Global and Northern-High-Latitude Terrestrial carbon sinks in the 21st century from CMIP6 experiments

Climate warming is accelerating the changes in the global terrestrial ecosystems and particularly those in the northern high latitudes (NHL), and rendering the land-atmosphere carbon exchange highly uncertain. 10 The Coupled Model Intercomparison Project Phase 6 (CMIP6) employs the most updated climate models to estimate terrestrial ecosystem carbon dynamics driven by a new set of socioeconomic and climate change pathways. By analyzing the future (2015-2100) carbon fluxes estimated by ten CMIP6 models, we quantitatively evaluated the projected magnitudes, trends and uncertainties of global and NHL carbon fluxes under four scenarios plus the role of NHL in the global terrestrial ecosystem carbon dynamics. Overall, the models suggest that the global and NHL 15 terrestrial ecosystems will be consistent carbon sinks in the future, and the extent of the carbon sinks is projected to be larger under scenarios with higher radiative forcing. By the end of this century, the models by average estimate the NHL net ecosystem productivity (NEP) as 0.54±0.77, 1.01±0 . 98, 0.97±1.62, and 1.05±1.83 PgC/yr under SSP126, SSP245, SSP370 and SSP585, respectively. The uncertainties are not substantially reduced compared with earlier results, e.g., the Coupled Climate Carbon Cycle Model Intercomparison Project (C4MIP). Although NHL 20 contributes a small fraction of the global carbon sink (~13%), the relative uncertainties of NHL NEP are much larger than the global level. Our results provide insights into future carbon flux evolutions under future scenarios and highlight the urgent need to constrain the large uncertainties associated with model projections for making better climate mitigation strategies. et al., 2016b). Guided by the goals to facilitate integrated research on the impact of future scenarios over natural and human systems, and to help quantify 75 uncertainties in future projections based on multi-model simulations, the most devoted MIP - the Scenario Model Intercomparison Project (ScenarioMIP; (O’Neill et al., 2016) incorporate a broad range of future scenarios with various combinations of Representative Concentration Pathways (RCPs) which was initially adopted in CMIP5 and newer Shared Socioeconomic Pathways (SSPs). These integrations allow a comprehensive assessment of plausible future climate conditions covering a wide span of mitigation and adaptation options (Riahi et al., 2017; van Vuuren 80 et al., 2014), and represent the most updated understanding of the climate change and carbon cycle in the next few by future SSPs a of five future global Four future scenarios with different 8.5 future historical period (1980-2014) and under the four future scenarios (2015-2100) in our analyses. We aggregated NEP, NPP and RH from different models and scenarios to global and 125 NHL scales as the area-weighted sum of all belonging grids, whereas for TS, area-weighted mean was used. The bottom (i.e., nearest to the land surface) layer atmospheric CO 2 concentration was employed and aggregated into global and NHL scales. Note that only four out of the ten models have available CO 2 data to date. The calculated monthly values were further aggregated into the yearly scale for analysis. The annual model outputs with various spatial resolutions were resampled with a consistent grid resolution of 1 degree (mesh size: 320 ×160) for generating 130 the spatial trend maps. The ensemble model projections and uncertainties of NEP, NPP, RH and TS were evaluated by calculating the multi-model mean (µ) and standard deviations (SD, σ) of the yearly model outputs at both the global and NHL scale. Meanwhile, the contribution of model SD relative to the mean is quantified by the coefficient of variation (CV), CV = σ/µ, to interpret the relative magnitude of the model uncertainty. We estimated the temporal trends of µ and SD using linear least square regression method to quantitatively analyze the ensembled model 135 behavior as a function of time. Meanwhile, the historical annual NEP was evaluated with estimates from the Global century under SSP126, such as CanESM5 and EC-Earth3-Veg. In the NHL, while most models suggest a carbon sink, the model BCC-CSM2-MR estimates a carbon source even though it shows the global ecosystem as a consistent carbon sink, irrespective of the model scenarios. under SSP126. The trends are positive under SSP245 at 8.93 Tg C/year 2 at the 175 global scale, and 2.54 Tg C/year 2 for NHL. Under SSP370 and SSP585, the positive trends become more prominent: they are 20.08 and 44.40 Tg C/year 2 at the global scale, and 3.08 and 4.27 Tg C/year 2 in the NHL under SSP370 and SSP585, respectively. NPP and The uncertainties (measured are for NPP and RH and radiative for NEP radiative forcing five-fold of the contribution estimated by the multi-model means for NEP under SSP370 and SSP585. land-use CMIP6 models All CMIP6 model results in this do rising NPP and RH in response to temperature rise in the future but with 325 divergent trends and patterns. Consequently, large uncertain or even irreconcilable NEP results in the NHL is shown among different models.