Abstract. Arctic-Boreal terrestrial ecosystems are rapidly changing under amplified high-latitude warming, including widespread expansion of shrubs, with consequences for regional carbon and energy balances. Yet, high-latitude vegetation diversity and vegetation-climate interactions remain under-represented in many global land surface models. In ORCHIDEE, the land surface component of the IPSL Earth system model, high-latitude vegetation is represented primarily as boreal trees or grasslands, omitting explicit shrubs. Here, we implement three high-latitude shrub plant functional types (PFTs) (tall deciduous, low deciduous, and evergreen dwarf shrubs) in ORCHIDEE (revision 9269). Following literature recommendations, this classification combines phenology and stature to capture key functional contrasts while keeping the number of new PFTs limited. The implementation builds on ORCHIDEE's existing woody vegetation scheme by recalibrating a targeted set of parameters controlling allometry, carbon allocation, recruitment, mortality and phenology. Parameter values are constrained using synthesised pan-Arctic observations to obtain regionally representative shrub traits. Shrub spatial distributions are prescribed with updated PFT maps that combine ESA CCI products with Arctic and regional shrub mapping information. The resulting shrub PFTs reproduce observed ranges of shrub size and biomass allocation across the Arctic–Boreal domain. Introducing shrubs reduces simulated total aboveground biomass in the Arctic-Boreal region from 54 to 46.7 P g C (-13.5 %) and mean annual gross primary productivity from 498 to 481 g C m−2 yr−1 (-3.4 %) over the simulated period 1992-2020, with a stronger reduction in the tundra region (4.6 to 3 P g C (-34.8 %); and 334 to 289 g C m−2 yr−1 (-13.5 %)), increasing agreement with benchmarking datasets. A key strength of our implementation is its simplicity, as it builds on ORCHIDEE's existing woody vegetation framework. In addition, the use of synthesised pan-Arctic observations provides regionally representative observational constraints, making the methodological choices transferable beyond ORCHIDEE. Overall, this work provides a data-constrained shrub representation in ORCHIDEE with minimal added process complexity and establishes a foundation for future development of shrub-climate interactions and dynamic shrubification processes.

Predicting the responses of terrestrial ecosystem carbon to future global change strongly relies on our ability to model accurately the underlying processes at a global scale. However, terrestrial biosphere models representing the carbon and nitrogen cycles and their interactions remain subject to large uncertainties, partly because of unknown or poorly constrained parameters. Parameter estimation is a powerful tool that can be used to optimise these parameters by confronting the model with observations. In this paper, we identify sensitive model parameters from a recent version of the ORgainzing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model that includes the nitrogen cycle. These sensitive parameters include ones involved in parameterisations controlling the impact of the nitrogen cycle on the carbon cycle and, in particular, the limitation of photosynthesis due to leaf nitrogen availability. We optimise these ORCHIDEE parameters against carbon flux data collected on sites from the FLUXNET network. However, optimising against present-day observations does not automatically give us confidence in future projections of the model, given that environmental conditions are likely to shift compared to the present day. Manipulation experiments give us a unique look into how the ecosystem may respond to future environmental changes. One such type of manipulation experiment, the Free Air CO2 Enrichment (FACE) experiment, provides a unique opportunity to assess vegetation response to increasing CO2 by providing data under ambient and elevated CO2 conditions. Therefore, to better capture the ecosystem response to increased CO2, we add the data from two FACE sites to our optimisations, in addition to the FLUXNET data. We use data from both CO2 conditions of FACE, which allows us to gain extra confidence in the model simulations using this set of parameters. We find that we are able to improve the magnitude of modelled productivity. Although we are unable to correct the interannual variability fully, we start to simulate possible progressive nitrogen limitation at one of the sites. Using an idealised simulation experiment based on increasing atmospheric CO2 by 1 % yr(-1) over 100 years, we find that optimising against only FLUXNET data tends to imply a large fertilisation effect, whereas optimising against FLUXNET and FACE data (with information about nutrient limitation and acclimation of plants) decreases it significantly.

