THE Island’s average sea temperature has been increasingly gradually over the past 60 years and is continuing its upwards trend as the global climate warms, according to Jersey Met.
Islanders who have ventured into the water in recent months may have noticed it feeling warmer than usual at this time of the year – that’s because it is, and it may not all be good news.
A surface sea temperature (SST) of 18.3°C was recorded in St Helier Harbour this week, which senior forecaster Matt Winter said was more in line with what would be expected on average for August (18.2°C) or September (17.8°C).
The sea was around 1°C higher than average for the time of year which was also the case for April, May and June 2025.
The average for July (17.8°C) was also higher than the long-term climate average, which is calculated over a 30-year period, of 16.9°C.
Mr Winter said the average SST in Jersey “has been increasing gradually over the course of the last 60 years or so, and continues to show an upward trend”.
The trend in Jersey is part of a “wider warming trend which is being seen across much of the globe”, according to a blog published last year by Jersey Met, which added that this was “in line with what we would expect to see over the land and sea in a globally warming climate”.
Forecasters wrote: “We don’t look at any single year in isolation. One year may be particularly warm or particularly cold, and therefore would only reflect what was happening within that year – not whether this was part of a longer-term trend.
“Instead, we look at changes between our long-term average data sets to try and see what’s happening.”
Jersey Met added: “Looking back through these data sets, we do notice a warming trend since the 1960s, with the average 30-year temperature increasing by around 0.2°C every time we move forward by a decade, and there is an overall difference of 0.7°C between the first and the latest long-term average reference periods.”
There were also some signs the warming trend may be increasing, according to the blog.
Jersey experienced its hottest June day on record this year, with temperatures of 33.4°C recorded on 30 June.
The Island experienced its seventh-hottest year on record in 2024.
The four hottest years to be recorded have happened over the past 11 years, with the record highest average from 2022 of 13.56°C.
Official records began in 1894.
