Effects of an advanced land-use scheme and dynamic vegetation on the terrestrial carbon cycle in EC-Earth

<p>With human land-use activities expected to increase in the future it is important to understand how LULCC (Land-Use Land-Cover Change) activities affect the Earth’s surface, climate and biogeochemical cycles. Here we use the CMIP6 version of the EC-Earth3 Earth System Model (ESM) to assess the impacts of LULCC on surface fluxes of carbon (C) and nitrogen (N). EC-Earth is one of the first ESMs to interactively couple a 2nd generation dynamical vegetation model (LPJ-GUESS) with mechanistic C-N dynamics in soil and vegetation to an atmospheric model. The size, age structure, temporal dynamics and spatial heterogeneity of the vegetated landscape are represented and simulated dynamically in LPJ-GUESS. Such functionality has been argued to be essential to correctly capture biogeochemical and biophysical land-atmosphere interactions on longer timescales and has been shown to improve their representation compared to more common area-based vegetation schemes. The patch based structure of LPJ-GUESS also makes it possible to represent the history (soil, litter status) of a single patch as it might have been involved in several land-use transitions. We focus on the effects of gross land-use transitions (“land-hist”), net land-use transitions (“land-noShiftcultivate”) and fixing land-use at 1850 levels (“land-noLu”).</p><p> </p><p>Global carbon pools increase in simulations without LUC while they decline in those applying LUC, with gross-transitions resulting in values around 3% (or 75 Pg) lower than simulations with net-transitions. This is mainly driven by vegetation carbon changes in the tropical to mid-latitude regions where gross-transitions lead to a significantly higher decrease in high vegetation cover. Furthermore, this is reflected differently for different species, e.g. while there is no change in the LAI of boreal needleleaf trees in net-transitions, their presence is significantly reduced in gross transition scenarios giving way to the growth of fast-growing shade-intolerant species. Moreover, fire-fluxes, which in these experiments are mainly driven by fuel-availability, are also lowest in the gross-transition simulations.</p><p>Finally, we will show that the level of complexity with which shifting cultivation is treated has implications for the biogeophysical feedbacks in ESMs resulting from changes to surface albedo and latent heat exchange.</p><p>The experiments we conducted clearly indicate the benefits of dynamic vegetation and the importance of using gross transitions in land use-change (LUC) studies. </p>

The Role of Nonlinear Drying above the Boundary Layer in the Mid-Holocene African Monsoon

Paleoclimatic proxies indicate that significant summertime rainfall reached the Sahara region during the mid-Holocene, presumably in response to stronger summertime heating in the Northern Hemisphere. Climate models generally do not replicate the enhanced precipitation. As a step toward understanding the response and possible role of model errors, a series of idealized experiments were conducted with the Community Earth System Model in which local atmospheric heat sources of increasing magnitude were applied in the boundary layer over the Sahel and Sahara. In response to this local heating, the cold and moist southwesterly monsoon inflow encroaches farther northward. A source strength of roughly 1 K day−1 produces responses similar to those in a simulation with mid-Holocene orbital forcing imposed globally, while that of 1.5 K day−1 produces a precipitation response similar to that from paleoproxies. The precipitation increases nonlinearly, with a jump at heating of around 1 K day−1, even though the low-level monsoon inflow increases linearly. Competition at low-to-middle levels between drying by a shallow return flow just above the boundary layer and moistening by vertical advection within the layer affects convection and determines the northward extension of precipitation. When the heating becomes 1.5 K day−1, the boundary layer flow encroaches sufficiently northward to weaken the shallow return flow, further aiding precipitation. This novel nonlinear mechanism operates without biogeophysical feedbacks, and suggests that poor representation of the local thermodynamic processes may hamper a model’s ability to simulate dynamical feedbacks and hence the strength and poleward extension of monsoon rains under forcings like those during the mid-Holocene.