New (a)biotic conditions resulting from climate change are expected to change disturbance dynamics, such as windthrow, forest fires, droughts, and insect outbreaks, and their interactions. These unprecedented natural disturbance dynamics might alter the capability of forest ecosystems to buffer atmospheric CO2 increases, potentially leading forests to transform from sinks into sources of CO2. This study aims to enhance the ORCHIDEE land surface model to study the impacts of climate change on the dynamics of the bark beetle, Ips typographus, and subsequent effects on forest functioning. The Ips typographus outbreak model is inspired by previous work from Temperli et al. (2013) for the LandClim landscape model. The new implementation of this model in ORCHIDEE r8627 accounts for key differences between ORCHIDEE and LandClim: (1) the coarser spatial resolution of ORCHIDEE; (2) the higher temporal resolution of ORCHIDEE; and (3) the pre-existing process representation of windthrow, drought, and forest structure in ORCHIDEE. Simulation experiments demonstrated the capability of ORCHIDEE to simulate a variety of post-disturbance forest dynamics observed in empirical studies. Through an array of simulation experiments across various climatic conditions and windthrow intensities, the model was tested for its sensitivity to climate, initial disturbance, and selected parameter values. The results of these tests indicated that with a single set of parameters, ORCHIDEE outputs spanned the range of observed dynamics. Additional tests highlighted the substantial impact of incorporating Ips typographus outbreaks on carbon dynamics. Notably, the study revealed that modeling abrupt mortality events as opposed to a continuous mortality framework provides new insights into the short-term carbon sequestration potential of forests under disturbance regimes by showing that the continuous mortality framework tends to overestimate the carbon sink capacity of forests in the 20- to 50-year range in ecosystems under high disturbance pressure compared to scenarios with abrupt mortality events. This model enhancement underscores the critical need to include disturbance dynamics in land surface models to refine predictions of forest carbon dynamics in a changing climate.

Eight locations were selected which represent the range of climatic conditions within the distribution area of spruce in Europe (Picea Abies Karst L.) as shown in Table 4. Half-hourly weather data from the FLUXNET database (Pastorello et al., 2020) for these locations were used to drive ORCHIDEE. Some of these locations (FON, SOR, HES, COL, WET) are not populated with spruce but all are located within the species distribution. For each location, a pure spruce stand was simulated and the available FLUXNET data was looped to simulate a 100-year period. The study did not investigate the effect of species mixture in the simulation experiments. Other inputs, including soil texture, pH and soil color were obtained from the USDA map derived from Eswaran et al. (2003), for the corresponding pixel. The amount of fresh breeding woody substrate inputs used by the bark beetles to breed was controlled by modifying the maximum wind speed of a windthrow event in ORCHIDEE. Seven wind speeds ranging between 19 m/s and 40 m/s were selected (Table 3). This range is justified by the observation that mean wind speeds below 19 m/s could not trigger a windthrow event in ORCHIDEE (Chen et al., 2018) while for wind speeds exceeding 40 m/s, more than 60% of the trees are uprooted, leaving too few living trees to trigger a bark beetle outbreak within the same pixel. To investigate the impact of windthrow intensity and background climate on bark beetle outbreaks, the study conducted a total of 56 [8 sites x 7 wind speed intensities] simulations as given in table 3. The same 56 simulations were also used to analyze the sensitivity of the carbon balance of spruce forests to windthrow intensity and background climate. Where most land surface models use a turnover time to simulate continuous mortality (Thurner et al., 2014; Pugh et al., 2019), ecological reality is better described by abrupt mortality events. An idealized simulation experiment was used to qualify the impact of abrupt mortality on net biome productivity by changing from a framework in which mortality is approximated by a constant background mortality to a framework in which mortality occurs in abrupt, discrete events. To test the impact of a change in mortality framework two versions of ORCHIDEE were compared to create an idealized simulation experiment: (1) a version simulating mortality as a continuous process, labeled ”the continuous version”, and (2) the version capable of simulating abrupt mortality from windthrow and subsequent bark beetle outbreaks, labeled ”the abrupt version”. The effect of simulating abrupt mortality was evaluated over 20-, 50-, and 100-year time horizons. The effect of changing the framework of simulating mortality from continuous to abrupt was qualified on the basis of 112 simulations (8 sites x 7 wind speeds x 2 model versions) of 100 years each. The simulations with abrupt mortality were run first. Subsequently, the number of trees killed was quantified and used as a reference value for the continuous mortality set-up. This approach resulted in the same quantities of dead trees at the end of the simulation for both frameworks, which then differed only in the timing of the simulated mortality. This precaution is necessary to avoid comparing two different mortality regimes where the result would mainly be explained by the intensity of the mortality rather than by its underlying mechanisms.

