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1.	Chen, Jie, et al. (författare) Northwestward shift of the northern boundary of the East Asian summer monsoon during the mid-Holocene caused by orbital forcing and vegetation feedbacks 2021 Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 268 Tidskriftsartikel (refereegranskat)abstract The East Asian summer monsoon (EASM) northern boundary is a critical indicator of EASM variations. Movement of the boundary is modulated by both the EASM and the mid-latitude westerlies. Here, we use the Earth system model EC-Earth to quantify the contribution of orbital forcing and vegetation feedbacks in modulating the movement of EASM northern boundary. The results show that the simulated EASM northern boundary during the mid-Holocene shifts by a maximum of ∼213 km northwestward due to orbital forcing. When the model was coupled with a dynamic vegetation module LPJ-GUESS, the northern boundary shifts further northwestward by a maximum of ∼90 km, indicating the importance of vegetation feedbacks. During the mid-Holocene, temperature increased in the mid-latitude during the boreal summer due to insolation, leading to increased meridional air temperature differences (MTDs) over the region north of 45°N and to decreased MTDs to the south. The changes in the temperature gradient weakened the East Asian Westly Jet (EAWJ) and displaced it northward, resulting in an earlier transition of the Meiyu stage and a more prolonged Midsummer stage. The northward movement of EAWJ, combined with the enhanced southerly moisture flow from South China, caused more precipitation in North China and eventually to a northwestward shift of the northern boundary of the EASM. The coupled dynamic vegetation module LPJ-GUESS simulated more grassland and less forest over Northeast Asia during the mid-Holocene. The increased surface albedo tended to lower the temperature in the region, and further enhanced the MTDs in mid-latitude East Asia, leading to the further northward movement of the EAWJ and a northwestward shift of the EASM northern boundary. Although the simulated vegetation distribution in several regions may be not accurate, it reflects the substantial contribution of climate-vegetation interaction on modulating the EASM.
2.	Chen, Jie, et al. (författare) Reconciling East Asia's mid-Holocene temperature discrepancy through vegetation-climate feedback Ingår i: Science Bulletin. - 2095-9273. Tidskriftsartikel (refereegranskat)abstract 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 ice-sheet 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 highlight the crucial role of vegetation-climate feedback in addressing the Holocene temperature conundrum and emphasize its importance for simulating accurate climate scenarios.
3.	Chen, Jie, et al. (författare) The Contribution of Vegetation-Climate Feedback and Resultant Sea Ice Loss to Amplified Arctic Warming During the Mid-Holocene 2022 Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:18 Tidskriftsartikel (refereegranskat)abstract Understanding influence of vegetation on past temperature changes in the Arctic region would help isolate uncertainty and build understanding of its broader climate system, with implications for paleoclimate reconstructions and future climate change. Using an Earth system model EC-Earth, we conduct a series of simulations to investigate the impact of vegetation-climate feedback on the Arctic climate during the mid-Holocene. Results show Arctic greening induced by the warming resulting from stronger orbital forcing, further amplifies the Arctic warming. The increased vegetation contributes 0.33 degrees C of Arctic warming and 0.35 x 106 km2 of Arctic sea ice loss. Increased Arctic vegetation leads to reduced land surface albedo and increased evapotranspiration, both of which cause local warming in spring and summer. The resultant sea ice loss causes warming in the following seasons, with atmospheric circulation anomalies further amplifying the warming. Our results highlight the significant contribution of vegetation-climate feedback to Arctic climate under natural conditions.
4.	Cheng, Hai, et al. (författare) Milankovitch theory and monsoon 2022 Ingår i: The Innovation. - : Elsevier BV. - 2666-6758. ; 3:6 Forskningsöversikt (refereegranskat)abstract 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 (∼30° N to ∼30° S) 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. We suggest shifting the emphasis from the ultimate outcomes of the ice volume to the causal relationship between changes in northern high-latitude insolation and ice age termination events (or ice sheet melting rate) to help reconcile the classic “100-ka problem.” We discuss the discrepancies associated with the characterization of monsoon dynamics, particularly the so-called “sea-land precession-phase paradox” and the “Chinese 100-ka problem.” We suggest that many of these discrepancies are superficial and can be resolved by applying a holistic “monsoon system science” approach. Finally, we propose blending the conventional Kutzbach orbital monsoon hypothesis, which calls for summer insolation forcing of monsoons, with Milankovitch theory to formulate a combined “Milankovitch-Kutzbach hypothesis” that can potentially explain the dual nature of orbital hydrodynamics of the ice sheet and monsoon systems, as well as their interplays and respective relationships with the northern high-latitude insolation and inter-tropical insolation differential.
5.	Dong, Jiang, et al. (författare) Millennial-scale interaction between the East Asian winter monsoon and El Niño-related tropical Pacific precipitation in the Holocene 2021 Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182. ; 573 Tidskriftsartikel (refereegranskat)abstract Both the East Asian winter monsoon (EAWM) and El Niño (EN) activities are vital climate modes that regulate the Pacific hydrologic cycle. However, the Holocene interactions among the EAWM, EN activities, and tropical Pacific precipitation remain unclear due to the lack of appropriate EAWM proxies. Here, we present high-resolution grain size records from the East China Sea shelf along with a transient climate model simulation to study the Holocene EAWM evolution and compare the findings with paleo-EN precipitation-related proxies records. The millennial-scale oscillations of grain size records, which are indicative of the intensity of the EAWM-driven coastal current, reveal an anti-phase coupling between the EAWM and EN-related tropical Pacific precipitation on a millennial timescale since 5.8 ka. These results, which are consistent with simulation results, indicate that the intensified EAWM could not only reduce equatorial western Pacific precipitation by reducing the sea surface temperature but also likely change boundary conditions in the tropical Pacific (i.e., the east-west Pacific temperature gradient and westerly anomaly) to favor the formation of subsequent intensive EN activities. The enhanced EN activities, inferred by the positive tropical eastern Pacific precipitation anomalies, could subsequently suppress the EAWM through anomalous low-level anticyclones and associated southerly anomalies, thereby generating intensified tropical western Pacific (mainly tropical monsoon areas) precipitation. Our study highlights these intrinsic interactions during the mid- to late Holocene and has useful implications for understanding this millennial-scale climate oscillation, which may represent periodic atmospheric exchange between high- and low-latitude climate systems by mediating the EAWM.
