Baltic Sea as a time machine for environmental change
With its brackish and shallow waters, the Baltic sea serves as a globally useful case study.

The Baltic Sea is shallow with a low salinity, and its rate of exchange with the Atlantic Ocean is low. Consequently, it would not at first sight seem very interesting to oceanographers investigating global change.

First impressions are deceiving. An extensive dataset compiled by researchers working at more than 20 international research institutions proves that warming, acidification, eutrophication, oxygen deprivation and other changes, which in many regions are only expected to take place in the future, are already now in full force in the Baltic Sea. The reasons behind this are the small scale of the Baltic Sea’s water body and its very slow turnover, which amplify changes and speed up several processes.

Indeed, researchers are encouraging oceanographers and decision-makers to employ the Baltic Sea region as a global model for future change. In a sense, the Baltic Sea can serve as a time machine for measuring what’s ahead.

“Developments in the Baltic Sea provide indications on what kinds of challenges can be expected in other marine regions and also show how damaging trends can be reversed through conservation,” says Professor Kari Hyytiäinen from the University of Helsinki.

For instance, temperatures in the Baltic Sea have risen 1.5 degrees in the last 30 years, whereas other marine regions around the world have seen only an average increase of 0.5 degrees during the same period. In the last century, anoxic seafloor area in the Baltic Sea has increased tenfold. In the region, the pH value indicating acidity often reaches heights not expected in other marine regions until the next century.

Combined with a forceful human impact, the contours of the Baltic Sea basin and its extensive catchment basin have sped up the deterioration of the sea’s state. Intensive agriculture strains the sea with nutrients, and commercial fishing is shaping the food webs of the open sea areas.

The Baltic Sea under the microscope

Not all of the news is bad: the Baltic Sea is among the most closely studied marine regions, with the first observations and measurements made already in the beginning of the 20th century. The countries surrounding the Baltic Sea have also long conducted strong research collaboration, the results of which have enabled nature conservation and decision-making based on scientific knowledge.

Examples of conservation successes include a significant decrease in nutrient stress since the 1980s, the return of several apex predators in the food chain and checks on overfishing. Positive developments have been made possible through binding agreements between EU states and the Baltic Sea Action Plan of the Baltic Marine Environment Protection Commission.

Achieving a good status for the Baltic Sea is still a long-term objective, requiring constant investments in conservation.

“New methods in marine governance, initiative shown by various parties and a growing awareness of our environmental footprint are significant steps on the way forward. The Baltic Sea Challenge launched by the cities of Helsinki and Turku is excellent evidence of the power of networking in water protection,” notes Kari Hyytiäinen.


Reusch, T. H. B., J. Dierking, H. Andersson, E. Bonsdorff, J. Carstensen, M. Casini, M. Czajkowski, B. Hasler, K. Hinsby, K. Hyytiäinen, K. Johannesson, S. Jomaa, V. Jormalainen, H. Kuosa, S. Kurland, L. Laikre, B. R. MacKenzie, P. Margonski, F. Melzner, D. Oesterwind, H. Ojaveer, J. C. Refsgaard, A. Sandström, G. Schwarz, K. Tonderski, M. Winder, M. Zandersen (2018): The Baltic Sea as a time machine for the future coastal ocean. Sci. Adv. 2018;4:

The published scholarly article was the collaborative result of various projects under BONUS, a research programme focused on the Baltic Sea funded by the EU and national funders of scientific research. In Finland, the research was funded by the Academy of Finland.