The impact of the growing cruise ship industry on air quality levels was investigated at the port of Copenhagen, Denmark. In 2018, 345 cruise ships visited Copenhagen, emitting 291 tons of NO x near the city centre. A spatiotemporal cruise ship emission inventory was developed for 2018 based on port list information, engine data, main and auxiliary engine power functions, and NO x emission factors, and was implemented in the OML-Multi atmospheric dispersion model. Evident plume effects from the cruise ships, which were traced by introducing the concept of likely concentration contribution, were obtained in the modelled and measured concentrations at Langelinie Quay, which is the busiest cruise ship terminal in Copenhagen port. Hourly peak values of NO x well above 200 μ g m −3 were obtained at the top of a residential building at Langelinie Quay. The emissions from cruise ships were increasing the annual concentration of NO 2 in the port area by up to 31% at ground level, and 86% 50 m above the ground in comparison to the urban background level. No exceedance of the European annual limit value of NO 2 was obtained. The short-term impact of cruise ships was more pronounced with local exceedances of the hourly European limit value for NO 2 . Increasing cruise ship activity in Copenhagen port leads to air quality deterioration on short time scales with implications for human health.

Forests mitigate climate change by storing carbon and reducing emissions via substitution effects of wood products. Additionally, they provide many other important ecosystem services (ESs), but are vulnerable to climate change; therefore, adaptation is necessary. Climate-smart forestry combines mitigation with adaptation, whilst facilitating the provision of many ESs. This is particularly challenging due to large uncertainties about future climate. Here, we combined ecosystem modeling with robust multi-criteria optimization to assess how the provision of various ESs (climate change mitigation, timber provision, local cooling, water availability, and biodiversity habitat) can be guaranteed under a broad range of climate futures across Europe. Our optimized portfolios contain 29% unmanaged forests, and implicate a successive conversion of 34% of coniferous to broad-leaved forests (11% vice versa). Coppices practically vanish from Southern Europe, mainly due to their high water requirement. We find the high shares of unmanaged forests necessary to keep European forests a carbon sink while broad-leaved and unmanaged forests contribute to local cooling through biogeophysical effects. Unmanaged forests also pose the largest benefit for biodiversity habitat. However, the increased shares of unmanaged and broad-leaved forests lead to reductions in harvests. This raises the question of how to meet increasing wood demands without transferring ecological impacts elsewhere or enhancing the dependence on more carbon-intensive industries. Furthermore, the mitigation potential of forests depends on assumptions about the decarbonization of other industries and is consequently crucially dependent on the emission scenario. Our findings highlight that trade-offs must be assessed when developing concrete strategies for climate-smart forestry.

Land surface models are used to provide global estimates of soil organic carbon (SOC) changes after past and future land use change (LUC). To evaluate how well the models capture decadal scale changes in SOC after LUC, we provide the first consistent comparison of simulated time series of LUC by six land models all of which participated in the Coupled Model Intercomparison Project Phase 6 (CMIP6) with soil carbon chronosequences (SCC). For this comparison we use SOC measurements of adjacent plots at four high-quality data sites in temperate and tropical regions. We find that initial SOC stocks differ among models due to different approaches to represent SOC. Models generally meet the direction of SOC change after reforestation of cropland but the amplitude and rate of changes vary strongly among them. Further, models simulate SOC losses after deforestation for crop or grassland too slow due to the lack of crop harvest impacts in the models or an overestimation of the SOC recovery on grassland. The representation of management, especially nitrogen levels is important to capture drops in SOC after land abandonment for forest regrowth. Crop harvest and fire management are important to match SOC dynamics but more difficult to quantify as SCC hardly report on these events. Based on our findings, we identify strengths and propose potential improvements of the applied models in simulating SOC changes after LUC.