Biogeophysical feedbacks enhance the Arctic terrestrial carbon sink in regional Earth system dynamics

Continued warming of the Arctic will likely accelerate terrestrial carbon (C) cycling by increasing both uptake and release of C. Yet, there are still large uncertainties in modelling Arctic terrestrial ecosystems as a source or sink of C. Most modelling studies assessing or projecting the future fate of C exchange with the atmosphere are based on either stand-alone process-based models or coupled climate–C cycle general circulation models, and often disregard biogeophysical feedbacks of land-surface changes to the atmosphere. To understand how biogeophysical feedbacks might impact on both climate and the C budget in Arctic terrestrial ecosystems, we apply the regional Earth system model RCA-GUESS over the CORDEX-Arctic domain. The model is forced with lateral boundary conditions from an EC-Earth CMIP5 climate projection under the representative concentration pathway (RCP) 8.5 scenario. We perform two simulations, with or without interactive vegetation dynamics respectively, to assess the impacts of biogeophysical feedbacks. Both simulations indicate that Arctic terrestrial ecosystems will continue to sequester C with an increased uptake rate until the 2060–2070s, after which the C budget will return to a weak C sink as increased soil respiration and biomass burning outpaces increased net primary productivity. The additional C sinks arising from biogeophysical feedbacks are approximately 8.5 Gt C, accounting for 22% of the total C sinks, of which 83.5% are located in areas of extant Arctic tundra. Two opposing feedback mechanisms, mediated by albedo and evapotranspiration changes respectively, contribute to this response. The albedo feedback dominates in the winter and spring seasons, amplifying the near-surface warming by up to 1.35 °C in spring, while the evapotranspiration feedback dominates in the summer months, and leads to a cooling of up to 0.81 °C. Such feedbacks stimulate vegetation growth due to an earlier onset of the growing season, leading to compositional changes in woody plants and vegetation redistribution.

Effects of regional afforestation on global climate

Carbon (C) sequestration following afforestation is regarded as economically, politically, and technically feasible for fighting global warming, whereas the afforested area which will contribute more efficiently as sinks for CO2 is still uncertain. To compare the benefits for C sequestration combined with its biogeochemical effects, an earth system model of intermediate complexity, the McGill Paleoclimate Model-2 (MPM-2) is used to identify the biogeophysical effects of regional afforestation on shaping global climate. An increase in forest in China has led to a prominent global warming during summer around 45° N. Conversely, the forest expansion in the USA causes a noticeable increase in global mean annual temperature during winter. Afforestation in the USA and China brings about a decrease in annual mean meridional oceanic heat transport, while the afforestation in low latitudes of the southern hemisphere causes an increase. These local and global impacts suggest that regional tree plantations may produce a differential effect on the Earth's climate, and even exert an opposite effect on the annual mean meridional oceanic heat transport; they imply that its spatial variation of biogeophysical feedbacks needs to be considered when evaluating the benefits of afforestation.

Biogeophysical feedbacks enhance Arctic terrestrial carbon sink in regional Earth system dynamics

Continued warming of the Arctic will likely accelerate terrestrial carbon (C) cycling by increasing both uptake and release of C. There are still large uncertainties in modelling Arctic terrestrial ecosystems as a source or sink of C. Most modelling studies assessing or projecting the future fate of C exchange with the atmosphere are based an either stand-alone process-based models or coupled climate–C cycle general circulation models, in either case disregarding biogeophysical feedbacks of land surface changes to the atmosphere. To understand how biogeophysical feedbacks will impact on both climate and C budget over Arctic terrestrial ecosystems, we apply the regional Earth system model RCA-GUESS over the CORDEX-Arctic domain. The model is forced with lateral boundary conditions from an GCMs CMIP5 climate projection under the RCP 8.5 scenario. We perform two simulations with or without interactive vegetation dynamics respectively to assess the impacts of biogeophysical feedbacks. Both simulations indicate that Arctic terrestrial ecosystems will continue to sequester C with an increased uptake rate until 2060s–2070s, after which the C budget will return to a weak C sink as increased soil respiration and biomass burning outpaces increased net primary productivity. The additional C sinks arising from biogeophysical feedbacks are considerable, around 8.5 Gt C, accounting for 22% of the total C sinks, of which 83.5% are located in areas of Arctic tundra. Two opposing feedback mechanisms, mediated by albedo and evapotranspiration changes respectively, contribute to this response. Albedo feedback dominates over winter and spring season, amplifying the near-surface warming by up to 1.35 K in spring, while evapotranspiration feedback dominates over summer exerting the evaporative cooling by up to 0.81 K. Such feedbacks stimulate vegetation growth with an earlier onset of growing-season, leading to compositional changes in woody plants and vegetation redistribution.

