This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. ARTICLE HISTORY

Questions: (1) Do 17 seres studied proceed towards corresponding potential natural vegetation;(2) what are the similarities between seral and potential natural vegetation, and is it possible to estimate how long it takes to reach potential natural vegetation; and (3) do primary and secondary seres differ?Location: Extracted peatlands, corridors of the former iron curtain, artificial fishpond islands and barriers, sedimentary basins, various spoil heaps after mining, various stone quarries, forest clearings, burned-down forests, road verges, sand and gravel-sand pits, river gravel bars and abandoned arable fields located in various parts of the Czech Republic.Methods: Seral stages were sampled by phytosociological relev es (2602). The following categories of successional age were considered: early (1-10 yrs), intermediate (11-25 yrs) and late (>25 yrs). Phytosociological relev es (386) representing corresponding potential natural vegetation were extracted from the National Phytosociological Database. DCA and CCA ordinations were performed to compare the pattern of seral stages with potential natural vegetation and between primary and secondary seres. Dissimilarity between seral stages of primary and secondary successions and the corresponding potential natural vegetation was further assessed using the Bray-Curtis dissimilarity measure. Extrapolation was performed to estimate when the seres will reach the stage corresponding to potential natural vegetation. Results:The ordination showed that successions proceeded towards the corresponding potential natural vegetation and reflected substrate pH, site moisture and successional age. The estimated average time needed to reach potential natural vegetation was about 180 yrs for primary successions and about 260 yrs for secondary successions, considering presence-absence species data, and 200 and 250 yrs, respectively, considering cover data. All species recorded in potential natural vegetation (421) were also recorded in seral vegetation. Conclusions:In the general view across the high number of seres spread over the whole country, successions advanced in the direction of the corresponding potential natural vegetation. The extrapolated recovery of potential natural vegetation is faster in primary seres than in secondary ones, and seres sooner resemble the corresponding potential natural vegetation in species composition than in vegetation structure.

The Baltic Sea is suffering from eutrophication caused by nutrient discharges from land to sea, and these loads might change in a changing climate. We show that the impact from climate change by mid-century is probably less than the direct impact of changing socioeconomic factors such as land use, agricultural practices, atmospheric deposition, and wastewater emissions. We compare results from dynamic modelling of nutrient loads to the Baltic Sea under projections of climate change and scenarios for shared socioeconomic pathways. Average nutrient loads are projected to increase by 8% and 14% for nitrogen and phosphorus, respectively, in response to climate change scenarios. In contrast, changes in the socioeconomic drivers can lead to a decrease of 13% and 6% or an increase of 11% and 9% in nitrogen and phosphorus loads, respectively, depending on the pathway. This indicates that policy decisions still play a major role in climate adaptation and in managing eutrophication in the Baltic Sea region.Electronic supplementary materialThe online version of this article (10.1007/s13280-019-01243-5) contains supplementary material, which is available to authorized users.

Abstract. Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge of the effects of global warming on past and future changes in climate of the Baltic Sea region is summarised and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focuses on the atmosphere, land, cryosphere, ocean, sediments, and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in palaeo-, historical, and future regional climate research, we find that the main conclusions from earlier assessments still remain valid. However, new long-term, homogenous observational records, for example, for Scandinavian glacier inventories, sea-level-driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution, and new scenario simulations with improved models, for example, for glaciers, lake ice, and marine food web, have become available. In many cases, uncertainties can now be better estimated than before because more models were included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth system have been studied, and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication, and climate change. New datasets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal timescales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first palaeoclimate simulations regionalised for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA), and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics are dominated by tides, the Baltic Sea is characterised by brackish water, a perennial vertical stratification in the southern subbasins, and a seasonal sea ice cover in the northern subbasins.

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