High temperatures across Europe this month have unleashed a prolonged marine heatwave in the Mediterranean Sea that could ravage ecosystems and kill off several species in the coming weeks, scientists have warned.

The extreme heat in recent weeks has already sparked wildfires and led to thousands of heat-related deaths in Europe, but that warmth hasn't been limited to the land.

The warmer air along with shifting ocean currents and a stable sea surface have warmed coastal Mediterranean waters several degrees Celsius beyond the average temperature of 24°C to 26°C for this time of year.

Waters between Spain's Balearic Islands and the Italian coast were up to 5°C warmer than at the same time last year, Spain's AMET weather agency said on Friday – while also warning that temperatures around the Spanish coast would be 3°C to 4°C higher until at least mid-August.

Spain's ports' authority said in a statement the water in Cabo de Gata, in the country's southeastern corner, registered a ten-year temperature record of nearly 28°C on Monday.

Marine heatwaves, which are far less researched than heatwaves on land, are becoming more frequent due to climate change, adding pressure to ecosystems already struggling from over-fishing and plastic pollution.

Ocean scientist Jean-Pierre Gattuso told Reuters that water near the French coastal city of Nice was measured at 29.2°C on July 25 - around 3.5°C higher compared to the same day last year.