6.	Gao, Yu, et al. (författare) Dynamic effect of last glacial maximum ice sheet topography on the east asian summer monsoon 2020 Ingår i: Journal of Climate. - 0894-8755. ; 33:16, s. 6929-6944 Tidskriftsartikel (refereegranskat)abstract The effect of ice sheet topography on the East Asian summer monsoon (EASM) during the Last Glacial Maximum is studied using CCSM3 in a hierarchy of model configurations. It is found that receding ice sheets result in a weakened EASM, with the reduced ice sheet thickness playing a major role. The lower ice sheet topography weakens the EASM through shifting the position of the midlatitude jet, and through altering Northern Hemisphere stationary waves. In the jet shifting mechanism, the lowering of ice sheets shifts the westerly jet northward and decreases the westerly jet over the subtropics in summer, which reduces the advection of dry enthalpy and in turn precipitation over the EASM region. In the stationary wave mechanism, the lowering of ice sheets induces an anomalous stationary wave train along the westerly waveguide that propagates into the EASM region, generating an equivalent-barotropic low response; this leads to reduced lower-tropospheric southerlies, which in turn reduces the dry enthalpy advection into East Asia, and hence the EASM precipitation.
7.	Githumbi, Esther, et al. (författare) Holocene quantitative pollen-based vegetation reconstructions in Europe for climate modelling: LandClim II 2019 Konferensbidrag (refereegranskat)abstract Understanding land use and land cover (LULC) change through time is an important aspect when attempting to interpret human-environment interactions through time. Palaeoenvironmental techniques have been crucial in bridging this gap by providing information that has been used to estimate climate change, vegetation change, sea level change etc. through time using a variety of proxies. Producing quantitative land-cover reconstructions has been an aim and a challenge with several methods attempted during the decades. In this project, we use the REVEALS model has been tested and validated in several regions of the world.We use REVEALS-based quantitative reconstructions of vegetation change to investigate the biogeochemical and biogeophysical forcings of land-cover change on climate. In the first phase of this project, LandClim I, quantitative vegetation reconstructions were produced for Europe (Mediterranean area excluded) focusing on five time windows of the Holocene between 6ka BP and present. The results from a regional climate model showed that the impact of the reconstructed LULC between 6 ka and 0.2 ka BP via biogeophysical forcing varied geographically and seasonally. We present the REVEALS quantitative pollen-based vegetation reconstruction from the ongoing second phase of the project LandClim II “Quantification of the biogeophysical and biogeochemical forcings from anthropogenic deforestation on regional Holocene climate in Europe”. This reconstruction covers entire Europe and is transient over the Holocene with a time resolution of 500 years between 11.2 and 0.7ka BP, and 100 to 300 years from 0.7ka BP to modern time.
8.	Gravgaard Askjær, Thomas, et al. (författare) Multi-centennial Holocene climate variability in proxy records and transient model simulations 2022 Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 296 Tidskriftsartikel (refereegranskat)abstract Variability on centennial to multi-centennial timescales is mentioned as a feature in reconstructions of the Holocene climate. As more long transient model simulations with complex climate models become available and efforts have been made to compile large proxy databases, there is now a unique opportunity to study multi-centennial variability with greater detail and a large amount of data than earlier. This paper presents a spectral analysis of transient Holocene simulations from 9 models and 120 proxy records to find the common signals related to oscillation periods and geographic dependencies and discuss the implications for the potential driving mechanisms. Multi-centennial variability is significant in most proxy records, with the dominant oscillation periods around 120–130 years and an average of 240 years. Spectra of model-based global mean temperature (GMT) agree well with proxy evidence with significant multi-centennial variability in all simulations with the dominant oscillation periods around 120–150 years. It indicates a comparatively good agreement between model and proxy data. A lack of latitudinal dependencies in terms of oscillation period is found in both the model and proxy data. However, all model simulations have the highest spectral density distributed over the Northern hemisphere high latitudes, which could indicate a particular variability sensitivity or potential driving mechanisms in this region. Five models also have differentiated forcings simulations with various combinations of forcing agents. Significant multi-centennial variability with oscillation periods between 100 and 200 years is found in all forcing scenarios, including those with only orbital forcing. The different forcings induce some variability in the system. Yet, none appear to be the predominant driver based on the spectral analysis. Solar irradiance has long been hypothesized to be a primary driver of multi-centennial variability. However, all the simulations without this forcing have shown significant multi-centennial variability. The results then indicate that internal mechanisms operate on multi-centennial timescales, and the North Atlantic-Arctic is a region of interest for this aspect.