Although coastal regions only amount to 7 % of the global oceans, their contribution to the global oceanic air–sea CO2 exchange is proportionally larger, with fluxes in some estuaries being similar in magnitude to terrestrial surface fluxes of CO2. Across a heterogeneous surface consisting of a coastal marginal sea with estuarine properties and varied land mosaics, the surface fluxes of CO2 from both marine areas and terrestrial surfaces were investigated in this study together with their impact in atmospheric CO2 concentrations by the usage of a high-resolution modelling framework. The simulated terrestrial fluxes across the study region of Denmark experienced an east–west gradient corresponding to the distribution of the land cover classification, their biological activity and the urbanised areas. Annually, the Danish terrestrial surface had an uptake of approximately −7000 GgC yr−1. While the marine fluxes from the North Sea and the Danish inner waters were smaller annually, with about −1800 and 1300 GgC yr−1, their sizes are comparable to annual terrestrial fluxes from individual land cover classifications in the study region and hence are not negligible. The contribution of terrestrial surfaces fluxes was easily detectable in both simulated and measured concentrations of atmospheric CO2 at the only tall tower site in the study region. Although, the tower is positioned next to Roskilde Fjord, the local marine impact was not distinguishable in the simulated concentrations. But the regional impact from the Danish inner waters and the Baltic Sea increased the atmospheric concentration by up to 0.5 ppm during the winter months.

In this Letter, in “About 75% of this reduction is expected to come from emission reductions and the remaining 25% from land use, land-use change and forestry”, ‘25%’ should read ‘1%’ and '75%' should read '99%'. In the sentence “The carbon-sink-maximizing portfolio has a small negative effect on annual precipitation (−2 mm) and no effect on air temperature (Table 1)” the word ‘precipitation’ was omitted. Denmark was accidentally deleted during the conversion of Fig. 1. The original Letter has been corrected online.

(1) Laboratoire des Sciences du Climat et de l’Environnement, Sant-Aubin, France, (2) Department of Physical Geography and Ecosystem Science, Lund University, SE-22362 Lund, Sweden, (3) Department of Forestry and Wood Technology, Linnaeus University, Växjö, Sweden, (4) CEREA, ENPC, 6-8 Avenue Blaise Pascal, Cité Descartes, Champs-sur-Marne 77455 Marne la Vallée, Cedex 2, France, (5) National Physical Laboratory, Teddingtog, London, UK, (6) Department of Land in the Earth Systems, Max Planck Institute for Meteorology, 20146 Hamburg, Germany, (7) Research Center for Environmental Change (RCEC), Academia Sinica, Taipei, Taiwan, (8) Climate Service Center Germany, Helmholtz-Zentrum Geesthacht, 20095 Hamburg, Germany, (9) Institut Pierre Simon Laplace, 75010 Paris, France, (10) Global Ecology Unit CREAF-UAB, Cerdanyalo del Vallès, 08193 Catalonia, Spain, (11) Department of Ecological Science, Free University Amsterdam, 1081 HV Amsterdam, Netherlands

The Paris Agreement promotes forest management as a pathway towards halting climate warming through the reduction of carbon dioxide (CO2) emissions1. However, the climate benefits from carbon sequestration through forest management may be reinforced, counteracted or even offset by concurrent management-induced changes in surface albedo, land-surface roughness, emissions of biogenic volatile organic compounds, transpiration and sensible heat flux2–4. Consequently, forest management could offset CO2 emissions without halting global temperature rise. It therefore remains to be confirmed whether commonly proposed sustainable European forest-management portfolios would comply with the Paris Agreement—that is, whether they can reduce the growth rate of atmospheric CO2, reduce the radiative imbalance at the top of the atmosphere, and neither increase the near-surface air temperature nor decrease precipitation by the end of the twenty-first century. Here we show that the portfolio made up of management systems that locally maximize the carbon sink through carbon sequestration, wood use and product and energy substitution reduces the growth rate of atmospheric CO2, but does not meet any of the other criteria. The portfolios that maximize the carbon sink or forest albedo pass only one—different in each case—criterion. Managing the European forests with the objective of reducing near-surface air temperature, on the other hand, will also reduce the atmospheric CO2 growth rate, thus meeting two of the four criteria. Trade-off are thus unavoidable when using European forests to meet climate objectives. Furthermore, our results demonstrate that if present-day forest cover is sustained, the additional climate benefits achieved through forest management would be modest and local, rather than global. On the basis of these findings, we argue that Europe should not rely on forest management to mitigate climate change. The modest climate effects from changes in forest management imply, however, that if adaptation to future climate were to require large-scale changes in species composition and silvicultural systems over Europe5,6, the forests could be adapted to climate change with neither positive nor negative climate effects. Simulations of commonly proposed forest-management portfolios for Europe show that no single portfolio would meet all the requirements of the Paris Agreement, and climate benefits from forest management would be modest and local.

