated in coral bleaching events and major (though usually temporary) shifts in fish distribution”.
Source: 16_fish_2020.pdf (mccip.org.uk)
https://statics.teams.cdn.office.net/evergreen-assets/safelinks/1/atp-safelinks.html
Prof Piers Forster, Professor of Climate Physics, at the University of Leeds, said:
“Both Met Office and NOAA analyses* of sea-surface temperature show temperatures are at their highest ever level – and the average sea-surface temperature breached 21C for the first time in April. These high temperatures are mainly driven by unprecedented high rates of human-induced warming. Cleaning up sulphur from marine shipping fuels is probably adding to the greenhouse gas driven warming. The shift towards El Nino conditions is also adding to the heat. There is also evidence that there is less Saharan dust over the ocean this year. This normally reflects heat away from the ocean. So in all, oceans are being hit by a quadruple whammy – it’s a sign of things to come.”
*https://climatereanalyzer.org/clim/sst_daily/
Dr Phil Williamson, Honorary Associate Professor, School of Environmental Sciences, University of East Anglia, said:
“The prolonged period of exceptionally high seawater temperatures currently experienced in the North Atlantic (and particularly around the UK) is a cause of great concern to marine ecologists. Although no biological effects have yet been reported – as far as I am aware – these seem near-inevitable.
“Elsewhere in the world, similar recent marine heatwaves in temperate waters have killed many millions of fish and seabirds off New Zealand and in the North East Pacific, as well as being responsible for coral reef bleaching and its long-term loss in tropical seas. Over the past 50 years the global frequency of marine heat waves has doubled.
“The main problem for marine fish and shellfish is not a direct temperature effect but is caused by lack of oxygen. For shallow coastal waters, a potentially lethal combination of five factors is involved. First, marine organisms’ greater need for oxygen when temperatures are higher, increasing their metabolic rates (by as much as 70% for a 3deg C rise); second, the lower solubility of oxygen in warmer water; third, the depletion of oxygen, due to warmer temperatures initially stimulating growth by algae (phytoplankton) and their subsequent decomposition by microbes; fourth, the reduced mixing of the water column during periods of warm, settled weather, preventing the natural re-supply of oxygen to the near seafloor; and fifth, the effects of warmer winters (due to long term heating-up of the ocean and coastal seas), reducing the main winter re-charge of oxygen throughout the water column, through storms during periods of cooler weather.
“The discharge of untreated sewage in British waters makes matters even worse. That has a high ‘oxygen demand’ as it is broken down by bacteria, resulting in a further lowering of oxygen levels.
“Ocean warming and increased frequency of damaging marine heat waves is, unfortunately, an inevitable effect of human-driven climate change. That is because around 93% of the extra heat caused by the greenhouse effect goes into the sea.”
Further background:
Dr Ali Mashayek, Associate Professor of Climate Dynamics, University of Cambridge, said:
What is happening and why is it happening?
“The global sea surface temperatures have been unprecedently high in April and May due to a combination of internal variability in the climate system and the background anthropogenic warming trend. While the strong combined warming is global, it can be more severe regionally.”
Is this unprecedented?
“Yes, the signal is record-breaking. This highlights how the continuous warming of the Earth due to greenhouse gas emissions creates the background for extreme events. When the entire system is heating up, the positive anomalies (i.e. warming due to natural climate variability) can add up to the background warming to result in unprecedented warming. In other words, without anthropogenic warming, very likely we would be experiencing some warming due to natural variability but not as extreme as revealed in this report.”
What does it mean for marine life?
