New ocean temperature record in 2023: Global warming continues impacting the Mediterranean Sea

• Climate change brought new records in 2023 for the ocean temperatures in the Mediterranean Sea, which were associated with records on marine heatwaves, ocean salinity and sea level rise.
• Web-based applications implemented by the Balearic Islands Coastal Observing and Forecasting System (SOCIB) allow monitoring the ocean state, variability and changes in near real-time and over the last four decades.

Mélanie Juza, physical oceanographer at SOCIB. After the record-breaking temperatures for the global ocean in 2022 (Cheng et al., 2023), global warming continues impacting ocean state and variability. 2023 has been the warmest year on record for the global-average surface air temperatures going back to 1850, close to 1.5ºC above the pre-industrial level as well as for the global-average sea surface temperature (SST) according to the Copernicus Global Climate Highlights 2023. Added to the long-term warming of the planet are the natural climate variations such as El Niño contributing to the ocean warming in 2023 (NASA news release 2023). The unprecedented SSTs were associated with marine heatwaves around the globe, in particular in the Mediterranean Sea. In this semi-enclosed sea, climate change brought new records in 2023 for the ocean temperatures but also for salinity and sea level affecting marine life and communities.

Record ocean temperatures: How does the temperature of the Mediterranean Sea vary?

Spatially-averaged annual mean SSTin the Mediterranean Sea were the warmest over the last six years ever registered since 1982 (https://apps.socib.es/subregmed-indicators/ocean_temperature.htm). Mediterranean warming continues its acceleration with local trends varying between 0.19 to 0.62ºC/decade (Figure 1). In 2023, SST was the warmest on record with an annual mean of 20.97ºC corresponding to a mean anomaly of 1.36ºC (Figure 2). Satellites observed the highest seasonal SST values ever registered over the last 42 years. In the western Mediterranean Sea, spring 2023 was the warmest spring ever registered at sea in the region while in the eastern Mediterranean, autumn 2023 was the warmest autumn on record. In both sub-basins, winter and summer 2023 were the second warmest on record. These records in SST have been associated with records on marine heatwaves (MHW) total days in 2023 reaching regionally averaged values of 208, 212 and 205 days in the Mediterranean Sea, western and eastern sub-basins, respectively (see Figure 2).

Figure 1. Linear trends of SST (top) and SLA (bottom) over the periods 1982-2023 and 1993-2023, respectively in the Mediterranean Sea. SOCIB.

Figure 2. Annual SST (top) and MHW total days (bottom) averaged in the Mediterranean Sea, western and eastern sub-basins from 1982 to 2023. SOCIB.

MHW properties show strong spatio-temporal variations at sub-regional scales (see Figure 3, https://apps.socib.es/subregmed-marine-heatwaves/). In 2023, the sub-regional MHW mean and maximum intensities (above the mean), mean duration, frequency and total days ranged over 1.67-2.32ºC, 3.24-5.18ºC, 19-58 days, 5-10 events and 143-256 days, respectively. Locally, MHW mean and maximum intensities, mean duration, frequency and total days reached maximum values of 3.16ºC (in the north Adriatic), 6.18ºC (in the Sicily Channel), 176 days (in the south Levantine), 15 events (in the Ligurian Sea) and 351 days (in the Alboran Sea), respectively (Figure 3). It is worth mentioning that the Alboran Sea, which is the region connecting with the Atlantic Ocean, suffered unprecedented warm SSTs and very intense MHWs during several weeks reaching the highest sub-regional and local MHW total days. Differently from the western Mediterranean, but also subjected to extreme temperatures, the eastern Mediterranean also experienced intense and long-lasting MHWs in winter, summer and autumn 2023.

Figure 3. MHW maximum intensity (in ºC, top) and total days (bottom) in 2023 in the Mediterranean Sea. SOCIB.

• Balearic Islands: In this region, SST continued to increase with a warming rate of 0.4ºC/decade over the period 1982-2023. In 2023, the region experienced its second warmest year on record for SST (after 2022) and suffered very intense and long-lasting MHWs (see Figure 4).

Figure 4. Daily SST averaged in the Balearic Islands region in 2023. MHWs are indicated in orange. SOCIB.

Record ocean temperatures: What are their impacts?

