Unusually high sea temperatures, persisting for several weeks, have been observed along Finland's Baltic Sea coastline in recent months, Veera Haapaniemi, a marine expert at Finland's Meteorological Institute (FMI), told AFP.

"We are witnessing an exceptionally warm period in the Baltic Sea", Haapaniemi said.

According to the weather agency, sea temperatures measured in the Gulf of Finland -- south of Finland -- had exceeded average temperatures for September by 4-5C.

"Today we measured 17C in the Gulf of Finland when the normal temperature for this time of the year is around 13C", Haapaniemi said.

The Baltic Sea is surrounded by Germany, Poland, Russia, Finland, Sweden and the Baltic states.

The sea's relatively shallow waters are known to be extremely sensitive to changes in the environment and climate and have already been warming at twice the pace of global oceans generally.

The whole of the Baltic Sea has seen marine heatwaves this summer, according to Haapaniemi.

In the Gulf of Finland, the current heatwave has continued for 50 days, while other Finnish coastal waters saw unusually high temperatures for more than three weeks.

"These kind of extreme weather events are expected to become more frequent and stronger as a result of climate change", Haapaniemi said, adding an approximately 20-day long heatwave also had occurred in the beginning of summer.

"The exceptional temperatures of course impact the marine ecosystems", she added.

Finland's MET institute's reference data is based on sea temperatures collected between 1991 and 2020.