This special issue is intended to be a comprehensive and up-to-date overview of the scientific topics which Baltic Earth had defined in 2013 as important fields for research. Baltic Earth is an international and interdisciplinary network of scientists and institutions around the Baltic Sea, striving for an improved understanding of the Earth system of the Baltic Sea region (baltic.earth). The research components of Baltic Earth encompass processes in the atmosphere, on land and in the sea, as well as processes and impacts related to the anthroposphere.

This special issue summarizes the current state of knowledge in regional climatology, oceanography, meteorology, hydrology and biogeochemistry, as defined by the Baltic Earth Science Plan and aims to identify gaps and inconsistencies in that knowledge. Hence, the special issue wraps together the currently available knowledge on the Grand Challenges and related topics, as defined in the Baltic Earth Science Plan 2017 (see baltic.earth).

The Baltic Earth Grand Challenges are currently the following:

• salinity dynamics in the Baltic Sea
• land–sea biogeochemical interlinkages in the Baltic Sea region
• natural hazards and extreme events in the Baltic Sea region
• sea level and coastal dynamics in the Baltic Sea
• regional variability of water and energy exchanges in the Baltic Sea region
• multiple drivers of regional Earth system changes in the Baltic Sea region.

Contributions should be in line or strongly related to the themes of the Baltic Earth Science Plan (2017) and must adhere to the following principles: BEAR wrap together the currently available published scientific knowledge. They do not document unpublished research results. The essence of the work is the synthesis of material drawn comprehensively from the available scientifically legitimate literature (e.g. peer-reviewed literature, conference proceedings and reports of scientific institutes). The work should encompass the knowledge about what scientists agree on but also identify cases of disagreement or knowledge gaps.