9.	Han, Zixuan, et al. (författare) The changes in ENSO-induced tropical Pacific precipitation variability in the past warm and cold climates from the EC-Earth simulations 2020 Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 55, s. 503-519 Tidskriftsartikel (refereegranskat)abstract The El Nino-Southern Oscillation (ENSO) is one of the most significant climate variability signals. Studying the changes in ENSO-induced precipitation variability (ENSO precipitation) in the past climate offers a possibility to a better understanding of how they may change under future climate conditions. This study uses simulations performed with the European community Earth-System Model (EC-Earth) to investigate the relative contributions of dynamic effect (the circulation anomalies together with the climatological specific humidity) and thermodynamic effect (the specific humidity anomalies together with the climatological circulation) on the changes in ENSO precipitation in the past warm and cold climates, represented by the Pliocene and the Last Glacial Maximum (LGM), respectively. The results show that the changes in ENSO precipitation are intensified (weakened) over the tropical western Pacific but weakened (intensified) over the tropical central Pacific in Pliocene (LGM) compared with the pre-industrial (PI) simulation. Based on the decomposed moisture budget equation, these changes in ENSO precipitation patterns are highly related to the dynamic effect. The mechanism can be understood as follows: the zonal gradient of the mean sea surface temperature (SST) over the tropical Indo-Pacific is increased (reduced) during the Pliocene (LGM), leading to the strengthening (weakening) of Pacific Walker Circulation as well as a westward (eastward) shift. In the Pliocene, the westward shift of Walker Circulation results in an increased (decreased) ENSO-induced low-level vertical velocity variability in the tropical western Pacific (central Pacific), and, in turn, favoring convergent (divergent) moisture transport through a dynamic process, and then causing intensified (weakened) ENSO precipitation there. The opposite mechanism exists in LGM. These results suggest that changes in the zonal SST gradient over tropical Indo-Pacific under different climate conditions determine the changes in ENSO precipitation through a dynamic process.
10.	Hou, Yandong, et al. (författare) Sahara's surface transformation forced an abrupt hydroclimate decline and Neolithic culture transition in northern China 2024 Ingår i: The Innovation. - 2666-6758. ; 5:1 Tidskriftsartikel (refereegranskat)abstract 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 modu-lation 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 mi-grations of the northern boundary of the EASM. Our reconstruction, informed by a water-and-energy balance model, indicates that annual precipitation reached -700 mm during 8-5 ka, followed by a rapid decline to -550 mm be-tween 5 and 4 ka. This shift coherently aligns with a significant -300 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 corrob-orated 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 Ho-locene was accompanied by a corresponding southward retraction of monsoon influence. These dramatic hydroclimate changes during -5-4 ka likely triggered or at least contributed to a shift in Neolithic cultures and societal transformation in northern China. With decreasing agricultural pro-ductivity, 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.
11.	Jin, Yishuai, et al. (författare) Controls of Spring Persistence Barrier Strength in Different ENSO Regimes and Implications for 21st Century Changes 2020 Ingår i: Geophysical Research Letters. - 0094-8276. ; 47:11 Tidskriftsartikel (refereegranskat)abstract This paper investigates potential factors that control the El Niño–Southern Oscillation (ENSO) Spring Persistence Barrier (SPB) strength in two different ENSO regimes and apply it to explain the ENSO SPB strength modulation after the 21st century. In a damped, noise-driven model, the theoretical solution of SPB strength illustrates that a weaker ENSO growth rate strengthens SPB. In the self-sustained regime, as in the Cane-Zebiak model (chaotic system), the strengthened thermodynamic damping and weakened thermocline positive feedback lead to a more negative ENSO growth rate and, in turn, a stronger SPB. Therefore, in both ENSO regimes, a weaker ENSO growth rate intensifies the SPB. The application of the theory to the real world suggests that a more negative ENSO growth rate, corresponding to a more damped feedback system, is responsible for the stronger SPB in recent decades than in 1980–2000.
12.	Jin, Yishuai, et al. (författare) Seasonal Cycle of Background in the Tropical Pacific as a Cause of ENSO Spring Persistence Barrier 2019 Ingår i: Geophysical Research Letters. - 0094-8276. ; 46:22, s. 13371-13378 Tidskriftsartikel (refereegranskat)abstract Statistical model results suggest that the declining growth rate from autumn to spring is the key to cause El Niño-Southern Oscillation (ENSO) spring persistence barrier (SPB). Using a dynamical approach, we develop the physical mechanisms responsible for ENSO SPB in the framework of recharge oscillator by adding a seasonally varying Bjerknes (BJ) stability index and linking it with ENSO growth rate. By decomposing BJ index, it is indicated that seasonal thermodynamic damping and thermocline positive feedback play an important role in determining the ENSO SPB. We further show that the increasing/decreasing upper-level cloud/low-level cloud and the deepening thermocline from autumn to spring are the main factors to control the SPB of ENSO. Our proposed mechanisms also have useful implications for the understanding of ENSO prediction.
13.	Kukla, Tyler, et al. (författare) The resilience of Amazon tree cover to past and present drying 2021 Ingår i: Global and Planetary Change. - : Elsevier BV. - 0921-8181. ; 202 Tidskriftsartikel (refereegranskat)abstract The Amazon forest is increasingly vulnerable to dieback and encroachment of grasslands and agricultural fields. Threats to these forested ecosystems include drying, deforestation, and fire, but feedbacks among these make it difficult to determine their relative importance. Here, we reconstruct the central and western Amazon tree cover response to aridity and fire in the mid-Holocene—a time of less intensive human land use and markedly drier conditions than today—to assess the resilience of tree cover to drying and the strength of vegetation-climate feedbacks. We use pollen, charcoal, and speleothem oxygen isotope proxy data to show that Amazon tree cover in the mid-Holocene was resilient to drying in excess of the driest bias-corrected future precipitation projections. Experiments with a dynamic global vegetation model (LPJ-GUESS) suggest tree cover resilience may be owed to weak feedbacks that act to amplify tree cover loss with drying. We also compare these results to observational data and find that, under limited human interference, modern tree cover is likely similarly resilient to mid-Holocene levels of aridification. Our results suggest human-driven fire and deforestation likely pose a greater threat to the future of Amazon ecosystems than drying alone.
