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The global carbon budget (GCB) - including fluxes of CO2 between atmosphere, land and ocean, and its atmospheric growth rate - show large interannual to decadal variations. Reconstructing and predicting the variable GCB is essential for tracing the fate of carbon and understanding the global carbon cycle in the changing climate. We use a novel approach to reconstruct and predict the next-years' variations in GCB based on our decadal prediction system enhanced with an interactive carbon cycle. By assimilating physical atmospheric and oceanic data products into the Max Planck Institute Earth system model (MPI-ESM), we can well reproduce the annual mean historical GCB variations from 1970-2018, with high correlations relative to the assessments from the Global Carbon Project of 0.75, 0.75 and 0.97 for atmospheric CO2 growth, air-land CO2 fluxes and air-sea CO2 fluxes, respectively. Such a fully coupled decadal prediction system, with an interactive carbon cycle enables representation of the GCB within a closed Earth system, and therefore provides an additional line of evidence for the ongoing assessments of the anthropogenic GCB. Retrospective predictions initialized from the assimilation simulation show high confidence in predicting the following year's GCB. The predictive skill is up to 5 years for the air-sea CO2 fluxes, and 2 years for the air-land CO2 fluxes and atmospheric carbon growth rate. This is the first study investigating the GCB variations and predictions with an emission-driven prediction system, such a system also enables the reconstruction and prediction of the evolution of atmospheric CO2 concentration changes. The earth system predictions in this study provide valuable inputs for understanding the global carbon cycle and informing climate relevant policy. [ABSTRACT FROM AUTHOR]

In 2018 and 2019, central Europe was stricken by two consecutive extreme dry and hot summers (DH2018 and DH2019). The DH2018 had severe impacts on ecosystems and likely affected vegetation activity in the subsequent year, for example though depletion of carbon reserves or damage from drought. Such legacies from drought and heat stress can further increase vegetation susceptibility to additional hazards. Temporally compound extremes such as DH2018 and DH2019 can, therefore, result in an amplification of impacts by preconditioning effects of past disturbance legacies. Here, we evaluate how these two consecutive extreme summers impacted ecosystems in central Europe and how the vegetation responses to the first compound event (DH2018) modulated the impacts of the second (DH2019). To quantify the modulating role of vegetation responses to the impacts of each compound event, we first train a set of statistical models for the period 2001-2017 to predict the impacts of DH2018 and DH2019 on Enhanced Vegetation Index (EVI) anomalies from MODIS. These estimates can be seen as the expected EVI anomalies, had the impacts of DH2018 and DH2019 been consistent with past sensitivity to climate. These can then be used to identify modulating effects by vegetation activity and composition or other environmental factors such as elevated CO2 or warming trends. We find two regions in which the impacts of the two DH events were significantly stronger than those expected based on previous climate-vegetation relationships. One region, largely dominated by grasslands and crops, showed much stronger impacts than expected in both DH events due to an amplification of their sensitivity to heat and drought, possibly linked to changing background CO2 and temperature conditions. A second region, dominated by forests, showed browning from DH2018 to DH2019, even though dry and hot conditions were partly alleviated in 2019. This browning trajectory was mainly explained by the preconditioning role of DH2018 to the observed response to DH2019 through legacy effects, and possibly by increased susceptibility to biotic disturbances, which are also promoted by warm conditions. Dry and hot summers are expected to become more frequent in the coming decades posing a major threat to the stability of European forests. We show that state-of-the-art process based models miss these legacy effects. These gaps may result in an overestimation of the resilience and stability of temperate ecosystems in future model projections. [ABSTRACT FROM AUTHOR]

