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This paper reviews the state of knowledge on past and present storms and marine coastal flooding (MCF) events of various origins within the Baltic Sea, which is an economically and environmentally important part of northwestern Europe. We show that the combination of sedimentary, historical and instrumental records provides the most comprehensive insight into the history of storms and MCF. The frequency and intensity of these events vary considerably throughout the region and over the time (past 7000 years). The southwestern and southern Baltic Sea coasts are identified as the area most vulnerable to hazard posed by storms and MCF, both in the past and in the future. The best records of storms come from urbanized areas where long tide-gauge and historical records are available, while storminess history is best reconstructed from inland sedimentary and peat archives. Archives of MCF have been preserved only in a few locations and represent local, but temporaly comprehensive record of the most severe events. However, it remains challenging to relate records of storms, storminess, and storm-induced MCF to each other.

Extreme sea surface waves pose significant risks to coastal infrastructure, ecosystems, and human activities, not only in oceans but also in semi-enclosed basins like the Baltic Sea, where limited fetch and complex bathymetry can amplify local impacts. This study presents a novel machine learning framework to project significant wave height at local scales across the Baltic Sea from 1850 to 2100. Using Random Forest models trained on observed wave data and ERA5 atmospheric reanalysis, we simulate 3-hourly maximum Hs time series driven by four CMIP6 climate models under the SSP2–4.5 climate scenario. While SSP2-4.5 is the primary focus due to its alignment with current policy trajectories, the EC-Earth model was selected for detailed projection analysis as it demonstrated the highest skill in reproducing historical wave conditions and synoptic-scale atmospheric patterns. To explore scenario uncertainty, additional simulations using SSP1-2.6 and SSP3-7.0 were also included. Results indicate a basin-wide decline in the 95th percentile of Hs from the early 20th century until the middle of the 21st century, followed by a stagnation shift, with spatial variability across sub-basins. This change appears largely independent of the carbon emission scenario for the future, suggesting a dominant role of internal atmospheric variability and large-scale dynamics patterns, though the limited ensemble size and absence of formal trend significance testing warrant cautious interpretation. An independent analysis of Lamb Weather Types, based on ERA5-reconstructed wave time series, reveals that extreme wave events are most frequently associated with cyclonic and westerly synoptic patterns, with some site-specific differences. In contrast, calm meteorological conditions are negatively correlated with wave extremes. An inter-model comparison reveals substantial variability in projected extremes, underscoring the importance of ensemble approaches. The methodology is transferable to other regions, though its performance may vary with local atmospheric and oceanographic conditions. These findings provide valuable insights for coastal risk assessments and adaptation planning in a changing climate.

This study presents a high-resolution shipping emission inventory for the Baltic Sea, assessing the environmental impacts of four fuel-based scenarios under a projected threefold increase in gross tonnage by 2050. The study evaluates how regulatory changes and alternative fuels, such as hydrogen and ammonia, can reduce emissions and advance sustainability in shipping. The study uses a bottom-up approach, combining activity data, fuel data, and emission factors to estimate tank-to-wake emissions. Comparative analysis indicates improved emissions prediction across all pollutants. While use of liquefied natural gas (LNG) and scrubber-equipped ships reduce sulphur oxides (SOx) emissions, they incur notable environmental trade-offs. By 2050, significant reductions in particulate matter (99%) and carbon dioxide are projected, while SOx emissions are expected to approach zero using hydrogen, ammonia, and methanol fuels. These reductions are helped by the decline in traditional fuels and technological progress. The current transition to cleaner marine fuels is insufficient to meet the IMO's 2030 and 2050 carbon reduction targets. While tank-to-wake contributes significantly toward emissions reduction, a broader focus on the well-to-wake approach is also critical for achieving net-zero emissions by 2050. Policy efforts should accelerate the adoption of green fuels and address challenges such as methane slip from LNG-powered ships.

Freshwater systems are important sources of atmospheric methane (CH₄). However, estimated emissions are associated with high uncertainties due to limited knowledge about the temporal variability in emissions and their associated controls, such as air–water gas transfer velocity. Here, we determined the gas transfer velocity of CH₄ based on a novel measurement setup that combines simultaneous eddy covariance flux measurements with continuously monitored CH₄ water- and air-side concentrations. Measurements were conducted during a 10-d campaign in a freshwater lake in mid-Sweden. The gas transfer velocity fell within the range of existing wind-speed-based parameterizations derived for carbon dioxide in other lakes. For wind speeds below 4 m s⁻¹, the gas transfer velocity for CH₄ followed parameterizations predicting faster gas exchange, while for wind speeds above 5 m s⁻¹, it aligned with those predicting relatively lower gas exchange. This pattern can be explained by ebullition. Extending the wind speed range for such combined eddy covariance measurements with continuously monitored CH₄ water- and air-side concentrations would improve model reliability.

Extreme sea levels are a major global concern due to their potential to cause fatalities and significant economic losses in coastal areas. Consequently, accurate projections of these extremes for the coming century are crucial for effective coastal planning. While it is well established that relative sea level rise driven by ongoing climate change is a key factor influencing future extreme sea levels, changes in storm surges resulting from shifts in storm climatology may also play a critical role. In this study, we project future daily maximum storm tides (the combination of storm surge and tides) using a random forest machine learning approach for 59 stations around the Baltic Sea, based on atmospheric variables such as surface pressure, wind speed, and wind direction derived from climate datasets. The results suggest both positive and negative changes, with sub-regional variations, in 50-year storm tide return levels across the Baltic Sea when comparing the period of 2070–2099 to 1850–1879. Localized increases of up to 10 cm are projected along the west coast of Sweden and the northern Baltic Sea, while decreases of up to 6 cm are anticipated along the south coast of Sweden, the Gulf of Riga, and the mouth of the Gulf of Finland. Negligible levels of change are expected in other parts of the Baltic Sea. The variability in atmospheric drivers across the four climate models contributes to a high degree of uncertainty in future climate projections.