14.	Li, Nannan, et al. (författare) Phytolith and simulation evidence for precipitation-modulated vegetation dynamics along the East Asian monsoon margin 2022 Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182. ; 590 Tidskriftsartikel (refereegranskat)abstract An improved understanding of past interactions between terrestrial vegetation and various forcings, such as climate change, human impact, and paleofire, is crucial for assessing impacts of future global change on terrestrial ecosystems. This study seeks to find the key factor or factors that have driven Holocene vegetation change along the East Asian monsoon margin. Several high-resolution pollen records are reviewed and new phytolith-based paleovegetation reconstructions and physical geochemical datasets are presented from a peatland in northeastern China. Using 108 modern topsoil samples as a training set, canopy cover and vegetation composition are estimated for the period since 5100 cal. yr BP. Variation partitioning analysis (VPA) is used to determine the relative importance of climate change, human impacts, and paleofire disturbance. The generalized dynamic vegetation model LPJ-GUESS is forced with climate anomaly output from an atmospheric general circulation model to simulate vegetation dynamics during the mid-Holocene and the pre-industrial era. The proxy-based estimates are compared to modelling output. Results indicate that regional tree cover varied from 10% to 40% during the past five millennia. The single-core, phytolith-based reconstructions are generally consistent with stacked tree pollen z-scores calculated from different records along the East Asian monsoon margin, implying that mid-Holocene tree cover decrease was persistent and almost synchronous over extensive areas. VPA demonstrates that long-term monsoon marginal vegetation successions were mainly caused by climate effects. Numerical modelling suggests that since the mid-Holocene the retreat of forests along the monsoon margin was primarily associated with precipitation deficits. Our investigation highlights that the precipitation associated with the East Asian monsoon system has exerted a stronger influence than the westerlies on the monsoon margin climate and vegetation change. With ongoing global change, close attention to variations in precipitation patterns and amounts should be especially helpful in efforts aimed at ecological monitoring, change prediction, and restoration.
15.	Lindgren, Amelie, et al. (författare) Millennial-scale analysis of land >23 ˚N as a carbon source and sink since the Last Glacial Maximum Annan publikation (övrigt vetenskapligt/konstnärligt)abstract The transfers of carbon between land, ocean and atmosphere, and their relation to temperature variability over glacial and interglacial cycles continue to intrigue the scientific community. Over the past four decades, many have focused on the role of the Southern Ocean to explain the atmospheric carbon dioxide (CO2) patterns seen in ice core records, but recent advances also include mentions of a possible terrestrial component. We quantify important terrestrial organic soil carbon (C) stocks north of 23˚, using palaeo-data and modeled climate to reconstruct terrestrial C dynamics from the Last Glacial Maximum until present at millennial time steps. During the deglaciation, C storage declined to reach a minimum around 10 kyr BP, a trend which then turned and led to progressively higher soil C stocks during the Holocene. Net changes in mineral soil C stocks are small, even though significant geographic shifts occurred; instead, deglacial and interglacial terrestrial C stock dynamics are dominated by losses from permafrost loess, inundation of continental shelves and gains in peatlands, processes commonly overlooked in complex Earth System Models.
16.	Lindgren, Amelie, et al. (författare) Reconstructing Past Global Vegetation With Random Forest Machine Learning, Sacrificing the Dynamic Response for Robust Results 2021 Ingår i: Journal of Advances in Modeling Earth Systems. - 1942-2466. ; 13:2 Tidskriftsartikel (refereegranskat)abstract Vegetation is an important component in the Earth system, providing a direct link between the biosphere and atmosphere. As such, a representative vegetation pattern is needed to accurately simulate climate. We attempt to model global vegetation (biomes) with a data‐driven approach, to test if this allows us to create robust global and regional vegetation patterns. This not only provides quantitative reconstructions of past vegetation cover as a climate forcing, but also improves our understanding of past land cover‐climate interactions which have important implications for the future. By using a Random Forest (RF) machine learning tool, we train the vegetation reconstruction with available biomized pollen data of present and past conditions to produce broad‐scale vegetation patterns for the preindustrial (PI), the mid‐Holocene (MH, ∼6,000 years ago), and the Last Glacial Maximum (LGM, ∼21,000 years ago). We test the method's robustness by introducing a systematic temperature bias based on existing climate model spread and compare the result with that of LPJ‐GUESS, an individual‐based dynamic global vegetation model. The results show that the RF approach is able to produce robust patterns for periods and regions well constrained by evidence (the PI and the MH), but fails when evidence is scarce (the LGM). The apparent robustness of this method is achieved at the cost of sacrificing the ability to model dynamic vegetation response to a changing climate.
