ALBERT

Description: We study an Earth-like terra-planet with an overland recycling mechanism bringing fresh water back from higher latitudes to the lower latitudes. By performing model simulations for such a planet we find two drastically different climate states for the same set of boundary conditions and parameter values: A Cold and Wet (CW) state (present-day Earth-like climate) with dominant low-latitude precipitation and, a Hot and Dry (HD) state with only high-latitude precipitation. We notice that for perpetual equinox conditions, both climate states are stable below a certain threshold value of background soil albedo while above the threshold only the CW state is stable. Starting from the HD state and increasing background soil albedo above the threshold causes an abrupt shift from the HD state to the CW state resulting in a sudden cooling of about 35 °C globally which is of the order of the temperature difference between the present-day and the Snowball Earth state. In contrast to the Snowball Earth instability, we find that the sudden cooling in our study is driven by the cloud albedo feedback rather than the snow-albedo feedback. Also, when albedo in the CW state is reduced back to zero the terra-planet does not display a closed hysteresis. This is due to the high cloud cover in the CW state hiding the surface from solar irradiation. As a result, this reduction of background surface albedo has only a minor effect on the top of the atmosphere radiation balance, thereby making it impossible to heat the planet sufficiently strongly to switch back to the HD state. Additional simulations point to a similar abrupt transition from HD state to the CW state for non-zero obliquity which is the only stable state in this configuration. Our study also has implications for the habitability of Earth-like terra-planets. At the inner edge of the habitable zone, the higher cloud cover in the CW state cools the planet and may prevent the onset of a runaway greenhouse state. At the outer edge, the resupply of water at lower latitudes stabilizes the greenhouse effect and keeps the planet in the HD state and may prevent water from getting trapped at higher latitudes in frozen form. Overall, the existence of bi-stability in the presence of an overland recycling mechanism hints at the possibility of a wider habitable zone for Earth-like terra-planets at lower obliquities.

Description: We present how variations in plant functional diversity affect climate–vegetation interaction towards the end of the African Humid Period (AHP) in coupled land–atmosphere simulations using the Max Planck Institute Earth system model (MPI-ESM). In experiments with AHP boundary conditions, the extent of the “green” Sahara varies considerably with changes in plant functional diversity. Differences in vegetation cover extent and plant functional type (PFT) composition translate into significantly different land surface parameters, water cycling, and surface energy budgets. These changes have not only regional consequences but considerably alter large-scale atmospheric circulation patterns and the position of the tropical rain belt. Towards the end of the AHP, simulations with the standard PFT set in MPI-ESM depict a gradual decrease of precipitation and vegetation cover over time, while simulations with modified PFT composition show either a sharp decline of both variables or an even slower retreat. Thus, not the quantitative but the qualitative PFT composition determines climate–vegetation interaction and the climate–vegetation system response to external forcing. The sensitivity of simulated system states to changes in PFT composition raises the question how realistically Earth system models can actually represent climate–vegetation interaction, considering the poor representation of plant diversity in the current generation of land surface models.

Description: Enhanced summer insolation during the early and mid-Holocene drove increased precipitation and widespread expansion of vegetation across the Sahara during the African Humid Period (AHP). While changes in atmospheric dynamics during this time have been a major focus of palaeoclimate modelling efforts, the transient nature of the shift back to the modern desert state at the end of this period is less well understood. Reconstructions reveal a spatially and temporally complex end of the AHP, with an earlier end in the north than in the south and in the east than in the west. Some records suggest a rather abrupt end, whereas others indicate a gradual decline in moisture availability. Here we investigate the end of the AHP based on a transient simulation of the last 7850 years with the comprehensive Earth System Model MPI-ESM1.2. The model largely reproduces the time-transgressive end of the AHP evident in proxy data, and indicates that it is due to the regionally varying dynamical controls on precipitation. The impact of the main rain-bringing systems, i.e. the summer monsoon and extratropical troughs, varies spatially, leading to heterogeneous seasonal rainfall cycles that impose regionally different responses to the Holocene insolation decrease. An increase in extratropical troughs that interact with the tropical mean flow and transport moisture to the Western Sahara during mid-Holocene delays the end of the AHP in that region. Along the coast, this interaction maintains humid conditions for a longer time than further inland. Drying in this area occurs when this interaction becomes too weak to sustain precipitation. In the lower latitudes of West Africa, where the rainfall is only influenced by the summer monsoon dynamics, the end of the AHP coincides with the retreat of the monsoonal rainbelt. The model results clearly demonstrate that non-monsoonal dynamics can also play an important role in forming the precipitation signal and should therefore not be neglected in analyses of North African rainfall trends.

