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Marine space is three dimensional, the turnover of life forms is rapid, defining a fourth dimension: time. The definition of ecologically significant spatial units calls for the spatio-temporal framing of significant ecological connections in terms of extra-specific (biogeochemical cycles), intra-specific (life cycles), and inter-specific (food webs) fluxes. The oceanic volume can be split in sub-systems that can be further divided into smaller sub-units where ecosystem processes are highly integrated. The volumes where oceanographic and ecological processes take place are splittable into hot spots of ecosystem functioning, e.g., upwelling currents triggering plankton blooms, whose products are then distributed by horizontal currents, so defining Cells of Ecosystem Functioning (CEFs), whose identification requires the collaboration of physical and chemical oceanography, biogeochemistry, marine geology, plankton, nekton and benthos ecology and biology, food web dynamics, marine biogeography. CEFs are fuzzy objects that reflect the instability of marine systems.

BACKGROUND There is ample evidence for human alteration of Europe’s regional seas, particularly the enclosed or partly enclosed Baltic, Black, Mediterranean, and North Seas. Accounts of habitat and biodiversity loss, pollution, and the decline of fish stocks in these economically, socially, and ecologically important seas demonstrate unsustainable use of the marine environment. At the same time, there is an insufficient quantity and quality of information to enable purely evidence-based management of Europe’s seas despite this being a declared goal of many decisionmakers; for example, less than 10% of the deep sea has been systematically explored (UNEP 2006). Evidence-based management alone is rarely possible in situations with complex value-laden policy options (Greenhalgh and Russell 2009), and unfortunately, many of the most pervasive problems in the marine environment are “wicked” second-order problems (Jentoft and Chuenpagdee 2009): they are complex in nature and their management will often involve both winners and losers. Solutions to these problems involve less politically attractive, valuebased choices and may require long time lags before tangible results are observed. Fisheries management, habitat and species protection, competition for marine space, and invasive species are all examples of “wicked” problems. These are some of the biggest issues facing Europe’s seas and are the major focus of this article and Special Feature. For the first time in European history, most countries have adopted a common maritime policy (the 2007 Integrated Maritime Policy) and a legally binding environmental directive (the 2008 Marine Strategy Framework Directive [MSFD]). These comprehensive policy vehicles encompass, or closely interface with, more specific measures, such as the recently reformed Common Fisheries Policy, the Water Framework Directive, the Habitats and Birds Directive, and a number of targeted policy instruments that deal with aspects of pollution control and coastal zone management. The overall array of measures has the potential to ensure the sustainable use of Europe’s seas and the restoration of marine environments, but the pathway between the current situation and the implementation of an ecosystem approach to management (the aspiration of the European Commission; see Our Approach to Research) is fraught with “wicked” problems. Science can help society resolve these problems, but in many cases this requires the broad and integrative vision of Odum’s (1971) “macroscope” rather than trying to piece together an ill-fitting jigsaw puzzle of discipline-focused information. This paper and the others in this Special Feature employ a systems approach. We describe the approach, how it can be applied practically, and some of the challenges in making it work. Though the work is based on research on Europe’s seas, it has much wider implications for regional seas throughout the world. OUR APPROACH TO RESEARCH ON MARINE SOCIALECOLOGICAL SYSTEMS The research described in this paper (and Special Feature) was conducted in the framework of the EU-FP7 funded project Knowledge-based Sustainable Management of Europe’s Seas (KnowSeas). The interdisciplinary research spanned 4 years and involved 33 institutions from 16 European countries (KnowSeas 2013). Its primary objective was to develop “a comprehensive scientific knowledge base and practical guidance for the application of the ecosystem approach to the sustainable development of Europe’s regional seas.” Given the knowledge gaps and uncertainties in the way Europe’s marine social-ecological systems function (e.g., unresolved causal links, poorly mapped habitats, nonlinear dynamics), an iterative approach to inquiry was adopted, based partly on the reasoning behind soft systems analysis (e.g., Checkland 2000).

