Extreme Sea Levels (ESLs) - Trends
The trends of ESLs, their components and the corresponding waves were examined for the period 1980-2014 (baseline) and for 9 different return periods (1, 5, 10, 20, 50, 100, 200, 500, 1000 years).
It is observed that the ESLs for Santorini and Kos are similar and range from ~0.6 (return period, Tr=1) to 1.04 (Tr=1000) m depending on the return period.
The corresponding significant wave height, during a storm, ranges between 1.47 – 1.78 m for Santorini and 1.32 – 1.86 m for Kos, while the wave period ranges between 4.89 – 5.64 sec for Santorini and between 4.55 – 5.64 sec for Kos, depending on the return period.
Extreme Sea Levels (ESLs) - Future Projections
Extreme Sea Levels (ESLs) are projected to increase over the course of the century, with projections varying based on greenhouse gas emission scenarios. Under the moderate RCP4.5 scenario, the 100-year extreme sea level (ESL₁₀₀) in Santorini and Kos is projected to increase by 0.11 to 0.58 m relative to the baseline ESL₁₀₀ (~0.90 m). In contrast, under the high-emission RCP8.5 scenario, the increase ranges from 0.18 to 0.79 m.
The trajectory of ESLs over time is increasingly influenced by the emission scenario, with a more substantial rise observed under RCP8.5 compared to RCP4.5. Regarding the contributions of the different ESL components and their temporal evolution, it appears that although the episodic sea level elevation (ηCE), driven by storm surges and wave setup, is the dominant factor, its influence diminishes over time, as it changes little with time compared to the accelerating relative sea level rise (RSLR).

Projections for Kos. (a) Temporal Evolution of Relative Sea Level Rise (RSLR) for 2 emission scenarios. (b) Temporal evolution of Extreme Sea Level (ESL) and episodic elevation (ηCE) due to storm surges and wave set-up, for 2 different return periods (Tr) and based on the RCP8.5 scenario.
The projected changes in the return periods of Extreme Sea Levels (ESLs) throughout the 21st century were analyzed, revealing a significant reduction over time for events of a given magnitude. In Santorini, the baseline 100-year ESL (ESL₁₀₀) is expected to occur approximately every 28 years under the RCP4.5 scenario and every 11 years under RCP8.5 by 2050. Similarly, in Kos, the same event is projected to occur approximately every 23 years under RCP4.5 and every 14 years under RCP8.5 by mid-century. By 2100, ESL₁₀₀ events are anticipated to become frequent, occurring multiple times per year.

Projections for Kos. (a) Temporal Evolution of Extreme Sea Level (ESL) for different emission scenarios and return periods and (b) temporal Evolution of corresponding return periods.
Meteorological parameters - 20-year Trends, 2003-2023
Santorini
By examining the monthly means of the last available 20-years period from the Copernicus ERA-5 dataset, no significant differentiations have been recorded for the investigated meteorological parameters (temperature, humidity/dew point, cloud coverade and precipitation). Monthly temperature ranges between 10 -26 °C, whereas humidity/dew point is found to range between 5 – 21°C. Similarly, cloud coverage and precipitation are found to be higher during the winter period with maximum values ranging of 0.6 και 0.1 – 0.2 mm, respectively. Slight differentiations are found in precipitation mean values of the winter months of November and December for the years 2006, 2010, 2012, 2018, 2019 and 2021, during which had the highest values (0.3 – 0.4 mm), while no significant differences are found regarding cloud coverage. Regarding the variance of monthly wave parameters, it is evident that significant wave heights ranged between 0.5 – 2.0 m, while mean wave period ranged between 3.5 – 6.0 s. Wave direction in most cases was found to be from the N and NW sectors (270° – 360°), while monthly mean wind velocity is found to range between 3 – 7 m/s .
Kos
When investigating the monthly mean values for the last 20-years period in Kos island, no significant differentiations have been recorded for the investigated meteorological parameters (temperature, humidity/dew point, cloud coverade and precipitation). Monthly temperature ranges between 10 -26 °C, whereas humidity/dew point is found to range between 5 – 21°C. Similarly, cloud coverage and precipitation are found to be higher during the winter period with maximum values ranging of 0.6 και 0.2 – 0.4 mm, respectively. Slight differentiations are found in precipitation mean values of the winter months of November and December for the years 2002, 2009, 2011 and 2021, during which had the highest values (0.3 – 0.4 mm), while no significant differences are found regarding cloud coverage. Regarding the variability of the wave parameters for the 2003-2023 period, significant wave height (Hs) is found to have maximum values (of about 1.5 m) during February, while the maximum value is found for February 2025 (at 2.1 m). In general, during winter months, wind velocity is found to range between 4-8 ms-1, while wave heights range between 0.8 – 1.5 m. Finally, the prevailing mean wave direction is found to be from the N and NW sectors (270° – 360°), which is the case for most months with the exception of March, April and May where the prevailing direction is from the W (225°).
Meteorological parameters - Future Projections
Santorini
Temperature projections for the reference years of 2050 and 2100 under the climate scenarios RCP 4.5 and RCP 8.5 from the Copernicus CORDEX e-database (3-hour step), are found to differ significantly compared to the records of 2022. More specifically, under the RCP 4.5 scenario, an increased temperature trend of about 2-4 °C is evident, whereas temperature increases of up to 12 °C is recorded for some days during January. Interestingly, January, March, and August 2050 are found to be warmer compared to the projections for 2100. Under RCP 8.5 , temperature is found to increase by 3-7°C compared to 2022. Regarding precipitation, projections under RCP 4.5 scenario show a significant increase (of about 0.5 kg m-2 s-1) for years 2050 and 2100, compared to 2022, during the winter and autumn periods. Similarly, under the RCP 8.5 scenario, projections show an increase for the months Febryary, June and December for 2050 and for the months February, April, October and November for 2100.
Kos
Under the RCP 4.5 scenario, temperature is projected to significantly increase between 2-3 °C, while there are cases of increase of up to 10 °C (during January). It hat to be noted that the first 10 days of February are found to be warmer by 2-3 °C in 2022 compared to the projections for 2050 and 2100. Under RCP 8.5. projections show an increase of about 3-5°C. 2050 is expected to be warmer by 2-3 °C during summer and autumn periods, as well as during December and January. For 2100, the increase is almost doubled, ranging between 4-6 °C, compared to 2022. Precipitation does not seem to be affected significantly. Under the RCP 4.5 scenario, increased trends are evident for the months June till September, which however is not the case when projections for RCP 8.5 are examined. An increase is also evident for months April and November 2100, while it has to be noted that a significant rain event is recorded during 2022 (3.0 kg/m2s).