17.	Lindgren, Amelie, et al. (författare) Reconstructing past vegetation with Random Forest Machine Learning, sacrificing the dynamic response for robust results Annan publikation (övrigt vetenskapligt/konstnärligt)abstract Vegetation is an important feature in the Earth system, providing a direct link between the biosphere and atmosphere. As such, a representative vegetation pattern is needed to accurately simulate climate. We attempt to reconstruct past and present vegetation with a data driven approach, to test if this allows us to create robust global and regional vegetation patterns. The motivation for this stems from the possibility of avoiding circular arguments when studying past time periods where vegetation is used to reconstruct climate, and climate is used to construct vegetation. By using the Random Forest machine learning tool, we train the vegetation reconstruction with available biomized pollen data of present and past conditions and are able to produce reasonable broad-scale vegetation patterns for the Pre-Industrial and the Mid-Holocene together with a few modeled climate variables. We test the methods robustness by introducing a systematic temperature bias based on existing climate model spread and compare the result with that of LPJ-GUESS, a process-based dynamic global vegetation model. Results prove that the Random Forest approach is able to produce robust patterns for periods and regions well constrained by evidence, but fails when evidence is scarce. The robustness is achieved by sacrificing a dynamic vegetation response to a changing climate.
18.	Long, Jingchao, et al. (författare) Large-scale photovoltaic solar farms in the Sahara affect solar power generation potential globally 2024 Ingår i: Communications Earth & Environment. - 2662-4435. ; 5 Tidskriftsartikel (refereegranskat)abstract Globally, solar projects are being rapidly built or planned, particularly in high solar potential regions with high energy demand. However, their energy generation potential is highly related to the weather condition. Here we use state-of-the-art Earth system model simulations to investigate how large photovoltaic solar farms in the Sahara Desert could impact the global cloud cover and solar generation potential through disturbed atmospheric teleconnections. The results indicate negative impacts on solar potential in North Africa (locally), Middle East, Southern Europe, India, Eastern China, Japan, Eastern Australia, and Southwestern US, and positive impacts in Central and South America, the Caribbean, Central & Eastern US, Scandinavia and South Africa, reaching a magnitude of ±5% in remote regions seasonally. Diagnostics suggest that large-scale atmospheric circulation changes are responsible for the global impacts. International cooperation is essential to mitigate the potential risks of future large-scale solar projects in drylands, which could impact energy production.
19.	Lu, Zhengyao, et al. (författare) A review of paleo El Niño-Southern Oscillation 2018 Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 9:4 Forskningsöversikt (refereegranskat)abstract The Earth has seen El Niño-Southern Oscillation (ENSO)-the leading mode of interannual climate variability-for at least millennia and likely over millions of years. This paper reviews previous studies from perspectives of both paleoclimate proxy data (from traditional sediment records to the latest high-resolution oxygen isotope records) and model simulations (including earlier intermediate models to the latest isotope-enabled coupled models). It summarizes current understanding of ENSO's past evolution during both interglacial and glacial periods and its response to external climatic forcings such as volcanic, orbital, ice-sheet and greenhouse gas forcings. Due to the intrinsic irregularity of ENSO and its complicated relationship with other climate phenomena, reconstructions and model simulations of ENSO variability are subject to inherent difficulties in interpretations and biases. Resolving these challenges through new data syntheses, new statistical methods, more complex climate model simulations as well as direct model-data comparisons can potentially better constrain uncertainty regarding ENSO's response to future global warming.
20.	Lu, Zhengyao, et al. (författare) Examining El Niño in the Holocene : Implications and challenges 2018 Ingår i: National Science Review. - : Oxford University Press (OUP). - 2095-5138 .- 2053-714X. ; 5:6, s. 807-809 Tidskriftsartikel (refereegranskat)
21.	Lu, Zhengyao, et al. (författare) Gigantic solar farms of the future might impact how much solar power can be generated on the other side of the world 2024 Ingår i: The Conversation. - 2201-5639. Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract The Sun’s energy is effectively limitless. While resources such as coal or gas are finite, if you are able to capture and use solar power it doesn’t prevent anyone else from also using as much sunshine as they need.Except that isn’t quite the full story. Beyond a certain size, solar farms become large enough to affect the weather around them and ultimately the climate as a whole. In our new research we have looked at the effect such climate-altering solar farms might have on solar power production elsewhere in the world.We know that solar power is affected by weather conditions and output varies through the days and seasons. Clouds, rain, snow and fog can all block sunlight from reaching solar panels. On a cloudy day, output can drop by 75%, while their efficiency also decreases at high temperatures.In the long term, climate change could affect the cloud cover of certain regions and how much solar power they can generate. Northern Europe is likely to see a solar decrease for instance, while there should be a slight increase of available solar radiation in the rest of Europe, the US east coast and northern China.
22.	Lu, Zhengyao, et al. (författare) Impacts of Large-Scale Sahara Solar Farms on Global Climate and Vegetation Cover 2021 Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 48:2 Tidskriftsartikel (refereegranskat)abstract Large-scale photovoltaic solar farms envisioned over the Sahara desert can meet the world's energy demand while increasing regional rainfall and vegetation cover. However, adverse remote effects resulting from atmospheric teleconnections could offset such regional benefits. We use state-of-the-art Earth-system model simulations to evaluate the global impacts of Sahara solar farms. Our results indicate a redistribution of precipitation causing Amazon droughts and forest degradation, and global surface temperature rise and sea-ice loss, particularly over the Arctic due to increased polarward heat transport, and northward expansion of deciduous forests in the Northern Hemisphere. We also identify reduced El Niño-Southern Oscillation and Atlantic Niño variability and enhanced tropical cyclone activity. Comparison to proxy inferences for a wetter and greener Sahara ∼6,000 years ago appears to substantiate these results. Understanding these responses within the Earth system provides insights into the site selection concerning any massive deployment of solar energy in the world's deserts.