The current downturn of the arctic cryosphere, such as the strong loss of sea ice, melting of ice sheets and glaciers, and permafrost thaw, affects the marine and terrestrial carbon cycles in numerous interconnected ways. Nonetheless, processes in the ocean and on land have been too often considered in isolation while it has become increasingly clear that the two environments are strongly connected: Sea ice decline is one of the main causes of the rapid warming of the Arctic, and the flow of carbon from rivers into the Arctic Ocean affects marine processes and the air-sea exchange of CO2. This review, therefore, provides an overview of the current state of knowledge of the arctic terrestrial and marine carbon cycle, connections in between, and how this complex system is affected by climate change and a declining cryosphere. Ultimately, better knowledge of biogeochemical processes combined with improved model representations of ocean-land interactions are essential to accurately predict the development of arctic ecosystems and associated climate feedbacks.

Abstract. The land and ocean absorb on average just over half of the anthropogenic emissions of carbon dioxide (CO2) every year. These CO2 "sinks" are modulated by climate change and variability. Here we use a suite of nine dynamic global vegetation models (DGVMs) and four ocean biogeochemical general circulation models (OBGCMs) to estimate trends driven by global and regional climate and atmospheric CO2 in land and oceanic CO2 exchanges with the atmosphere over the period 1990–2009, to attribute these trends to underlying processes in the models, and to quantify the uncertainty and level of inter-model agreement. The models were forced with reconstructed climate fields and observed global atmospheric CO2; land use and land cover changes are not included for the DGVMs. Over the period 1990–2009, the DGVMs simulate a mean global land carbon sink of −2.4 ± 0.7 Pg C yr−1 with a small significant trend of −0.06 ± 0.03 Pg C yr−2 (increasing sink). Over the more limited period 1990–2004, the ocean models simulate a mean ocean sink of −2.2 ± 0.2 Pg C yr−1 with a trend in the net C uptake that is indistinguishable from zero (−0.01 ± 0.02 Pg C yr−2). The two ocean models that extended the simulations until 2009 suggest a slightly stronger, but still small, trend of −0.02 ± 0.01 Pg C yr−2. Trends from land and ocean models compare favourably to the land greenness trends from remote sensing, atmospheric inversion results, and the residual land sink required to close the global carbon budget. Trends in the land sink are driven by increasing net primary production (NPP), whose statistically significant trend of 0.22 ± 0.08 Pg C yr−2 exceeds a significant trend in heterotrophic respiration of 0.16 ± 0.05 Pg C yr−2 – primarily as a consequence of widespread CO2 fertilisation of plant production. Most of the land-based trend in simulated net carbon uptake originates from natural ecosystems in the tropics (−0.04 ± 0.01 Pg C yr−2), with almost no trend over the northern land region, where recent warming and reduced rainfall offsets the positive impact of elevated atmospheric CO2 and changes in growing season length on carbon storage. The small uptake trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counter\\-act the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the magnitude and sign of modelled carbon trends in several regions, as well as regarding the influence of land use and land cover changes on regional trends.

AbstractExtreme weather events can have strong negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme, short‐lived, winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (for instance, 2–10 °C for 2–14 days) but upon return to normal winter climate exposes the ecosystem to much colder temperatures due to the loss of insulating snow. Single events have been shown to reduce plant reproduction and increase shoot mortality, but impacts of multiple events are little understood as are the broader impacts on community structure, growth, carbon balance, and nutrient cycling. To address these issues, we simulated week‐long extreme winter warming events – using infrared heating lamps and soil warming cables – for 3 consecutive years in a sub‐Arctic heathland dominated by the dwarf shrubsEmpetrum hermaphroditum, Vaccinium vitis‐idaea(both evergreen) andVaccinium myrtillus(deciduous). During the growing seasons after the second and third winter event, spring bud burst was delayed by up to a week forE. hermaphroditumandV. myrtillus, and berry production reduced by 11–75% and 52–95% forE. hermaphroditumandV. myrtillus, respectively. Greater shoot mortality occurred inE. hermaphroditum(up to 52%),V. vitis‐idaea(51%), andV. myrtillus(80%). Root growth was reduced by more than 25% but soil nutrient availability remained unaffected. Gross primary productivity was reduced by more than 50% in the summer following the third simulation. Overall, the extent of damage was considerable, and critically plant responses were opposite in direction to the increased growth seen in long‐term summer warming simulations and the ‘greening’ seen for some arctic regions. Given the Arctic is warming more in winter than summer, and extreme events are predicted to become more frequent, this generates large uncertainty in our current understanding of arctic ecosystem responses to climate change.