Reconciling East Asia’s mid-Holocene temperature discrepancy through vegetation-climate feedback

The term“Holocene temperature conundrum”refers to the inconsistencies between proxy-based reconstructions and transient model simulations,and it challenges our understanding of global temperature evolution during the Holocene.Climate reconstructions indicate a cooling trend following the Holocene Thermal Maximum,while model simulations indicate a consistent warming trend due to icesheet retreat and rising greenhouse gas concentrations.Various factors,such as seasonal biases and overlooked feedback processes,have been proposed as potential causes for this discrepancy.In this study,we examined the impact of vegetation-climate feedback on the temperature anomaly patterns in East Asia during the mid-Holocene(~6 ka).By utilizing the fully coupled Earth system model EC-Earth and performing simulations with and without coupled dynamic vegetation,our objective was to isolate the influence of vegetation changes on regional temperature patterns.Our findings reveal that vegetation-climate feedback contributed to warming across most of East Asia,resulting in spatially diverse temperature changes during the mid-Holocene and significantly improved model-data agreement.These results high-light the crucial role of vegetation-climate feedback in addressing the Holocene temperature conundrum and emphasize its importance for simulating accurate climate scenarios.

Progress of vegetation modelling and future research prospects

Terrestrial vegetation is a crucial component of the Earth system,and its changes not only represent one of the most distinct aspects of climate change but also exert significant feedback within the climate system by exchanging energy,moisture,and carbon dioxide.To quantitatively and mechanistically study climate-vegetation feedback,numerical vegetation models have been developed on the theory of ecophysiological constraints on plant functional types.The models eventually can simulate vegetation distribution and succession across different spatial and temporal scales,and associated terrestrial carbon cycle processes by categorizing vegetation into biomes according different plant functional types and their associated environmental factors.Here we review the developing history of vegetation models and provide recent advances and future directions.Before 21st century,static vegetation models,as developed statistical models,can only simulate equilibrated characteristics of vegetation distribution.In last several decades,Dynamic Global Vegetation Models(DGVMs)have been developed to simulate instantaneous responses of vegetation to climate change and associated dynamics,and can be coupled with Earth system models to investigate interactions among atmosphere,ocean,and land.DGVMs are also widely applied to investigate the dynamics accounting for changes in the geographic distribution patterns of land surface vegetation at different spatial and temporal scales and to assess the impacts of terrestrial carbon and water fluxes and land use changes.We suggest that future vegetation modeling could integrate with machine learning,and explore vegetation transient response and feedback as well as impacts of process hierarchies and human activities on climate and ecosystem.

Sahara’s surface transformation forced an abrupt hydroclimate decline and Neolithic culture transition in northern China

The remote forcing from land surface changes in the Sahara is hypothesized to play a pivotal role in modulating the intensity of the East Asian summer monsoon(EASM)through ocean-atmospheric teleconnections.This modulation has far-reaching consequences,particularly in facilitating societal shifts documented in northern China.Here,we present a well-dated lake-level record from the Daihai Lake Basin in northern China,providing quantitative assessments of Holocene monsoonal precipitation and the consequent migrations of the northern boundary of the EASM.Our reconstruction,informed by a water-and-energy balance model,indicates that annual precipitation reached700 mm during 8–5 ka,followed by a rapid decline to tween 5 and 4 ka.This shift coherently aligns with a signi550 mm be-ficant300 km northwestward movement of the EASM northern boundary during the Middle Holocene(MH),in contrast to its current position.Our findings underscore that these changes cannot be entirely attributed to orbital forcing,as corroborated by simulation tests.Climate model simulations deployed in our study suggest that the presence of the Green Sahara during the MH significantly strengthened the EASM and led to a northward shift of the monsoon rainfall belt.Conversely,the Sahara’s reversion to a desert landscape in the late Holocene was accompanied by a corresponding southward retraction of monsoon influence.These dramatic hydroclimate changes during5–4 ka likely triggered or at least contributed to a shift in Neolithic cultures and societal transformation in northern China.With decreasing agricultural productivity,communities transitioned from millet farming to a mixed rainfed agriculture and animal husbandry system.Thus,our findings elucidate not only the variability of the EASM but also the profound implications of a remote forcing,such as surface transformations of the Sahara,on climatic changes and cultural evolution in northern China.

Milankovitch theory and monsoon

The widely accepted“Milankovitch theory”explains insolation-induced waxing and waning of the ice sheets and their effect on the global climate on orbital timescales.In the past half century,however,the theory has often come under scrutiny,especially regarding its“100-ka problem.”Another drawback,but the one that has received less attention,is the“monsoon problem,”which pertains to the exclusion of monsoon dynamics in classic Milankovitch theory even though the monsoon prevails over the vast low-latitude(
30N to
30S)region that covers half of the Earth’s surface and receives the bulk of solar radiation.In this review,we discuss the major issues with the current form of Milankovitch theory and the progress made at the research forefront.