The explosive development of extratropical cyclones and atmospheric rivers plays a crucial role in driving extreme weather in the mid-latitudes, such as compound windstorm–flood events. Although both explosive cyclones and atmospheric rivers are well understood and their relationship has been studied previously, there is still a gap in our understanding of how a warmer climate may affect their concurrence. Here, we focus on evaluating the current climatology and assessing changes in the future concurrence between atmospheric rivers and explosive cyclones in the North Atlantic. To accomplish this, we independently detect and track atmospheric rivers and extratropical cyclones and study their concurrence in both ERA5 reanalysis and CMIP6 historical and future climate simulations. In agreement with the literature, atmospheric rivers are more often detected in the vicinity of explosive cyclones than non-explosive cyclones in all datasets, and the atmospheric river intensity increases in all the future scenarios analysed. Furthermore, we find that explosive cyclones associated with atmospheric rivers tend to be longer lasting and deeper than those without. Notably, we identify a significant and systematic future increase in the cyclones and atmospheric river concurrences. Finally, under the high-emission scenario, the explosive cyclone and atmospheric river concurrences show an increase and model agreement over western Europe. As such, our work provides a novel statistical relation between explosive cyclones and atmospheric rivers in CMIP6 climate projections and a characterization of their joint changes in intensity and location.

Sea surface temperature anomaly (SSTA) associated with mesoscale oceanic processes, which are prevalent throughout the ocean, can significantly influence the atmospheric boundary layer and consequently atmospheric systems. While its influences on tropical and extratropical cyclones have been well-documented, the influence of mesoscale SSTA on polar lows (PLs) remains unexplored. To bridge this knowledge gap, we conducted a series of sensitivity numerical experiments with different SST configurations. The simulation results indicate that, over the lifespan of a PL, SSTA does not significantly influence PL intensity but does enhance latent heat release. On a longer time scale, based on simulations of five winter seasons over the Nordic Sea, we found that the accumulated impact of mesoscale SSTA creates favorable environments for PL intensification, characterized by higher moisture levels and lower static stability. These results highlight the importance of considering high-resolution SST boundary conditions, i.e. resolving mesoscale SST, in climate simulations of PLs.

Moisture transport within atmospheric rivers is driven by a complex combination of processes, including the convergence of moisture from different sources, which change over the atmospheric river's life cycle. The water vapor budget (WVB) within an atmospheric river enables us to understand moisture sources and sinks (horizontal flux, evaporation, and precipitation). Here, we applied our new WVB approach throughout the life cycle of the exceptional atmospheric river associated with Storm Dennis, which led to record-breaking precipitation on 15 February 2020. We used the WRF model to simulate the event and performed two sets of sensitivity experiments: one reducing tropical moisture and the other modifying ocean evaporation to assess how these two main moisture sources affect the water vapor balance within the atmospheric river. We analyzed changes in the atmospheric river, cyclone, and associated precipitation at landfall in the sensitivity experiments. In the Dennis case study, tropical moisture played a prominent role in the early stages of the atmospheric river, whereas ocean evaporation became critical later. Additionally, the reduction of evaporation and also of tropical moisture is related to a decrease in precipitation over Europe. This study offers a new approach to understanding the evolution of atmospheric rivers and highlights the importance of different moisture processes. It provides a case study that helps unravel feedback mechanisms and the impact of different perturbations on the water vapor balance of atmospheric rivers.

Ocean surface gravity waves facilitate gas exchanges primarily in two ways: (1) the formation of bubbles during wave breaking increases the surface area available for gas exchange, promoting CO$$_2$$transfer, and (2) wave-current interaction processes alter the sea surface partial pressure of CO$$_2$$and gas solubility, consequently affecting the CO$$_2$$flux. This study tests these influences using a global ocean-ice-biogeochemistry model under preindustrial conditions. The simulation results indicate that both wave–current interaction processes and the sea-state-dependent gas transfer scheme–which explicitly accounts for bubble-mediated gas transfer velocity–influence the air–sea CO$$_2$$flux, with substantial spatial and seasonal variations. In the equatorial region (10$$^{\circ }$$S–10$$^{\circ }$$N), both processes enhance the CO$$_2$$outgassing flux, with comparable magnitudes (more than 10% on average). However, in the region between approximately 10$$^{\circ }$$and 35$$^{\circ }$$, the impact of ocean surface waves on the air-sea CO$$_2$$flux via the sea-state-dependent gas transfer velocity is greater than that of the wave-current interaction processes, with opposing directions of influence. During winter, the sea-state-dependent gas transfer velocity enhances the CO$$_2$$uptake flux, while in the summer season, it increases the CO$$_2$$outgassing flux. In regions poleward of 35$$^{\circ }$$, the impact of wave–current interaction processes on CO$$_2$$exchange dominates over that of the sea-state-dependent gas transfer velocity. It is worth noting that the impact of wave-current interaction processes on air-sea CO$$_2$$flux is primarily driven by changes in the ratio between the concentrations of dissolved inorganic carbon and total alkalinity, with variations in sea surface temperature exerting an opposite influence on pCO$$_2$$, albeit with a smaller magnitude. Overall, wave-related processes should be considered in Earth System Models to better model the carbon cycle.