“There are adverse impacts on marine life on multiple timescales. Extreme heat stress can almost immediately impact life (plants and animals) in the surface ocean layer. For example, it can impact nutrient availability to phytoplankton, the foundation of the aquatic food web, by making the surface mixed layer shallow. Or it can alter migration patterns for marine life and impact breeding patterns, or lead to hazardous algal blooms. On slightly longer timescales (years to decades), sustained heat stress can impact marine ecosystems (e.g. coral reefs, mangrove forests, etc) adversely, limiting nurseries for marine life such as fish populations and subsequently endangering the security of our marine food resources. Rising surface temperatures can also impact polar climates often faster than the rest of the globe (e.g. reduced sea ice cover, ice shelf collapse) which can limit the habitability zones for many polar animals in and out of sea. On the time scales of decades and longer, rising ocean temperature can change the large-scale ocean circulation patterns, modifying the distribution of nutrients in the ocean and impacting ocean biochemistry and the entire food chain built upon it. This will not only impact marine life but also potentially comprise the ability of the oceans to mitigate against climate change by uptaking significant anthropogenic heat and carbon.”
Prof Tom Rippeth, Professor of Physical Oceanography, University of Bangor, said:
What is happening and why is it happening?
“According to the UK Met Office (and others) Global sea surface temperatures for April and May are the highest on record. This is particularly so in the North Atlantic Ocean, where average sea surface temperatures are nearly a degree and a half higher than is the norm for this time of year.
“Why is it happening? Partly due to natural variations such as the El Nino event which is currently beginning, and partly due to climate change, for example, linked to the declining sea ice in the Arctic Ocean, which acts as a giant mirror, keeping the northern hemisphere cool in the summer but which has declined greatly in recent years.
“This year in particular, the other factors have also been at play. Weaker winds mean that there is less Saharan dust in the atmosphere, this dust is reflective, reflecting some of the sun’s rays back to space. Less dust = more of the sun’s energy reaches to the Earth’s surface. Less dust = more warming of the sea surface (and all else being equal, higher sea surface temperatures).
“Also, the relatively quiescent conditions of late has meant relatively few storms to deepen the ocean surface mixed layer, so the energy put into the ocean by the sun is concentrated into a thinner (than usual) surface mixed layer, resulting in a larger rise in sea surface temperature.”
Is this unprecedented?
“Yes – according to the Met Office: https://blog.metoffice.gov.uk/2023/06/16/sea-surface-temperatures-breaking-records/”
What does it mean for marine life?
“Marine species have evolved to life within a specific range of conditions, as the ocean has warmed over recent years there is a lot of evidence that mobile marine species have moved poleward to find the optimal conditions for their survival. This is particularly the case in the Arctic Ocean, where warming driven by the increasing influence of Atlantic water heat has also introduced many temperate marine species to a region once the preserve of polar species (but also lots of reported cases of more tropical species sighted around the UK coast).”
When warmer than average temperatures constitute a heatwave?
“A marine heatwave is a period (usually 5 or more days?) of unusually high ocean temperatures* and is defined by its length of the extreme temperatures and by how much the water temperature is above the norm for that time of year.
“A common way to measure the temperature, is to make sea surface temperature maps from satellite data. As such the “unusually high temperatures” used to define marine heat waves tend to be unusually high sea surface temperatures.
“The most severe marine heat wave on the planet is currently taking place in the shallow seas around the UK where sea surface temperatures are currently 4 – 5 deg C above the seasonal average.”
Dr Melissa Lazenby, Lecturer in Climate Change at the University of Sussex, said:
What is happening and why is it happening?
“We are currently experiencing very high global sea surface temperatures (SST’s) and have experienced the highest recorded SSTs for April and May since 1850. The two main reasons for these extremely high SSTs are due to 1.) human induced climate change and 2.) natural climate variability of the developing El Niño state – a naturally warmer phase due to changes in winds and ocean circulation patterns.
“There are likely other contributing factors too such as reduced Saharan dust which causes cooling over the Atlantic Ocean as a result of sunlight being reflected back and additionally the reduced Artic and particularly Antarctic ice extent for this time of year. It is important to note that the climate system is complex and no one element acts in isolation, for example if there is less sea ice in the Antarctic than usual, there will be less reflection of sunlight due to ice acting as a large reflector and therefore warming is enhanced – known as a positive feedback loop.
“In a warming world due to the high levels of greenhouse gases in the atmosphere, it is no surprise that we are seeing new record-breaking SSTs globally and regionally. We cannot change the natural oscillations of the climate system such as El Niño but we can change our human impact by reducing emissions to lower global warming.”