The carbon flux due to land-use and land-cover change (net LULCC flux) historically contributed to a large fraction of anthropogenic carbon emissions while at the same time being associated with large uncertainties. This study aims to compare the contribution of several sensitivities underlying the net LULCC flux by assessing their relative importance in a bookkeeping model (BLUE) based on a LULCC dataset including uncertainty estimates (the LUH2 dataset). The sensitivity experiments build upon the approach of Hurtt et al. (2011) and compare the impacts of LULCC uncertainty (a high, baseline and low landuse estimate), the starting time of the bookkeeping model simulation (850, 1700 and 1850), net area transitions versus gross area transitions (shifting cultivation) and neglecting wood harvest on estimates of the net LULCC flux. Additional factorial experiments isolate the impact of uncertainty from initial conditions and transitions on the net LULCC flux. Finally, historical simulations are extended with future land-use scenarios to assess the impact of past LULCC uncertainty in future projections. Over the period 1850–2014, baseline and low LULCC scenarios produce a comparable cumulative net LULCC flux while the high LULCC estimate initially produces a larger net LULCC flux which decreases towards the end of the period and even becomes smaller than in the baseline estimate. LULCC uncertainty leads to slightly higher sensitivity in the cumulative net LULCC flux (up to 22 %, reference are the baseline simulations) compared to the starting year of a model simulation (up to 15 %). The contribution from neglecting wood harvest activities (up to 28 % cumulative net LULCC flux) is larger than from LULCC uncertainty and the implementation of land-cover transitions (gross or net transitions) exhibits the smallest sensitivity (up to 13 %). At the end of the historical LULCC dataset in 2014, the LULCC uncertainty retains some impact on the net LULCC flux (±0.15 PgC yr-1 at an estimate of 1.7 PgC yr-1). Of the past uncertainties in LULCC, a small impact persists in 2099, mainly due to uncertainty of harvest remaining in 2014. However, compared to the uncertainty range of the LULCC flux estimated today, the estimates in 2099 appear to be indistinguishable. These results, albeit from a single model, are important for CMIP6 as they compare the relative importance of starting year, uncertainty of LULCC, applying gross transitions and wood harvest on the net LULCC flux. For the cumulative net LULCC flux over the industrial period the uncertainty of LULCC is as relevant as applying wood harvest and gross transitions. However, LULCC uncertainty matters less (by about a factor three) than the other two factors for the net LULCC flux in 2014 and historical LULCC uncertainty is negligible for estimates of future scenarios. [ABSTRACT FROM AUTHOR]

ABSTRACT Some of the major drivers of extreme temperatures in Australia are related to large-scale variations in the climate. While many of the relationships between climate variability and heatwaves have been described, the preceding climate conditions and the circulation patterns responsible for interannual heatwave variations are still poorly understood. We have used rotated principal component analysis ( PCA) to describe the variance of frequency, duration, magnitude and timing of Australian summer heatwaves. This provides a simple means of investigating both the spatial patterns of circulation and the preceding climate mode conditions of summer heatwaves. We find that spatial patterns of heatwave variability associated with El Niño-Southern Oscillation ( ENSO) were focused in the north and northeast of Australia for heatwave occurrence. Patterns of heatwave amplitude in southeastern Australia were also weakly related to ENSO. The PCA did not find any direct, concurrent Indian Ocean Dipole ( IOD) or Southern Annular Mode ( SAM) signal. However, lead correlations indicate that winter IOD conditions can have a weak indirect effect on heatwave occurrence in northeast Australia and similarly, winter SAM conditions can affect heatwave timing in central eastern Australia. Regressions reveal that ENSO related heatwave duration and frequency in the northeast are related to the movement of the Walker circulation, while southeastern heatwave amplitude is associated with a north-south movement of the Tasman high pressure system. These results may lead to improvements in understanding the predictability and general preparedness for heatwaves in the summer season. [ABSTRACT FROM AUTHOR]

The drought affecting central Europe, UK and Scandinavia in 2018 was one of thestrongest in the past decade, and comparable in severity to the 2003, 2010 and 2015droughts. From the atmospheric perspective, the summer drought and heatwave in2018 differed from the previous events as it was preceded by unprecedented heightemperatures and lower than average rainfall from early in April that persisted until latesummer. Unlike the other events, this drought affected regions that seldom experience strongdrought (e.g. Scandinavia and UK). In some of the regions experiencing the strongestsummer drought anomalies, e.g. Germany and UK, preceding spring rainfall deficits werewithin the normal variability but the warming from April onwards was particularlyextreme. In these regions, ecosystem activity is limited by spring temperature, andtherefore the extreme warming in spring likely stimulated photosynthesis early in thegrowing-season. Increased productivity trends in spring due to earlier onset of the growing season in theNorthern Hemisphere have been associated with stronger water deficits and reducedproductivity in summer (Buermann et al., 2018). However, analyzing the heatwave in 2010,Flach et al. (2018) pointed to a north-south asymmetry in ecosystems response to spring andsummer heatwaves, with northern forests showing increased GPP even during the heatwaveand drought, possibly because of differentiated responses by different vegetation types towarming and drying. Here we analyze a series of land-surface model simulations using high-resolution climateforcing for the European region. We evaluate the impacts of the 2018 drought on the carbonand hydrologic processes in ecosystems, during spring and summer and compare them toobservation-based estimates of CO2 and water fluxes over the region. Finally, we quantify thecontribution of the spring extreme warming and the resulting hydro-ecological feedbacks tothe intensity of the summer drought. Buermann, Wolfgang, et al.: Widespread seasonal compensation effects of springwarming on northern plant productivity. Nature 562.7725 (2018): 110. Flach, M., Sippel, S., Gans, F., Bastos, A., Brenning, A., Reichstein, M., andMahecha, M. D.: Contrasting biosphere responses to hydrometeorological extremes:revisiting the 2010 western Russian heatwave. Biogeosciences, 15, 6067-6085,https://doi.org/10.5194/bg-15-6067-2018, 2018. [ABSTRACT FROM AUTHOR]