23.	Lu, Zhengyao, et al. (författare) Natural decadal variability of global vegetation growth in relation to major decadal climate modes 2023 Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326 .- 1748-9318. ; 18:1 Tidskriftsartikel (refereegranskat)abstract The ongoing climate change can modulate the behavior of global vegetation and influence the terrestrial biosphere carbon sink. Past observation-based studies have mainly focused on the linear trend or interannual variability of the vegetation greenness, but could not explicitly deal with the effect of natural decadal variability due to the short length of observations. Here we put the variabilities revealed by remote sensing-based global leaf area index (LAI) from 1982 to 2015 into a long-term perspective with the help of ensemble Earth system model simulations of the historical period 1850-2014, with a focus on the low-frequency variability in the global LAI during the growing season. Robust decadal variability in the observed and modelled LAI was revealed across global terrestrial ecosystems, and it became stronger toward higher latitudes, accounting for over 50% of the total variability north of 40 degrees N. The linkage of LAI decadal variability to major natural decadal climate modes, such as the El Nino-Southern Oscillation decadal variability (ENSO-d), the Pacific decadal oscillation (PDO), and the Atlantic multidecadal oscillation (AMO), was analyzed. ENSO-d affects LAI by altering precipitation over large parts of tropical land. The PDO exerts opposite impacts on LAI in the tropics and extra-tropics due to the compensation between the effects of temperature and growing season length. The AMO effects are mainly associated with anomalous precipitation in North America and Europe but are mixed with long-term climate change impacts due to the coincident phase shift of the AMO which also induces North Atlantic basin warming. Our results suggest that the natural decadal variability of LAI can be largely explained by these decadal climate modes (on average 20% of the variance, comparable to linear changes, and over 40% in some ecosystems) which also can be potentially important in inducing the greening of the Earth of the past decades.
24.	Lu, Zhengyao, et al. (författare) Orbital modulation of ENSO seasonal phase locking 2019 Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 52:7-8, s. 4329-4350 Tidskriftsartikel (refereegranskat)abstract Modern El Niño-Southern Oscillation (ENSO) events are characterized by their phase locking of variability to the seasonal cycle and tend to peak at the end of calendar year. Here, we show that in an idealized NCAR-CCSM3 simulation of the climate of the last 300,000 years, ENSO seasonal phase locking is shifted periodically following the precessional forcing: ENSO tends to peak in boreal winter when perihelion is near vernal equinox, but to peak in boreal summer when perihelion lies in between autumnal equinox and winter solstice. The mechanism for the change of ENSO’s phase locking is proposed to be caused by the change of seasonality of the growth rate, or the intensity of ocean–atmosphere feedbacks, of ENSO. It is found that the December peak of ‘winter ENSO’ is caused by the continuous growth of ENSO anomaly from June to November, while the May–June peak of ‘summer ENSO’ appears to be caused jointly by the seasonal shift of higher growth rate into spring and stronger stochastic noise towards the first half of the year. Furthermore, the change of the seasonal cycle of feedbacks is contributed predominantly by that of the thermodynamic damping. The summer peak of ENSO is proposed to be caused by the following mechanism. A perihelion in the late fall to early winter leads to a cooling of the surface eastern equatorial Pacific (EEP) due to reduced insolation in spring. This cooling, reinforced by an oceanic process, reduces the latent heat flux damping in spring, and therefore favors the growth of the eastern Pacific-like ENSO (as opposed to the central Pacific-like ENSO). This EEP cooling is also likely to generate more effective short wave-cloud-SST feedback and, in turn, increased instability. Ultimately, the weakened thermodynamic damping in spring, combined with relatively intensive stochastic forcing, benefits the subsequent summer peak of ENSO.
25.	Lu, Zhengyao, et al. (författare) Prominent precession‐band variance in ENSO intensity over the last 300,000 years 2019 Ingår i: Geophysical Research Letters. - 1944-8007. ; 46:16, s. 9786-9795 Tidskriftsartikel (refereegranskat)
26.	Lu, Zhengyao, et al. (författare) Solar panels in Sahara could boost renewable energy but damage the global climate – here’s why 2021 Ingår i: The Conversation. - 2201-5639. Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
27.	Lu, Zhengyao, et al. (författare) Vegetation Pattern and Terrestrial Carbon Variation in Past Warm and Cold Climates 2019 Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:14, s. 8133-8143 Tidskriftsartikel (refereegranskat)abstract Understanding the transition of biosphere-atmosphere carbon exchange between glacial and interglacial climates can constrain uncertainties in its future projections. Using an individual-based dynamic vegetation model, we simulate vegetation distribution and terrestrial carbon cycling in past cold and warm climates and elucidate the forcing effects of temperature, precipitation, atmospheric CO2 concentration (pCO(2)), and landmass. Results are consistent with proxy reconstructions and reveal that the vegetation extent is mainly determined by temperature anomalies, especially in a cold climate, while precipitation forcing effects on global-scale vegetation patterns are marginal. The pCO(2) change controls the global carbon balance with the fertilization effect of higher pCO(2) linking to higher vegetation coverage, an enhanced terrestrial carbon sink, and increased terrestrial carbon storage. Our results indicate carbon transfer from ocean and permafrost/peat to the biosphere and atmosphere and highlight the importance of forest expansion as a driver of terrestrial ecosystem carbon stock from cold to warm climates.
28.	Pausata, Francesco S. R., et al. (författare) Greening of the Sahara suppressed ENSO activity during the mid-Holocene 2017 Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8 Tidskriftsartikel (refereegranskat)abstract The evolution of the El Nino-Southern Oscillation (ENSO) during the Holocene remains uncertain. In particular, a host of new paleoclimate records suggest that ENSO internal variability or other external forcings may have dwarfed the fairly modest ENSO response to precessional insolation changes simulated in climate models. Here, using fully coupled ocean-atmosphere model simulations, we show that accounting for a vegetated and less dusty Sahara during the mid-Holocene relative to preindustrial climate can reduce ENSO variability by 25%, more than twice the decrease obtained using orbital forcing alone. We identify changes in tropical Atlantic mean state and variability caused by the momentous strengthening of the West Africa Monsoon (WAM) as critical factors in amplifying ENSO's response to insolation forcing through changes in the Walker circulation. Our results thus suggest that potential changes in the WAM due to anthropogenic warming may influence ENSO variability in the future as well.