Description: We evaluate the radiative forcing of forests and the feedbacks triggered by forests in a warm, basically ice-free climate and in a cool climate with permanent high-latitude ice cover using the Max Planck Institute for Meteorology Earth System Model. As a paradigm for a warm climate, we choose the early Eocene, some 54 to 52  million years ago, and for the cool climate, the pre-industrial climate, respectively. To isolate first-order effects, we compare idealised simulations in which all continents are covered either by dense forests or by deserts with either bright or dark soil. In comparison with desert continents covered by bright soil, forested continents warm the planet for the early Eocene climate and for pre-industrial conditions. The warming can be attributed to different feedback processes, though. The lapse-rate and water-vapour feedback is stronger for the early Eocene climate than for the pre-industrial climate, but strong and negative cloud-related feedbacks nearly outweigh the positive lapse-rate and water-vapour feedback for the early Eocene climate. Subsequently, global mean warming by forests is weaker for the early Eocene climate than for pre-industrial conditions. Sea-ice related feedbacks are weak for the almost ice-free climate of the early Eocene, thereby leading to a weaker high-latitude warming by forests than for pre-industrial conditions. When the land is covered with dark soils, and hence, albedo differences between forests and soil are small, forests cool the early Eocene climate more than the pre-industrial climate because the lapse-rate and water-vapour feedbacks are stronger for the early Eocene climate. Cloud-related feedbacks are equally strong in both climates. We conclude that radiative forcing by forests varies little with the climate state, while most subsequent feedbacks depend on the climate state.

Description: Easter Island underwent a rapid deforestation several hundred years ago. The causes have been discussed in depth. However, the effect of the deforestation on the near-surface climate of Easter Island and possible feedbacks have not yet been studied. Here we use the limited-area model COSMO to simulate a series of typical weather situations for a fully tree-covered, grass-covered and a bare soil Easter Island, respectively. We find that the top soil layer of the deforested island becomes much warmer and the wind speed roughly doubles, thereby enhancing the erosion on the deforested island. During a drought spell, evapotranspiration decreases much more slowly over a forested area. If the soil has become dry, then the tree-covered island triggers convective precipitation much more efficiently than the bare-soil or grass-covered island could do. This is caused by the higher surface roughness and stronger sensible heat flux which lead to a deeper boundary layer and an enhanced moisture flux convergence over the forested island. Hence, the climate of a deforested Easter Island appears to be significantly less resistant to drought than a forested island and thus, deforestation has probably exacerbated the effects of past climate drought spells on Easter Island socio-ecological systems.

Description: Marine sediment records reveal an abrupt and strong increase in dust deposition in the North Atlantic at the end of the African Humid Period about 4.9 to 5.5 ka ago. The change in dust flux has been attributed to varying Saharan land surface cover. Alternatively, the enhanced dust accumulation is linked to enhanced surface winds and a consequent intensification of coastal upwelling. Here we demonstrate for the first time the direct link between dust accumulation in marine cores and changes in Saharan land surface. We simulate the mid-Holocene (6 ka BP) and pre-industrial (1850 AD) dust cycle as a function of Saharan land surface cover and atmosphere-ocean conditions using the coupled atmosphere–aerosol model ECHAM6.1-HAM2.1. Mid-Holocene surface characteristics, including vegetation cover and lake surface area, are derived from proxy data and simulations. In agreement with data from marine sediment cores, our simulations show that mid-Holocene dust deposition fluxes in the North Atlantic were two to three times lower compared with pre-industrial fluxes. We identify Saharan land surface characteristics to be the main control on dust transport from North Africa to the North Atlantic. We conclude that the increase in dust accumulation in marine cores is directly linked to a transition of the Saharan landscape during the Holocene and not due to changes in atmospheric or ocean conditions alone.

Description: The large variety of atmospheric circulation systems affecting the East Asian climate is reflected by the complex Asian vegetation distribution. Particularly in the transition zones of these circulation systems, vegetation is supposed to be very sensitive to climate change. Since proxy records are scarce, hitherto a mechanistic understanding of the past spatio-temporal climate-vegetation relationship is lacking. To assess the Holocene vegetation change and to obtain an ensemble of potential mid-Holocene biome distributions for Eastern Asia, we forced the diagnostic biome model BIOME4 with climate anomalies of different transient Holocene climate simulations performed in coupled atmosphere-ocean(-vegetation) models. The simulated biome changes are compared with pollen-based biome records for different key regions. In all simulations, substantial biome shifts during the Holocene are confined to the high-northern latitudes and the Monsoon-Westerlies transition zone, but the temporal evolution and amplitude of change strongly depends on the climate forcing. Large parts of the southern tundra are replaced by taiga during the mid-Holocene due to a warmer growing season and the boreal treeline in northern Asia is shifted northward by approx. 4° in the ensemble mean, ranging from 1.5° to 6° in the individual simulations, respectively. This simulated treeline shift is in agreement with pollen-based reconstructions from northern Siberia. The desert fraction in the transition zone is reduced by 21 % during the mid-Holocene compared to pre-industrial due to enhanced precipitation. The desert- steppe margin is shifted westward by 5° (1°–9° in the individual simulations). The forest biomes are expanded north-westward by 2° ranging from 0°–4° in the single simulations. These results corroborate pollen-based reconstructions indicating an extended forest area in north Central China during the mid-Holocene. According to the model, the forest-to-non-forest and steppe-to-desert changes in the climate transition zones are spatially not uniform and not linear during the Holocene.

