Ocean Temperature Bulletin - 2025 Mid-year highlights

Key takeaways

The global ocean experienced its second warmest first semester in 2025 despite neutral ENSO (El Niño Southern Oscilation) conditions (not an El Niño year) which are typically not associated with record-high Sea Surface Temperatures (SSTs).
In the Mediterranean Sea, the first half of 2025 was the warmest on record with a mean temperature of 18.50 ± 0.12°C.
The eastern North Atlantic, off the coast of France and the United Kingdom showed a widespread very warm region with 4 to 6 months of record monthly mean SSTs.
88% of the Mediterranean Sea was affected by strong or higher Marine Heatwaves (MHWs) – the second wider extent after 2023 (95%).

Read the full bulletin for the first semester of 2025 for the Mediterranean Sea and North Atlantic regions.

Global Ocean

January to June 2025: SST and SST anomaly

Figure 1: Top: Daily sea surface temperatures averaged for the global ocean between 1991-2020 using ESA’s Climate Change Initiative (gray shades), and between 2021-2025 (colored shades). 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 2nd warmest first semester on record, despite neutral ENSO conditions (not an El Niño year) which are typically not associated with record-high Sea Surface Temperatures (SSTs).
The mean SST over the global ocean (between 60°S and 60°N) reached 20.89 ± 0.06°C, surpassed only by the 2024 record of 21.04 ± 0.06 °C. For this period, mean SSTs were above average for 82% of the global ocean (between 60°S and 60°N) (figure 2).
The European region shows strong SSTs reaching more than 1°C above the long-term average (from 1993 to 2022).
The Pacific Ocean shows high SSTs at midlatitude in both hemispheres, with large regions with SSTs reaching 1.5°C to 2°C above average. Along the equator, below average SSTs are observed reaching almost 1°C below average.
The same is observed for SSTs in the Indian Ocean with above average SST between Australia and Madagascar reaching almost 2°C along the Australian coast.

Highest first semester mean SST for Europe and the west Pacific

Figure 2: Rankings of 2025 mid-annual mean SST since 1993 using daily data from Mercator Ocean International’s GLO12 analysis from 2021 to 2025 and from GLORYS12 reanalysis from 1993 to 2020. Blank areas mean that 2025 mean SST doesn’t rank in the 4 warmest or 4 coolest mean SST since 1993

January to June 2025 SST mean values are compared to those from previous years (since 1993). For each area of the global ocean, 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.

The first semester of 2025 was the warmest since 1993 for 10% of the global ocean with the European region and the Southeast Asia region most affected (figure 3).
The first half of 2025 was amongst the warmest 4 years for 28% of the global ocean (between 60°S and 60°N). In contrast, less than 0.2% of the global ocean ranked among the 4 coldest years.

2025 Monthly sea surface temperature

*Figure 3: Number of months with record-breaking mean SST between January and June 2025. Blank areas mean that no month ranked as the warmest since 1993*

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.
The eastern North Atlantic off the coast of France and the United Kingdom showed a widespread very warm region with 4 to 6 months of record monthly mean temperatures.
Overall, 26% of the global ocean (between 60°S and 60°N) experienced at least one month with a record-breaking mean SST.

Marine Heatwaves

Marine heatwaves (MHWs) are extreme rises in ocean temperature for an extended period of time (at least 5 consecutive days). 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

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, ocean 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.*

Figure 5: Evolution of the extent of the highest MHW category. For each year, the length of the bar represents the percentage of the region impacted by MHW, while the colors indicate the proportion of the highest categories reached within that region. The horizontal dotted line represents the average surface of the region impacted by MHW during the semester between 1993 and 2025. The region displayed is the global ocean (60°S to 60°N).

79% of the global ocean was affected by MHW during the first semester of 2025 – the third largest extent impacted by MHW after 2016 (79%) and 2024 (83%).
Focusing on strong or higher category MHW events, 42% of the global ocean was affected by such MHWs, the second widest extent after 2024 (58%).

Total number of MHW days between Jan-June 2025

We monitor the number of MHW days to highlight the region often impacted or impacted by long MHWs. 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 events occurred between January and June 2025. For instance, ocean 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.

Figure 6: 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.

Figure 7: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 global ocean (60°S to 60°N) exposed to a strong or higher MHW. The colours indicate the number of days impacted by such MHW.

Between January and June 2025, 14% of the global ocean (between 60°S and 60°N) was exposed to intense MHW conditions (i.e. strong or higher categories) for more than 15 days (not necessarily consecutive days). The most impacted regions are east of Madagascar, west of Australia, the southeast Asia region, the south Pacific off the coast of Chili, the Caribbean and the European region.

January- June 2025 marine heatwave statistics

Figure 8: Mid-Annual MHW Characteristics from 1993 to 2025. Duration, intensity, extent and activity averaged for each half-year between 1993 and 2025 for the global ocean. 2025 is highlighted with a blue edge.

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 February 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 previous Februarys (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.