Global sea temperatures in June hottest since records began

The marine heatwave in the Atlantic west of Ireland was classified as "Beyond Extreme" by NOAA, reaching its peak on 21 June.
June 2023 had the hottest global sea surface temperatures ever recorded. Photo: Adobe Stock.
June 2023 had the hottest global sea surface temperatures ever recorded. Photo: Adobe Stock.

Ocean temperatures worldwide are moving into "uncharted territory", with the hottest June global sea temperatures registered since records began, according to the Copernicus Climate Change Service (CCCS), which today released figures for June 2023.

The north Atlantic in particular experienced "unprecedented" high temperatures last month, including several "extreme" and "beyond extreme" marine heatwaves, as defined by the US National Oceanic and Atmospheric Agency (NOAA).

These, according to the newly-released analysis from the CCCS, were caused by a combination of "short-term changes in atmsopheric circulation and longer-term changes in the ocean".

Sea surface temperature anomaly (°C) for the month of June 2023, relative to the 1991-2020 reference period. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

Marine heatwaves a global threat to seafood production

Marine heatwaves pose significant threats to fisheries and aquaculture, with the situation worsening the longer the anomalous temperatures persist.

According to the NOAA, "Weeks, months, or years of unusually warm waters can cause mass die-offs of fish, marine mammals, and seabirds, disrupt food webs and fisheries, bleach corals, spur harmful algal blooms and wipe out seaweeds. Billions of dollars are lost in such events around the world each year."

During May, global sea surface temperatures were already higher than the same month any previous year since records began. But the thermometer climbed even higher in June, as a rash of marine heatwaves hit oceans across the globe. In the Pacific, marine heatwaves were recorded in the Sea of Japan and in the ocean southeast of New Zealand. Meanwhile, the Indian Ocean zone southeast of Madagascar was also struck by similarly anomalous high temperatures.

However, one of the most severe anomalies was a record-breaking marine heatwave affecting the north-east Atlantic, particularly around Ireland and Scotland. From the end of May, sea temperatures steadily increased, with the marine heatwave peaking on 21 June.

North Atlantic marine heatwave categorised as "Beyond Extreme"

NOAA's Marine Heatwave Watch classified the heatwave in the north Atlantic west of Ireland as a Category 5 ("Beyond Extreme"). At its peak, the heatwave produced sea surface temperatures as high as 5°C above average.

"Category 1 marine heatwave conditions were already present in this area since early June, reaching extreme conditions between 15 and 24 June before returning to category 1 at the end of the month," the CCCS observed.

Around the coasts of Ireland, Scotland and England, and in the Baltic Sea, the NOAA identified marine heatwave conditions at Category 4 "extreme" level, while "strong" heatwave conditions persisted further south in the Atlantic, including the Bay of Biscay, the northwest coast of Africa, as well as the subtropical and tropical Atlantic.

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