29.	Power, Katherine, et al. (författare) Impacts of large-scale Saharan solar farms on the global terrestrial carbon cycle 2023 Ingår i: Environmental Research Letters. - 1748-9326. ; 18:10 Tidskriftsartikel (refereegranskat)abstract Amassing the available solar energy over the Sahara desert, through the installation of a large-scale solar farm, would satisfy the world's current electricity needs. However, such land use changes may affect the global carbon cycle, possibly offsetting mitigation efforts. Here a fully coupled Earth System model EC-Earth was used to investigate the impact of a Saharan solar farm on the terrestrial carbon cycle, simulated with prescribed reduced surface albedo approximating the albedo effect of photovoltaic solar panels over the Sahara desert. The resulting changes to the carbon cycle were an enhancement of the carbon sink across Northern Africa, particularly around the Sahel but a simultaneous weakening of the carbon sink in the Amazon basin. This is observed through spatial pattern changes to the values of net biome production (NBP), more evident during Northern Hemisphere summer season. NBP changes are contributed by competing responses in the net primary production and heterotrophic respiration rates. These changes to carbon exchange correspond to a wetter and warmer climate occurring in Northern Africa and a drier and warmer climate in the Amazon, with stronger driving effects of precipitation. Due to these coupled responses and complex teleconnections, thorough investigation of remote impacts of solar farms are needed to avoid unintended consequences on the terrestrial carbon cycle.
30.	Tang, Xiaojie, et al. (författare) Orbital hydroclimate variability revealed by grain-size evidence in the tropical Pacific Islands since 140 ka 2024 Ingår i: Global and Planetary Change. - 0921-8181. ; 236 Tidskriftsartikel (refereegranskat)abstract The past evolution of precipitation and atmospheric convection in the Western Pacific Warm Pool (WPWP) is critical for global climate changes but is under debate because of its forcing mechanisms. Here, we present a high temporal resolution (∼156 years) grain-size record of core MD01–2385 over the last 140 kyr, in offshore northern New Guinea to reveal sediment dynamics as a proxy for precipitation changes. End-member analysis revealed that a two-endmember model was optimal. The end-member 1/end-member 2 (EM1/EM2) ratio could represent the variation in grain size and exhibited significant precessional cycles changes in phase with modelled Niño 3 SST anomaly from a global climate model transient simulation. From these data, we inferred orbital fluctuations in precipitation from tropical western Pacific islands, with general precipitation peaks during the time of perihelion at the boreal autumnal equinox (midpoint from a low to high precession index), corresponding to La Niña-like conditions and vice versa. Comparisons of our new record with published precipitation records showed that orbital precipitation changes in the WPWP are mainly dominated by El Niño-Southern Oscillation-like (ENSO-like) oscillations in the precession band, while the Intertropical Convergence Zone (ITCZ) mainly controls the distribution of precipitation over a larger spatial area.
31.	Wen, Qin, et al. (författare) Investigating the role of the tibetan plateau in ENSO variability 2020 Ingår i: Journal of Climate. - 0894-8755. ; 33:11, s. 4835-4852 Tidskriftsartikel (refereegranskat)abstract The role of the Tibetan Plateau (TP) in El Niño-Southern Oscillation (ENSO) variability is investigated using coupled model experiments with different topography setups. Removing the TP results in weakened trade winds in the tropical Pacific, an eastward shift of atmospheric convection center, a shallower mixed layer in the equatorial Pacific, and a flattened equatorial thermocline, which leads to an El Niño-like sea surface temperature (SST) response. In association with these mean climate changes in the tropical atmosphere- ocean system, the ENSO variability exhibits a much stronger amplitude in the world without the TP. Detailed diagnoses reveal that in the absence of the TP, both thermocline feedback in the eastern equatorial Pacific and Ekman pumping feedback in the central-eastern equatorial Pacific are enhanced substantially, leading to stronger ENSO variability. The changes of these two feedbacks are caused by the eastward shift of the atmospheric convection center and enhanced ocean sensitivity; the latter is due to the shallower mixed layer and flattened thermocline. This study suggests that the presence of the TP may be of fundamental importance for modern-day tropical climate variability; namely, the TP may have played a role in suppressing ENSO variability.
32.	Wu, Mousong, et al. (författare) Regional Responses of Vegetation Productivity to the Two Phases of ENSO 2024 Ingår i: Geophysical Research Letters. - 0094-8276. ; 51:8 Tidskriftsartikel (refereegranskat)abstract The two phases of El-Niño-Southern Oscillation (ENSO) influence both regional and global terrestrial vegetation productivity on inter-annual scales. However, the major drivers for the regional vegetation productivity and their controlling strengths during different phases of ENSO remain unclear. We herein disentangled the impacts of two phases of ENSO on regional carbon cycle using multiple data sets. We found that soil moisture predominantly accounts for ∼40% of the variability in regional vegetation productivity during ENSO events. Our results showed that the satellite-derived vegetation productivity proxies, gross primary productivity from data-driven models (FLUXCOM) and observation-constrained ecosystem model (Carbon Cycle Data Assimilation System) generally agree in depicting the contribution of soil moisture and air temperature in modulating regional vegetation productivity. However, the ensemble of weakly constrained ecosystem models exhibits non-negligible discrepancies in the roles of vapor pressure deficit and radiation over extra-tropics. This study highlights the significance of water in regulating regional vegetation productivity during ENSO.