Surface heterogeneity can be challenging to fully encompass by modelling studies of ::::::: Although :::::: coastal ::::::: regions :::: only :::::: amount :: to ::: 7% ::: of ::: the ::::: global ::::::: oceans, :::: their ::::::::::: contribution :: to ::: the ::::: global ::::::: oceanic :::::: air-sea : CO2 surface exchanges, especially when it comes to land-sea boarders. The relative importance of the marine and the :::::::: exchange :: is :::::::::::::: in-proportionally ::::: larger :::: with :::::: fluxes :: in :::: some :::::::: estuaries ::::: being :::::: similar :: in ::::::::: magnitude :: to : terrestrial surface fluxes on the atmospheric concentration were examined by developing a mesoscale modelling framework capable of simulating surface exchanges at a high spatiotemporal resolution. 5 This study exploits the complexity of the Danish landscape and the many land-sea boarders found along the nation’s 7,300 km of coastline. :: of CO2:. An atmospheric transport model, DEHM, with a horizontal spatial resolution of 5.6 km × 5.6 km constituted the basis of the modelling framework. A mechanistic biosphere model, SPA, was coupled to DEHM in order to simulate terrestrial surface exchanges applying a tiling approach with the seven most dominant land-use classes in Denmark to account for sub-grid 10 heterogeneity. Detailed surface fields of p :::::: Across : a :::::::::::: heterogeneous ::::::: surface ::::::::: consisting :: of : a ::::::: coastal ::::::: marginal :::: sea :::: with ::::::: esturine :::::::: properties :::: and ::::: varied :::: land :::::::: mosaics, ::: the :::::: surface :::::: fluxes :: of CO2 were used to simulate the air-sea :::: from:::: both:::::: marine::::: areas:::: and :::::::: terrestrial ::::::: surfaces :::: were :::::::::: investigated :: in ::: this ::::: study ::::::: together :::: with :::: their :::::: impact :: in :::::::::: atmospheric CO2 exchange for the study region. Monthly mean diurnal cycles of surface water pwere imposed onto these, in order to include short-term variability in surface

Surface heterogeneity can be challenging to fully encompass by modelling studies of surfaces exchanges of CO 2 . By developing a mesoscale modelling framework capable of simulating surface exchanges at a high spatiotemporal resolution, this study examines the relative importance of the marine and the terrestrial surface fluxes on the atmospheric CO 2 concentration during 2011-2014 across the complex landscape of Denmark – a country which contains many land-sea boarders along its 7300 km of coastline. An atmospheric transport model, DEHM, with a horizontal spatial resolution of 5.6 km x 5.6 km consti-tuted the basis of the modelling framework. A mechanistic biosphere model, SPA, was coupled to DEHM in order to simulate

