ALBERT

Description: Dynamic vegetation models simulate global vegetation in terms of fractional coverages of a few plant functional types (PFTs). Although these models often share the same concept, they differ with respect to the number and kind of PFTs, complicating the comparability of simulated vegetation distributions. Pollen-based reconstructions are initially only available in form of time-series of individual taxa that are not distinguished in the models. Thus, to evaluate simulated vegetation distributions, the modelling results and pollen-based reconstructions have to be converted into a comparable format. The classical approach is the method of biomisation, but hitherto, PFT-based biomisation methods were only available for individual models. We introduce and evaluate a simple, universally applicable technique to harmonize PFT-distributions by assigning them into nine mega-biomes that follow the definitions commonly used for vegetation reconstructions. The method works well for all state-of the art dynamic vegetation models, independent of the spatial resolution or the complexity of the models. Large biome belts (such as tropical forest) are well represented, but regionally confined biomes (warm-mixed forest, Savanna) are only partly captured. Overall, the PFT-based biomisation is able to keep up with the conventional biomisation approach of forcing biome models (here: BIOME1) with the background climate states. The new method has, however, the advantage that it allows a more direct comparison and evaluation of the vegetation distributions simulated by Earth System Models. Thereby, the new method provides a powerful tool for the evaluation of Earth System Models in general.

Description: Highlights: • Multiproxy record from S Ethiopia extends knowledge about environment and climate of past 116,000 yrs during human expansion. • Hydroclimate during Marine Isotope Stage (MIS) 5 was much more variable (frequency and amplitude) than during MIS 3 and 4. • Earth system models and model simulations of intermediate complexity emulate corresponding amplitude shifts in hydroclimate. • Environment was arid during MIS 3 and 4, but permanent lake water bodies existed as inferred from our biological proxies. Abstract: Archaeological findings, numerical human dispersal models and genome analyses suggest several time windows in the past 200 kyr (thousands of years ago) when anatomically modern humans (AMH) dispersed out of Africa into the Levant and/or Arabia. From close to the key hominin site of Omo-Kibish, we provide near continuous proxy evidence for environmental changes in lake sediment cores from the Chew Bahir basin, south Ethiopia. The data show highly variable hydroclimate conditions from 116 to 66 kyr BP with rapid shifts from very wet to extreme aridity. The wet phases coincide with the timing of the North African Humid Periods during MIS5, as defined by Nile discharge records from the eastern Mediterranean. The subsequent record at Chew Bahir suggests stable regional hydrological setting between 58 and 32 kyr (MIS4 and 3), which facilitated the development of more habitable ecosystems, albeit in generally dry climatic conditions. This shift, from more to less variable hydroclimate, may help account for the timing of later dispersal events of AMH out of Africa.