Biogeophysical feedbacks trigger shifts in the modelled vegetation-atmosphere system at multiple scales

Terrestrial vegetation influences climate by modifying the radiative-, momentum-, and hydrologic-balance. This paper contributes to the ongoing debate on the question whether positive biogeophysical feedbacks between vegetation and climate may lead to multiple equilibria in vegetation and climate and consequent abrupt regime shifts. Several modelling studies argue that vegetation-climate feedbacks at local to regional scales could be strong enough to establish multiple states in the climate system. An Earth Model of Intermediate Complexity, PlaSim, is used to investigate the resilience of the climate system to vegetation disturbance at regional to global scales. We hypothesize that by starting with two extreme initialisations of biomass, positive vegetation-climate feedbacks will keep the vegetation-atmosphere system within different attraction domains. Indeed, model integrations starting from different initial biomass distributions diverged to clearly distinct climate-vegetation states in terms of abiotic (precipitation and temperature) and biotic (biomass) variables. Moreover, we found that between these states there are several other steady states which depend on the scale of perturbation. From here global susceptibility maps were made showing regions of low and high resilience. The model results suggest that mainly the boreal and monsoon regions have low resiliences, i.e. instable biomass equilibria, with positive vegetation-climate feedbacks in which the biomass induced by a perturbation is further enforced. The perturbation did not only influence single vegetation-climate cell interactions but also caused changes in spatial patterns of atmospheric circulation due to neighbouring cells constituting in spatial vegetation-climate feedbacks. Large perturbations could trigger an abrupt shift of the system towards another steady state. Although the model setup used in our simulation is rather simple, our results stress that the coupling of feedbacks at multiple scales in vegetation-climate models is essential and urgent to understand the system dynamics for improved projections of ecosystem responses to anthropogenic changes in climate forcing.

Biogeophysical feedbacks trigger shifts in the modelled climate system at multiple scales

Terrestrial vegetation influences climate by modifying the radiative-, momentum-, and hydrologic-balance. This paper contributes to the ongoing debate on the question whether positive biogeophysical feedbacks between vegetation and climate may lead to multiple equilibria in vegetation and climate and consequent abrupt regime shifts. Several modelling studies argue that vegetation-climate feedbacks at local to regional scales could be strong enough to establish multiple states in the climate system. An Earth Model of Intermediate Complexity, PlaSim, is used to investigate the resilience of the climate system to vegetation disturbance at regional to global scales. We hypothesize that by starting with two extreme initialisations of biomass, positive vegetation-climate feedbacks will keep the vegetation-atmosphere system within different attraction domains. Indeed, model integrations starting from different initial biomass distributions diverged to clearly distinct climate-vegetation states in terms of abiotic (precipitation and temperature) and biotic (biomass) variables. Moreover, we found that between these states there are several other steady states which depend on the scale of perturbation. From here global susceptibility maps were made showing regions of low and high resilience. The model results suggest that mainly the boreal and monsoon regions have low resiliences, i.e. instable biomass equilibria, with positive vegetation-climate feedbacks in which the biomass induced by a perturbation is further enforced. The perturbation did not only influence single vegetation-climate cell interactions but also caused changes in spatial patterns of atmospheric circulation due to neighbouring cells constituting in spatial vegetation-climate feedbacks. Large perturbations could trigger an abrupt shift of the system towards another steady state. Although the model setup used in our simulation is rather simple, our results stress that the coupling of feedbacks at multiple scales in vegetation-climate models is essential and urgent to understand the system dynamics for improved projections of ecosystem responses to anthropogenic changes in climate forcing.