Abstract. Surface heterogeneity can be challenging to fully encompass by modelling studies of CO2 surface exchanges, especially when it comes to land-sea boarders. The relative importance of the marine and the terrestrial surface fluxes on the atmospheric CO2 concentration were examined by developing a mesoscale modelling framework capable of simulating surface exchanges at a high spatiotemporal resolution. This study exploits the complexity of the Danish landscape and the many land-sea boarders found along the nation's 7300 km of coastline. An atmospheric transport model, DEHM, with a horizontal spatial resolution of 5.6 km × 5.6 km constituted the basis of the modelling framework. A mechanistic biosphere model, SPA, was coupled to DEHM in order to simulate terrestrial surface exchanges applying a tiling approach with the seven most dominant land-use classes in Denmark to account for sub-grid heterogeneity. Detailed surface fields of pCO2 were used to simulate the air-sea CO2 exchange for the study region. Monthly mean diurnal cycles of surface water pCO2 were imposed onto these, in order to include short-term variability in surface water pCO2. The Danish biospheric fluxes simulated by the SPA-DEHM model system experienced an east-west gradient corresponding to the distribution of the land-use classes and their biological activity. The relative importance of the seven land-use classes varied throughout the year according to their individual growth patterns. A major contribution to the monthly net ecosystem exchange (NEE) through all seasons came from grasslands, while the influence from croplands increased from March to July. Grasslands had, on an annual basis, the largest impact on the biospheric net uptake with −1423 GgC yr−1. The total Danish biospheric uptake for 2011 was −6302 GgC yr−1. Relating the annual natural biospheric surface fluxes to the CO2 emitted by fossil fuel combustions and industrial processes by Denmark, the Danish terrestrial uptake corresponded to 52 % of these, while the Danish annual marine uptake was negligible in comparison, although hiding larger seasonal variations. During 2013–2014, the simulated atmospheric CO2 concentrations compared well with measurements made at the Risø tall tower located on the shore of Roskilde Fjord (R = 0.88 and RMSE = 4.87 ppm). The origin of the simulated CO2 concentrations at Risø varied between seasons with biospheric fluxes and fossil fuel emissions having the largest impact on the variations. Impact from Roskilde fjord was difficult to detect in the simulated CO2 concentrations. These difficulties in simulating the local impact from the Roskilde Fjord might arise from (i) the fjord not being adequately resolved in the constructed model system, (ii) the lack of a realistic representation of the surface water pCO2 dynamics, or (iii) that the fjord is not in the simulated footprint and only had a modest impact on the simulated atmospheric CO2 at the Risø tall tower.

Coastal seas and estuarine systems are highly variable in both time and space and with their heterogeneity difficult to capture with measurements. Models are useful tools in obtaining a better spatiotemporal coverage or, at least, a better understanding of the impacts such heterogeneity has in driving variability in coastal oceans and estuaries. A modelbased sensitivity study is constructed in this study in order to examine the effects of short-term variability in surface water pCO2 on the annual air–sea CO2 exchange in coastal regions. An atmospheric transport model formed the basis of the modelling framework for the study of the Baltic Sea and the Danish inner waters. Several maps of surface water pCO2 were employed in the modelling framework. While a monthly Baltic Sea climatology (BSC) had already been developed, the current study further extended this with the addition of an improved near-coastal climatology for the Danish inner waters. Furthermore, daily surface fields of pCO2 were obtained from a mixed layer scheme constrained by surface measurements of pCO2 (JENA). Short-term variability in surface water pCO2 was assessed by calculating monthly mean diurnal cycles from continuous measurements of surface water pCO2, observed at stationary sites within the Baltic Sea. No apparent diurnal cycle was evident in winter, but diurnal cycles (with amplitudes up to 27 μatm) were found from April to October. The present study showed that the temporal resolution of surface water pCO2 played an influential role on the annual air–sea CO2 exchange for the coastal study region. Hence, annual estimates of CO2 exchanges are sensitive to variation on much shorter time scales, and this variability should be included for any model study investigating the exchange of CO2 across the air–sea interface. Furthermore, the choice of surface pCO2 maps also had a crucial influence on the simulated air–sea CO2 exchange.

Future changes in population exposures to ambient air pollution are inherently linked with long-term trends in outdoor air quality, but also with changes in the building stock. Moreover, the burden of disease is further driven by the ageing of the European populations. This study aims to assess the impact of changes in climate, emissions, building stocks and population on air pollution related human health impacts across Europe in the future. Therefore an integrated assessment model combining atmospheric models and health impacts has been setup for projections of the future developments in air pollution related premature mortality. The focus is here on the regional scale impacts of exposure to surface ozone (O3), Secondary Inorganic Aerosols (SIA) and primary particulate matter (PPM).