Key takeaways
- The first half of 2025 was the warmest first semester ever recorded for the Mediterranean Sea, with a mean Sea Surface Temperature (SST) of 18.50 ± 0.12°C.
- 98% of the Mediterranean Sea was affected by marine heatwaves, while 88% of the basin experienced strong or higher categories of these extreme events.
Mediterranean Sea
January to June 2025: SST and SST anomaly
Figure 1: Top: Daily sea surface temperatures averaged for the Mediterranean Sea between 1991-2020 using ESA’s Climate Change Initiative (gray shades), and between 2021-2025 (colored shades) using Mercator Ocean International’s GLO12 analysis. Bottom: Jan to Jun 2025 Mean SST Anomaly relative to a 30-year climatology (1993-2022), calculated using daily data from Mercator Ocean International’s GLO12 analysis for 2024 and from GLORYS12 reanalysis for the climatological mean.
- The first half of 2025, from January to June, was the warmest first semester ever recorded for the Mediterranean Sea, with a mean Sea Surface Temperature (SST) of 18.50 ± 0.12°C, breaking 2024 record of 18.46 ± 0.26 °C.
- For this period, mean SSTs are above average for 97% of the Mediterranean Sea, with almost half of the basin (49%) exceeding the long-term average (from 1993 to 2022) by at least 1°C (figure 2).
- The highest values are reached in the western part.
Highest first semester mean SST for Western Mediterranean Sea
January to June 2025 SST mean values are compared to those from previous years (since 1993). For each area of the Mediterranean Sea, we show where the SST for the first semester of 2025 corresponded to the highest observed, warmer than usual (being part of the 4 warmest years), or inversely corresponded to the lowest SST, or was colder than usual (amongst 4 coldest years) using the 34-year period of study.
- In 2025, more than half of the Mediterranean Sea (54%) experienced its warmest 1st semester since 1993 (figure 2).
- The western part is the most impacted.
- The first half of 2025 was amongst the warmest 4 years for 84% of the Mediterranean Sea. In contrast, less than 0.2% of the basin ranked among the 4 coldest years.
2025 Monthly sea surface temperature
We also look at the number of months of 2025 between January and June for which record monthly means were recorded (Figure 4). For instance, regions in green correspond to areas where SST records were broken for 4 months between January and June 2025.
70% of the Mediterranean Sea experienced at least one month with a record-breaking mean SST, and almost a quarter of the basin (23%) experienced at least 3 months of record-breaking mean SST.
Marine Heatwaves
Marine heatwaves (MHWs) are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean, and their magnitude and frequency have increased over the last couple of decades, with harmful impacts on ecosystems, and human activities.
Highest Marine Heatwave categories reached in 2024
MHW can be classified based on their intensity into 4 categories (moderate, strong, severe, or extreme). Here we look at the highest category reached in each region during the first semester of 2025. For instance, mediterranean regions in orange correspond to areas where the highest MHW category reached during 2025 is strong (meaning that no severe nor extreme MHWs occurred in those regions). We then measure the extent of the highest category and monitor its evolution since 1993.
Figure 4: Highest MHW Category reached between Jan and Jun 2025. Blank areas mean that no MHW occurred in these regions in the first semester of 2025.
- 98% of the Mediterranean Sea was affected by MHW during the first semester of 2025 – it is the second largest extent impacted by MHW after 2024 (99%).
- Focusing on strong or higher category MHW events, 88% of the Mediterranean Sea was affected by such MHWs, the second wider extent after 2024 (95%).
Total number of MHW days during 2025 first semester
Figure 6: Left: 2025 first semester MHW days of strong and higher category. Number of days during which a MHW of category strong, severe or extreme occurred during the first semester of 2025. Blank areas mean that no MHW of strong or higher category happened in 2025. Right: Year by year evolution of the surface impacted by strong and higher MHW for the period January to June. The size of the bar represents the proportion of the Mediterranean Sea exposed to a strong or higher MHW. The colours indicate the number of days impacted by such MHW.
