The western Mediterranean reaches record highs after a summer of extreme marine heat

Three consecutive months of record temperatures made summer 2026 the warmest recorded in the western Mediterranean since 1982, with an average sea surface temperature of 26.9 ºC, 2.9 ºC above the 1982–2015 reference-period average.

The western Mediterranean ended the summer of 2026 with the warmest conditions recorded since 1982, following three consecutive months of record-breaking temperatures. The mean sea surface temperature reached 26.87 °C, 2.85 °C above the average for the 1982–2015 reference period, according to the 2026 summer report prepared by the Balearic Islands Coastal Observing System (SOCIB).

The summer was also marked by marine heatwaves, defined as periods of at least five consecutive days during which sea temperatures exceed the 90th percentile of historical values recorded for that area and time of year. From January to September, the western Mediterranean experienced a cumulative total of 158 marine heatwave days, with a maximum intensity of almost 6 °C above historical values.

In July, the mean sea surface temperature reached 26.59 °C (+2.99 °C compared with 1982–2015); in August, 27.79 °C (+2.95 °C); and in September, 26.23 °C (+2.59 °C). August 2026 was the warmest August and the month with the highest mean sea surface temperature ever recorded in the western Mediterranean, exceeding the previous regional monthly record, set in August 2024, by 0.81 °C.

Between June and September, all subregions of the western Mediterranean experienced high-intensity marine heatwaves that persisted throughout the summer. In the Balearic Islands, the regional mean sea surface temperature reached a maximum of 29.83 °C on 15 August. Coastal observations also recorded all-time highs of 33.02 °C at Dragonera, 32.36 °C off Maó, 32.5 °C in the Port of Pollença, and 30.9 °C in the Bay of Palma. Temperatures above 30 °C were recorded on 22 days at Dragonera, off Maó and in the Bay of Palma, and for as many as 42 consecutive days in Pollença.

“The data from this summer show not only exceptionally high temperatures, but also the intensity and persistence of marine heatwaves in the western Mediterranean. In the Balearic Islands, these extremes have been observed both at the regional scale and in coastal waters,” explains Mélanie Juza, SOCIB researcher and lead author of the report.

To monitor sea temperature, SOCIB combines satellite data from the Copernicus Marine Service with in situ observations collected through its network of buoys, gliders and profilers. The latter make it possible to observe warming from the surface down to intermediate and deep layers, reaching depths of 2,000 metres.

According to the SOCIB report, warming of the upper ocean layers can hinder water mixing, limit nutrient supply and reduce the amount of oxygen available in deeper layers. In addition, as seawater warms, it expands and contributes to sea-level rise. In the western Mediterranean, sea level shows an average regional upward trend of 3.9 centimetres per decade between 1999 and 2025.

Extreme temperatures and marine heatwaves also affect ecosystems. Previous events have been associated with the decline of seagrass meadows, such as those formed by Posidonia oceanica, mass mortality events among marine organisms, and changes in species reproduction, growth, abundance and distribution.

“The recurrence of intense and prolonged marine heatwaves over several consecutive years, combined with the gradual warming of the ocean, is placing unprecedented thermal stress on Mediterranean ecosystems. Maintaining continuous observations is essential to understand how these changes are evolving and to assess their long-term effects,” says Juza.

Thermal conditions at the end of summer are also relevant as autumn begins. The report notes that abnormally warm waters can transfer more heat and moisture to the atmosphere and contribute to the intensification of extreme weather events, including severe storms and heavy rainfall, which can cause flooding, beach erosion and damage to infrastructure.

“The Balearic Islands are on the front line of the changes we are observing in the western Mediterranean. SOCIB has been monitoring the evolution of the ocean since 2010 and now has more than 15 years of observations that allow us to better understand these changes and anticipate how they may evolve. Strengthening this observing and forecasting capacity is essential to provide the knowledge that science and public administrations need to manage risks and advance climate change adaptation,” says Emma Heslop, Director of SOCIB.

The results and annual reports are available through SOCIB’s open-access applications: Subregional Mediterranean Indicators and Subregional Mediterranean Marine HeatWaves.