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A carbon budget for the northwest European continental shelf seas (NWES) was synthesized using available estimates for coastal, pelagic and benthic carbon stocks and flows. Key uncertainties were identified and the effect of future impacts on the carbon budget were assessed. The water of the shelf seas contains between 210 and 230 Tmol of carbon and absorbs between 1.3 and 3.3 Tmol from the atmosphere annually. Offshelf transport and burial in the sediments account for 60-100 and 0-40% of carbon outputs from the NWES, respectively. Both of these fluxes remain poorly constrained by observations and resolving their magnitudes and relative importance is a key research priority. Pelagic and benthic carbon stocks are dominated by inorganic carbon. Shelf sediments contain the largest stock of carbon, with between 520 and 1600 Tmol stored in the top 0.1 m of the sea bed. Coastal habitats such as salt marshes and mud flats contain large amounts of carbon per unit area but their total carbon stocks are small compared to pelagic and benthic stocks due to their smaller spatial extent. The large pelagic stock of carbon will continue to increase due to the rising concentration of atmospheric CO 2 , with associated pH decrease. Pelagic carbon stocks and flows are also likely to be significantly affected by increasing acidity and temperature, and circulation changes but the net impact is uncertain. Benthic carbon stocks will be affected by increasing temperature and acidity, and decreasing oxygen concentrations, although the net impact of these interrelated changes on carbon stocks is uncertain and a major knowledge gap. The impact of bottom trawling on benthic carbon stocks Frontiers in Marine Science | www.frontiersin.org 1 March 2020 | Volume 7 | Article 143Legge et al.Carbon on the Northwest European Shelf is unique amongst the impacts we consider in that it is widespread and also directly manageable, although its net effect on the carbon budget is uncertain. Coastal habitats are vulnerable to sea level rise and are strongly impacted by management decisions. Local, national and regional actions have the potential to protect or enhance carbon storage, but ultimately global governance, via controls on emissions, has the greatest potential to influence the long-term fate of carbon stocks in the northwestern European continental shelf.

European continental shelf seas have experienced intense warming over the last 30 33 years 1 . In the North Sea, fishes have been comprehensively monitored throughout 34 this period and resulting data provide a unique record of changes in distribution and 35 abundance in response to climate change 2,3 . We use these data to demonstrate the 36 remarkable power of Generalised Additive Models (GAMs), trained on data earlier in 37 the time-series, to reliably predict trends in distribution and abundance in later years. 38Then, challenging process-based models that predict substantial and ongoing 39 poleward shifts of cold-water species 4,5 , we find that GAMs coupled with climate 40 projections predict future distributions of demersal (bottom-dwelling) fish species 41 over the next 50 years will be strongly constrained by availability of habitat of suitable 42depth. This will lead to pronounced changes in community structure, species 43 interactions and commercial fisheries, unless individual acclimation or population-44 level evolutionary adaptations enable fish to tolerate warmer conditions or move to 45 previously uninhabitable locations. 46 47While the temperature of the world's oceans has gradually risen through the 20 th Century, 48 the northeast Atlantic has experienced particularly intense warming, resulting in the North 49 Sea mean annual sea-surface temperature increasing by 1.3°C over the last 30 years 1 , a 50 rate four times faster than the global average 6 . Predictions for the North Sea suggest a 51 further 1.8°C rise in sea-surface temperatures during the next five decades (Hadley Centre 52 QUMP_ens_00 model, unpublished data supplied by J. Tinker) (Fig. 1). Impacts of recent 53 warming on northeast Atlantic marine ecosystems have been diverse, including 54 reorganisation of the plankton community 7 , modification to the phenology of fish spawning 8,9 , 55 and alterations of ecosystem interactions 10,11 . Due to its longstanding economic importance 56 to fisheries (reported landings in 2007 valued at $1.2 billion 1 ) and other industries, the 57 ecology of the North Sea has been intensively monitored throughout this period of recent 58 warming. 59 60 3 Analyses of North Sea fish surveys have revealed northerly range expansions of warmer-61 water species 12 , population redistributions to higher latitudes 2 and deeper water 13 , and 62 widespread changes in local abundance associated with warming, with impacts on 63 community structure 3 . This substantial modification to fish community composition in the 64 region has had an observable economic impact on fisheries, with landings of cold-adapted 65 species halved but landings of warm-adapted species increasing 2.5 times since the 1980s 3 ; 66 a pattern also identified in other marine ecosystems 14 . With a uniquely rich fish abundance 67 time-series from the period of warming, it is possible to split these data to assess how 68 predictions made using data from earlier years match observations from later years; a 69 validation approach which has been...

a b s t r a c tThere are a number of sources of uncertainty that impact climate projections for regional seas. We have assessed the impact that uncertain large-scale climate forcings have on the projections for the north-west European shelf seas. An ensemble of global Atmosphere-Ocean climate model (GCM) projections made by perturbed (atmospheric) parameter model variants which were designed to span uncertainty in climate sensitivity, was dynamically downscaled with the shelf seas model POLCOMS. The simulations were run as transient experiments (from 1952 to 2098) under a medium emissions scenario (SRES A1B). This study has focused on centennial changes over the period 2069-2098 relative to 1960-1989, but also refers to the full transient simulation to assess the significance of projected changes given interannual and lowfrequency variability. The ensemble mean of the POLCOMS projections showed a shelf and annual mean Sea Surface Temperature (SST) rise of 2.90°C (±2r = 0.82°C), and a Sea Surface Salinity (SSS) freshening of À0.41 psu (±2r = 0.47 psu) between these periods. We described the spread in a field for a particular period using the variances associated with both the time mean ensemble dispersion (ensemble variance) and with the interannual variability. For SST in the present-day period, the magnitudes of both ensemble and interannual variance were fairly spatially homogenous. While the future interannual variance is of similar magnitude to that of the present day, the ensemble variance increased considerably into the future period. For SSS, both sources of variance were more spatially heterogeneous, and both increased into the future period. We investigated relationships between the projected shelf seas changes across the ensemble and changes in the large-scale climate forcing. We found that the near surface-air temperature from the driving GCM (averaged over the domain) and the GCM surface salinity to the west of the POLCOMS domain are good proxies for the changes within the shelf seas. We then compared these GCM indicators of shelf changes in our ensemble (under A1B) to the same measures across a number of CMIP5 models, under the RCP6.0 and RCP8.5 scenarios. The spread of these indicators, for our ensemble, fall within the range of the CMIP5 models (particularly under RCP8.5), suggesting our shelf projections would be consistent with an ensemble of projections driven by CMIP5 models.Crown