Is this unprecedented?
“Yes, this is unprecedented warming of SSTs since 1850. Whenever we break a record of warming whether it is in the atmosphere or ocean, we are stepping into unchartered territory. It is not surprising, however, since these higher temperatures have been projected by climate models based on human activity.”
What does it mean for marine life?
“I am not a marine expert therefore I cannot confidently say how marine life will be affected, but it is likely there will be effects to marine life that will have adverse impacts on industries such as fisheries due to the reduction in cold nutrient rich waters that are beneficial to the fishing industry, therefore having a knock on effect on the food supply chain.”
Prof Chris Merchant, Professor in Earth and Ocean Observation, of the NCEO (National Centre for Earth Observation) at the University of Reading, said:
“The warm sea surface in the north Atlantic that the Met Office has drawn attention to is confirmed in measurements of the surface temperature from space. The sea surface is warmer than usual for the time of year by between 2 and 4 degrees centigrade from the Cape Verde islands up to the seas around Scotland.
“At this time of year, the northern hemisphere oceans are warming up in the summer sunlight, and the prevailing weather has allowed that to happen faster than in an average year, so the sea is relatively warm. This weather-related variation is happening on top of the steady rise in temperatures across the planet from global warming.”
Prof Rick Stafford, Professor of Marine Biology and Conservation Co-Lead Centre for Ecology, Environment and Sustainability, Bournemouth University, said:
“The record sea surface temperatures are another indication that climate change is happening now. The immediate effects of these high temperatures are probably biggest in the tropics, where coral bleaching events are known to correlate strongly with warmer temperatures. However, warmer seas may also have major implications for temperate seas like the UK. There is evidence that our plankton communities will decrease as temperatures rise, meaning less food for fish, but ultimately less carbon capture and less oxygen production. Changes to our seas also have far reaching, global effects on land.”
Prof Daniela Schmidt, Professor of Earth Sciences, University of Bristol, said:
“The extreme and unprecedented temperatures show the power of the combination of human induced warming and natural climate variability like El Nino. While marine heatwaves are found in warmer seas like the Mediterranean, such anomalous temperatures this part of the North Atlantic they are unheard of. They have been linked to less dust from the Sahara but also the North Atlantic climate variability which will need further understanding to unravel.
“Heat, like on land, stresses marine organisms. In other parts of the world, we have seen several mass mortalities of marine plants and animals caused by ocean heatwave which have caused hundreds of millions of pounds of losses, in fisheries income, carbon storge, cultural values and habitat loss. 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.”
https://blog.metoffice.gov.uk/2023/06/16/sea-surface-temperatures-breaking-records/
Declared interests
Dr Dan Smale: “I have no competing interests to declare.”
Prof Tom Rippeth: “my research is funded by RCUK (mainly NERC) and I am an active member of the Lib Dems, and previously ran a campaign to stop a road in Northeast Wales.”
Dr Melissa Lazenby: “I declare I have no vested interests besides providing accurate and transparent climate science information as a climate scientist from the University of Sussex.”
Prof Piers Forster: “No conflict of interests.”
Dr Phil Williamson: “previously employed (to 2020) as a science coordinator by the UK Natural Environment Research Council, part of UKRI. Since then he has held Honorary positions at the University of East Anglia, currently Associate Professor. He is not in receipt of any other external income relevant to his continuing scientific interests in climate change and its impacts.”
Dr John Pinnegar: “No conflicts.”
Prof Daniela Schmidt: “No competing interests.”
Prof Rick Stafford: “I am a trustee of the British Ecological Society and chair the organisation’s policy committee. I receive research funding through Bournemouth University from Earthwatch (US) and Interreg (EU) for work on artificial reefs in both Indonesia and Europe.”
Dr Ali Mashayek: “No conflicts to declare.”
Prof Chris Merchant: “No conflicts to declare.”
For all other experts, no reply to our request for DOIs was received.