blog.gov.jeIf you are a sea swimmer, surfer, snorkeler or diver, you may well have noticed how warm the sea has been through most of 2025. The sea around Jersey is quite shallow and due to our local currents, a lot of the water remains swirling around the Islands, with cooler water to the north and west struggling to mix in. This, coupled with our proximity to the large landmass of France, which heats up significantly in the spring and summer, means that Jersey generally enjoys higher sea surface temperatures (SST) through much of the year than elsewhere in the British Isles.
Is the warm sea this year unusual? Have we seen temperatures this high before?
Well, the highest SST we’ve recorded in 2025 has been 19.4ºC earlier in August, and it’s currently sitting at 19.3ºC. We measure the temperature using an automated sensor at St. Helier harbour, taking readings twice a day* at high tide and then using the average value as the daily SST. This is the way we measure the SST and we have accurate and reliable daily data going back to 1990. During this period, we have seen the daily SST reach 20ºC or more on 9 days, with 2003 being the last time this occurred, when it peaked at 20.4 ºC on 9 August.
*Very occasionally a day will only have 1 high tide in the 24-hour period.
So, will the sea temperature reach 20ºC this year?
Well, that’s certainly possible, but not a given. Based on climatology, the SST usually peaks in late August or early September. At this time of year, the sea still receives more heat during the day from sunshine than it loses at night to radiation. So, the trend should still be generally upwards through the rest of this month, before things peak around the end of August, then slowly drop away through the autumn months. However, it only takes a short period of windier, more unsettled weather during the month to upset this balance and mix cooler deeper water with the warmer surface layer, lowering the SST. 2023 was a good example of this and quite an unusual year. The SST in 2023 took a significant dip through the summer when it should have been rising, due to very windy weather and choppy seas, before it rose through September as high pressure and sunshine returned, reaching a peak during the middle of that month. So, with further spells of settled and very warm weather to come this month, we may well get close to 20ºC in 2025.
Is the sea around Jersey warming due to climate change?
We wrote a blog on this very topic in June 2024, which you can read here (Sea Temperature Trends in Jersey Over the Last 60 Years – Official Government of Jersey Blog) and yes, this certainly seems to have been the case over the last 60 years, and the SST during the 2020s seem to be continuing this trend. You can see the daily SST for all years since 1990 plotted on the graph below, with older years in blue and more recent years in red. Generally speaking, there are more red (recent) years above the dotted black long-term average line, and more blue (older) years below this line, with this trend more pronounced in the winter months. 2025 (shown in black) sits near the top of the pack for the much of the year from March onwards. However, it’s not always as clear cut as this, with 1990 at the beginning of this data series, actually having some of the highest SST in August, when it reached 20.2 ºC.