Consequences on ocean system and marine life

The evaporation of warmer water increases the ocean salinity, in regions where there is a marked excess of evaporation over precipitation. In the eastern Mediterranean, the ocean salinity also reached new records in 2023 with sub-basin-averaged annual mean value exceeding 39 psu for the first time corresponding to an anomaly higher than 0.2 psu (https://apps.socib.es/subregmed-indicators/ocean_salinity.htm). Through thermal expansion of the oceanic water column, warm ocean waters also contribute to increase the sea level. Average in the Mediterranean Sea, sea level continued rising with linear trends over the period 1993-2023 of 2.3 cm/decade and reaching its highest anomaly on records since 1993, being this rise more than double in some sub-regions. Indeed, locally, the rate of sea level rise reached 5.3 cm/decade in the eastern Levantine basin (see Figure 1, https://apps.socib.es/subregmed-indicators/sea_level.htm). Changes in the upper ocean temperatures may also increase the stratification that can in turn modify vertical exchanges hindering nutrient supply, reducing heat and carbon absorption into the deep ocean and depriving deep ocean of oxygen, among other key processes.

• Balearic Islands: In this region, sea level continued to rise with a trend of 2.1cm/decade over the period 1993-2023. In 2023, the region experienced its record-breaking year for sea level anomaly.

Furthermore, increasing temperatures and extreme warm events in the ocean threaten marine ecosystems. Devastating consequences on marine species and habitats have been observed during and after previous events, such as coral bleaching, seagrass meadows declining, harmful algal blooms, mass mortality of marine organisms, or location shift of species. In the Mediterranean Sea, marine heatwaves strongly affect key species such as the endemic seagrass “Posidonia Oceanica”, which provide habitats and protection for marine species, produce oxygen, absorb an important part of anthropogenic carbon dioxide, and protect the coast from erosion (Guerrero-Meseguer et al., 2017; Marbà et al., 2014). In addition, the introduction of invasive and voracious species from the eastern sub-basin reaching the western basin has been observed contributing to the deterioration of habitats and distribution shift of marine species.

• Balearic Islands: Very stressful thermal conditions were observed during the last two consecutive years. In addition to very intense and long-lasting MHWs, SST values reached thresholds that strongly threaten marine ecosystems. Averaged in the region, daily SST values reached 29.1ºC in 2022 and 29ºC in 2023, while the mooring at Dragonera observed local temperatures warmer than 31ºC in August of both years.

Consequences on communities

The resulting biodiversity loss and marine life deterioration have impacts on essential goods and services offered by the oceans affecting key sectors of the Blue Economy (e.g. marine living resources and tourism) (Smith et al., 2021). Bringing more heat and moisture to the atmosphere, warm waters can contribute to the intensification of extreme events such as storms with heaving rainfalls that can cause flooding, beach loss or infrastructure destruction (Mitchell et al., 2006). Coastline will also be impacted by the sea level rise accelerated in the last decades with global warming. Finally, human health will also be affected by ocean warming and extreme event intensification through infectious disease, harmful algal blooms, proliferation of jellyfish or lack of food (UNEP/MAP and Plan Bleu, 2020).

The consequences of ocean warming and marine heatwaves on marine life and communities are becoming increasingly alarming and it is worth mentioning that they add to the severe degradation and perturbation effects from human activities such as pollution, overfishing, maritime traffic, natural resource extraction and coastal urbanization.
 

• Sources of information

Web-based applications
SOCIB has implemented open access web-based applications for the Mediterranean Sea to continuously monitor and visualize timely information on the ocean state and variability, from event detection to long-term variation monitoring, in the different sub-regions. See and access applications below:

1. Sub-regional Mediterranean Sea indicators
   
2. Sub-regional Mediterranean marine heat waves
   
3. TIAMAT Observatory - Cabrera Archipelago
   

References
Juza, M. and Tintoré, J. (2021). Multivariate sub-regional ocean indicators in the Mediterranean Sea: from event detection to climate change estimations, Frontiers in Marine Science, 8:610589, https://doi.org/10.3389/fmars.2021.610589.
Juza, M., Fernández-Mora, A., and Tintoré, J. (2022). Sub-regional marine heat waves in the Mediterranean Sea from observations: long-term surface changes, sub-surface and coastal responses, Frontiers in Marine Science, 785771, https://doi.org/10.3389/fmars.2022.785771.
Juza, M., de Alfonso, M., and Fernández-Mora, Á. (2024). Coastal ocean response during the unprecedented marine heat waves in the western Mediterranean in 2022, State Planet Discuss. [preprint], doi:10.5194/sp-2023-18, accepted.