Description: In Earth system model simulations we find different carbon cycle sensitivities for recent and glacial climate. This result is obtained by comparing the transient response of the terrestrial carbon cycle to a fast and strong atmospheric CO2 concentration increase (roughly 1000ppm) in C4MIP type simulations starting from climate conditions of the Last Glacial Maximum (LGM) and from Pre-Industrial times (PI). The sensitivity β to CO2 fertilization is larger in the LGM experiment during most of the simulation time: The fertilization effect leads to a terrestrial carbon gain in the LGM experiment almost twice as large as in the PI experiment. The larger fertilization effect in the LGM experiment is caused by the stronger initial CO2 limitation of photosynthesis, implying a stronger potential for its release upon CO2 concentration increase. In contrast, the sensitivity γ to climate change induced by the radiation effect of rising CO2 is larger in the PI experiment for most of the simulation time. Yet, climate change is less pronounced in the PI experiment, resulting in only slightly higher terrestrial carbon losses than in the LGM experiment. The stronger climate sensitivity in the PI experiment results from the vastly more extratropical soil carbon under those interglacial conditions whose respiration is enhanced under climate change. Comparing the radiation and fertilization effect in a factor analysis, we find that they are almost additive, i.e. their synergy is small in the global sum of carbon changes. From this additivity, we find that the carbon cycle feedback strength is more negative in the LGM than in the PI simulations.

Description: The large variety of atmospheric circulation systems affecting the eastern Asian climate is reflected by the complex Asian vegetation distribution. Particularly in the transition zones of these circulation systems, vegetation is supposed to be very sensitive to climate change. Since proxy records are scarce, hitherto a mechanistic understanding of the past spatio-temporal climate–vegetation relationship is lacking. To assess the Holocene vegetation change and to obtain an ensemble of potential mid-Holocene biome distributions for eastern Asia, we forced the diagnostic biome model BIOME4 with climate anomalies of different transient Holocene climate simulations performed in coupled atmosphere–ocean(–vegetation) models. The simulated biome changes are compared with pollen-based biome records for different key regions.In all simulations, substantial biome shifts during the last 6000 years are confined to the high northern latitudes and the monsoon–westerly wind transition zone, but the temporal evolution and amplitude of change strongly depend on the climate forcing. Large parts of the southern tundra are replaced by taiga during the mid-Holocene due to a warmer growing season and the boreal treeline in northern Asia is shifted northward by approx. 4° in the ensemble mean, ranging from 1.5 to 6° in the individual simulations, respectively. This simulated treeline shift is in agreement with pollen-based reconstructions from northern Siberia. The desert fraction in the transition zone is reduced by 21 % during the mid-Holocene compared to pre-industrial due to enhanced precipitation. The desert–steppe margin is shifted westward by 5° (1–9° in the individual simulations). The forest biomes are expanded north-westward by 2°, ranging from 0 to 4° in the single simulations. These results corroborate pollen-based reconstructions indicating an extended forest area in north-central China during the mid-Holocene. According to the model, the forest-to-non-forest and steppe-to-desert changes in the climate transition zones are spatially not uniform and not linear since the mid-Holocene.

Description: We present how variations in plant functional diversity affect climate–vegetation interaction towards the end of the African Humid Period (AHP) in coupled land-atmosphere simulations using the Max Planck Institute Earth System Model MPI-ESM. In experiments with AHP boundary conditions, the extent of the green Sahara varies considerably with changes in plant functional diversity. Differences in vegetation cover extent and Plant Functional Type (PFT) composition translate into significantly different land surface parameters, water cycling and surface energy budget. These changes have not only regional consequences but considerably alter large scale atmospheric circulation patterns and the position of the tropical rain belt. Towards the end of the AHP, simulations with the standard PFT set in MPI-ESM depict a gradual decrease of precipitation and vegetation cover over time, while simulations with modified PFT composition show either a sharp decline of both variables or an even slower retreat. Thus, not the quantitative but the qualitative PFT composition determines climate–vegetation interaction and the climate–vegetation system response to external forcing. The sensitivity of simulated system states to changes in PFT composition raises the question how realistically Earth system models can actually represent climate–vegetation interaction, considering the poor representation of plant diversity in the current generation of land surface models.