Longer and more regular MHWs increase heat stress on marine ecosystems, especially intense events. Here we focus on MHW of strong or higher category and look at the total number of days where such MHWs occurred between January and June 2025. For instance, Mediterranean regions in light green correspond to areas where a total of 30 to 45 days of strong or higher MHW were observed during the first semester of 2025. We then measure the extent of impacted areas and monitor its evolution since 1993.
- Between January and June 2025, 51% of the Mediterranean Sea was exposed to intense MHW conditions (i.e. strong or higher category) for more than 15 days (not necessarily consecutive days), and 25% for at least 30 days.
January – June 2025 marine heatwave statistics
Figure 7: Mid-Annual MHW Characteristics from 1993 to 2025. Duration, intensity, extent and activity averaged for each half-year between 1993 and 2025 for the Mediterranean Sea. 2025 is highlighted with a blue edge.The larger and the darker the bubble, and the more to the right and to the top, the more the year that the bubble represents is exposed to MHWs.
The main MHW mean statistics for a given year – duration, intensity, surface occupied and activity– are combined together in a single-colored bubble on a graph to compare the MHW landscape for different years.
- For the Mediterranean Sea, the first semester of 2025 was the third most extreme semester in terms of MHWs intensity (after 2023 and 2024) and the second most extreme in terms of duration (after 2024).
About the bulletin
This mid-year surface temperature bulletin reports on mean temperature and marine heatwave (MHW) conditions for the first semester of the year 2025. This bulletin focuses on the global ocean (between 60°S ans 60°N) and 2 regions: the Mediterranean Sea and the North Atlantic (between 0°N and 60°N). It includes:
- The mean Sea Surface Temperatures and anomalies.
- The highest category reached by MHWs during the first semester of the year,
- The total number of days of exposure to MHW (including only strong and higher intensity categories),
- A comparison of the MHW landscape for different years.
Image citation: European Union, Copernicus Marine Service Data 2025 © Mercator Ocean
All images in this article can be used freely and should be cited with the information above.
Datasets and Products
Sea Surface Temperature
The results are obtained using the Copernicus Marine Service global analysis and forecasting system together with observation-based products.
- GLOBAL_ANALYSISFORECAST_PHY_001_024 Global Ocean Physics Analysis and Forecast sea surface temperature. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00016
- ESA SST CCI reprocessed sea surface temperature analyses. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00169
- NOAA OI SST V2 High Resolution Dataset data provided by the NOAA PSL, Boulder, Colorado, USA, from their website at https://psl.noaa.gov, Huang, B., C. Liu, V. Banzon, E. Freeman, G. Graham, B. Hankins, T. Smith, and H.-M. Zhang, 2021:
- Improvements of the Daily Optimum Interpolation Sea Surface Temperature (DOISST) Version 2.1, Journal of Climate, 34, 2923-2939. doi: 10.1175/JCLI-D-20-0166.1
- Global Ocean OSTIA Sea Surface Temperature and Sea Ice Analysis. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00165
- ODYSSEA Global Sea Surface Temperature Gridded Level 4 Daily Multi-Sensor Observations. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00016
Notes: The global analysis system was used to study the ocean surface temperature from 2021 and 2025. To study surface temperature for previous years (1991 to 2020), it was used the ESA CCI data. The other observation-based sea surface temperature products (OSTIA, OI SST and Odyssea) were used for calculating monthly mean and uncertainty values for the past month in 2025.
Marine Heatwaves
The results are obtained using the Copernicus Marine Service global systems including the analysis and forecasting system together with the reanalysis product.
- GLOBAL_ANALYSISFORECAST_PHY_001_024 Global Ocean Physics Analysis and Forecast sea surface temperature. E.U. Copernicus Marine Service Information (CMEMS). Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00016
- GLOBAL_MULTIYEAR_PHY_001_030 Global Ocean Physics Reanalysis sea surface temperature. E.U. Copernicus Marine Service Information (CMEMS). Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00021
Notes: The global analysis system was used to study MHWs for 2024 and 2025. To study MHWs for this month in previous years (1993 to 2023), it was used the reanalysis system. The reanalysis product was used for generating a daily climatology using the 30-year period from 1993 to 2022.
