A significant temperature increase only occurred in the second half of July with the arrival of hot weather. Since then, the surface temperature of the Estonian coastal sea has been up to 2 degrees warmer than average, although there is some local variation. On August 21, a marine heatwave began to develop in the Gulf of Finland, and it continues to persist in a limited area in September. A more extensive marine heatwave took place in the northern part of the Bay of Bothnia, where it covered an area of nearly 40,000 km² and lasted from mid-July to the beginning of August.

This year's summer was different compared to last year, which is a common phenomenon in Estonia. As the saying goes, "no two years are the same," and this holds true even in the era of climate change. The spring and first half of the summer in 2025 were cooler than average, July was warmer, and August was cooler again. These changes in atmospheric temperature were, of course, also noticeable in the sea. The marine heatwave monitoring climate service from the Institute of Marine Systems at Tallinn University of Technology (TalTech), which was created as part of the AdaptEst project, provides us with real-time information about sea surface temperature and anomalous conditions.

Mariliis Kõuts, a researcher at the Institute of Marine Systems at TalTech, notes that while the May of the previous year shocked with a long-lasting heatwave that warmed the coastal sea and led to records in the history of Baltic Sea surface temperature measurements, May 2025 was as much as 1.9 degrees cooler than the long-term average. Consequently, the Estonian coastal sea warmed more slowly than usual. The true momentum for the sea surface temperature rise began with the hot weather in mid-July, and by early August, a marine heatwave covering nearly 40,000 km² had developed in the northern Bay of Bothnia. A smaller-scale and shorter-duration marine heatwave also emerged in the last third of August in the Gulf of Finland and, due to warm weather, is still ongoing in early September. A marine heatwave, or mereleitsak, is defined as an anomalous situation lasting at least five days, where the temperature of the sea's surface layer is higher than it has been on 90 percent of the days for that same time period in previous years.

The average surface temperature of the Estonian coastal sea rose above the long-term average in July, and the warmth persists in several locations even in early September, despite the changeable weather. The sea along Estonia's northern and southwestern coast, for example, in the Tallinn and Pärnu bays, is 2-3 degrees warmer than average. On the west coast of Saaremaa, however, the water temperature has dropped below the long-term average, which shows the importance of local processes in shaping water temperature. In the northern part of the Bay of Bothnia, the seawater is as much as four degrees warmer than the long-term average. For marine life, water that is warmer than usual means great discomfort, as it causes stress, reproductive difficulties, and in the worst-case scenario, death for marine organisms. The severity of the situation in turn depends on how intense the heatwave is, how long it lasts, and during which period it occurs. In the Estonian coastal sea, water temperature records are likely not to be broken this year, but the water may remain warm well into autumn if the weather allows.

TalTech Marine Heatwave Monitoring Service

To assess the potential impacts of heatwaves on the Baltic Sea ecosystem and the blue economy, a climate service was developed at the Institute of Marine Systems at TalTech, which allows for the monitoring of sea surface temperature and marine heatwaves. It is possible to observe the development of marine heatwaves in real-time in both time and space, and based on the collected data, long-term analyses can be conducted on the frequency and extent of the phenomenon. The web-based system relies on an autonomous data processing model that calculates sea surface temperature from satellite images and compares it to the long-term average to identify areas affected by heatwaves in the Baltic Sea. Service link: https://cmems.msi.ttu.ee/