Portugal’s Maritime Authority has released data showing that the seas off the country’s Algarve coast underwent a “marine heatwave” at the end of June and beginning of July 2025.
The measurements were taken from a coastal buoy placed by the Hydrographic Institute’s national monitoring network (MONIZEE) near Faro, the country’s southernmost district. They averaged 25.1°C, which is “significantly higher” than the last two decades’ average, the Portuguese Navy’s website says.
Average temperatures hit “Extreme” levels
The naval statement goes on to explain that the conditions for a “marine heatwave” are met when daily sea temperatures exceed the 90th percentile of averages for that region and season, for at least five consecutive days. In this instance, the data spanned 28 June to 9 July.
What’s more, the daily sea temperatures recorded “exceeded the average temperature by more than 5°C” and so fall into “the category of marine heatwave considered extreme”, the Navy website went on.
Since that high point, sea temperatures in the region have dropped back to normal, and now sit between 18 and 20°C, but periods of high sea temperatures have been found by scientists to have a catastrophic effect on ecosystems and habitats such as the coral reefs, sea grass meadows, and kelp forests that provide food and shelter for a wide array of marine life.
Here is a visualization of the remarkable marine heatwave in the Mediterranean.
— Andrew Austin-Adler (@WeathermanAAA_) July 7, 2025
These temperatures are incredibly anomalous and are among the highest ever observed for this time of year. pic.twitter.com/1EkrfbABhi
Risk to human life
The same study, published in the Proceedings of the National Academy of Sciences in April 2025, points out that as well as affecting aquatic creatures and organisms, high sea temperatures are behind supercharged storms and deadly weather systems that cause flooding and other risks to humanity – who are driving the problem by burning fossil fuels. Marine heatwaves are becoming not only more frequent but hotter, and half of them in the last quarter century would not have occurred without the effect of global warming, the researchers said.
As well as affecting agriculture and the fishing industry, waters that are warmer than usual are said to have contributed to recent disasters such as the Persian Gulf Floods of 2024, which cancelled over 1,200 flights at Dubai Airport, the sinking of the Bayesian yacht in the same year off the coast of Sicily, with the loss of seven lives, and flash floods in Jordan in 2025, that resulted in mass evacuations of tourists from Petra and claimed the lives of a Belgian mother and her son.
