Baltic Earth Working Group on Natural hazards and their impacts

• How do natural hazards in the Baltic Sea region arise from the interplay of large-scale circulation,
  regional processes, and human-induced climate change?
• How can long-term data, counterfactual approaches, and reanalysis products improve attribution of
  extreme events and compound hazards?
• How will changes in natural hazards affect society, ecosystems, and key sectors in the Baltic Sea region
  in a warming climate?

Natural hazards are rare but impactful events that pose risks to human society and the environment. Their occurrence is shaped by a complex interplay of stochastic variability, natural climate processes, and human induced drivers such as anthropogenic warming. Distinguishing the contributions of these different drivers is both scientifically challenging and essential for informed decision-making and raising public awareness.  A major limitation in this context is the lack of sufficiently long observational records, which often do not capture the tails of probability distributions where extreme events occur. Recent advances in atmospheric and oceanic reanalysis products, some extending back to the early 20th century, now make it possible to apply modern attribution techniques to Baltic Sea hazards. Counterfactual approaches – comparing real-world outcomes with hypothetical scenarios in which a specific factor such as anthropogenic climate change is absent – offer a promising way to quantify how the likelihood or intensity of extreme events have changed over time. RT3 is closely linked to RT1 (teleconnections and large-scale circulation), RT4 (sea-level extremes and inundation), RT5 (biogeochemistry and harmful algal blooms), and RT8 (future projections). Together, these topics address hydrometeorological hazards such as storms, heavy rainfall, droughts, heat waves, high river flows, and compound events. While RT1 focuses on large-scale circulation patterns and their changes (e.g., shifts in storm tracks), RT3 examines how regional characteristics shape extreme events and determine their environmental and societal impacts. Because natural hazards directly affect life, infrastructure, and the human environment, socioeconomic expertise is essential. Key knowledge gaps include limited understanding of how regional processes modulate large-scale drivers, how compound hazards interact, how natural variability and model biases affect projections of future extremes, and how these hazards translate into impacts on marine ecosystems and ecosystem services. The objective of RT3 is to advance understanding of the drivers, characteristics, and impacts of natural hazards in the Baltic Sea region by integrating long-term observations, reanalysis products, and modern attribution methods. RT3 seeks to clarify how regional processes interact with large-scale circulation and anthropogenic climate change to shape extreme and compound events. A further objective is to strengthen the capability to attribute past extremes and project future changes, including associated uncertainties. Ultimately, RT3 aims to provide a robust scientific basis for assessing societal and ecosystem impacts, thereby supporting climate resilient risk management and adaptation strategies in the Baltic Sea region.

Potential activities include continued investigation of marine heat waves in the Baltic Sea, focusing on their occurrence, duration, extent, intensity, and underlying drivers. This includes analysing past trends in observational and reanalysis datasets and assessing how the specific characteristics of the Baltic Sea – such as its proximity to the Arctic, limited water exchange, and strong stratification – shape marine heat wave dynamics. Further work will explore how marine heat waves are likely to evolve under future climate conditions, and how uncertainties arising from natural variability and model biases influence projections. Research will also investigate the ecological impacts of future marine heatwaves, informed by past events. Since temperature extremes affect dissolved oxygen levels (e.g., episodic hypoxia) as well as ecosystem structure and functioning, studies will consider interactions with other drivers such as eutrophication, sea-ice decline, and changing wind and wave climates. A similar framework – covering occurrence, duration, extent, intensity, drivers, and impacts – will be applied to other natural hazards, including storms, heavy precipitation, droughts, high river flows, and heatwaves on land, as well as compound events involving multiple interacting extremes.