33.	Xing, Xiuli, et al. (författare) Soil Moisture Assimilation Improves Terrestrial Biosphere Model GPP Responses to Sub-Annual Drought at Continental Scale 2023 Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 15:3 Tidskriftsartikel (refereegranskat)abstract Due to the substantial gross exchange fluxes with the atmosphere, the terrestrial carbon cycle plays a significant role in the global carbon budget. Drought commonly affects terrestrial carbon absorption negatively. Terrestrial biosphere models exhibit significant uncertainties in capturing the carbon flux response to drought, which have an impact on estimates of the global carbon budget. Through plant physiological processes, soil moisture tightly regulates the carbon cycle in the environment. Therefore, accurate observations of soil moisture may enhance the modeling of carbon fluxes in a model–data fusion framework. We employ the Carbon Cycle Data Assimilation System (CCDAS) to assimilate 36-year satellite-derived surface soil moisture observations in combination with flask samples of atmospheric CO2 concentrations. We find that, compared to the default model, the performance of optimized net ecosystem productivity (NEP) and gross primary productivity (GPP) has increased with the RMSEs reduced by 1.62 gC/m2/month and 10.84 gC/m2/month, which indicates the added value of the ESA-CCI soil moisture observations as a constraint on the terrestrial carbon cycle. Additionally, the combination of soil moisture and CO2 concentration in this study improves the representation of inter-annual variability of terrestrial carbon fluxes as well as the atmospheric CO2 growth rate. We thereby investigate the ability of the optimized GPP in responding to drought by comparing continentally aggregated GPP with the drought index. The assimilation of surface soil moisture has been shown to efficiently capture the influences of the sub-annual (≤9 months drought durations) and large-scale (e.g., regional to continental scales) droughts on GPP. This study highlights the significant potential of satellite soil moisture for constraining inter-annual models of the terrestrial biosphere’s carbon cycle and for illustrating how GPP responds to drought at a continental scale.
34.	Xiong, Zhifang, et al. (författare) Intensified aridity over the Indo-Pacific Warm Pool controlled by ice-sheet expansion during the Last Glacial Maximum 2022 Ingår i: Global and Planetary Change. - : Elsevier BV. - 0921-8181. ; 217 Tidskriftsartikel (refereegranskat)abstract The magnitude, direction and cause of precipitation changes across the Indo-Pacific Warm Pool (IPWP) during the Last Glacial Maximum (LGM) remain elusive. In particular, it is still inconclusive whether tropical or extra-tropical factors controlled such precipitation changes. Determining the spatio-temporal distribution of precipitation in the IPWP during the LGM is a valid strategy to address this issue, but the existing precipitation records are dominantly from maritime continents and marginal seas, with few data from pelagic oceans. In order to fill this gap, we analyzed the oxygen isotopic compositions of single Ethmodiscus rex diatom frustules (δ18OE. rex) from a sediment core (WPD-03) consisting of laminated diatom mats (LDMs) in the eastern Philippine Sea (EPS). δ18OE. rex was controlled mainly by sea-surface salinity variation and, thus, can reflect open-ocean precipitation changes across the IPWP. Our precipitation proxy records, in combination with existing published data, reveal spatial patterns of precipitation change that indicate overall drying across the IPWP during the LGM. Based on a comparison of paleoclimatic records with modeling results, we propose that extra-tropical factors (ice-sheet size) controlled precipitation variability in the IPWP during the LGM through a combination of zonal shifts of ENSO and meridional migration of the ITCZ. Strong aridity during the LGM prevented formation of a subsurface barrier layer and, hence, allowed accessing of sufficient nutrients to surface waters, stimulating blooms of E. rex and subsequent formation of LDMs in the IPWP. These findings suggest an important role for high-latitude climate in the tropical hydrological cycle during the LGM.
35.	Yang, Yiping, et al. (författare) A contracting Intertropical Convergence Zone during the Early Heinrich Stadial 1 2023 Ingår i: Nature Communications. - 2041-1723. ; 14:1 Tidskriftsartikel (refereegranskat)abstract Despite the fact that the response of tropical hydroclimate to North Atlantic cooling events during the Heinrich Stadial 1 (HS1) has been extensively studied in African, South American and Indonesia, the nature of such responses remains debated. Here we investigate the tropical hydroclimate pattern over the Indo-Asian-Australian monsoon region during the HS1 by integrating hydroclimatic records, and examining a δ18Oseawater record from Globigerinoides ruber (white) in the tropical Indian Ocean. Our findings indicate that tropical hydrological conditions were synchronously arid in both hemispheres during the early HS1 (~18.3-16.3 ka) in the Indo-Asian-Australian monsoon region, except for a narrow, wet hydrological belt in northern low latitudes, suggesting the existence of a contracted tropical precipitation belt at that time. This study reveals that the meltwater discharge and resulting changes in global temperatures and El Niño exerted a profound influence on the tropical hydroclimate in the Indo-Asian-Australian monsoon region during the early HS1.
36.	Zhang, Hongyue, et al. (författare) Asymmetric changes in temperature in the Arctic during the Holocene based on a transient run with the Community Earth System Model (CESM) 2023 Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 19:3, s. 665-680 Tidskriftsartikel (refereegranskat)abstract The Arctic temperature changes are closely linked to midlatitude weather variability and extreme events, which has attracted much attention in recent decades. Syntheses of proxy data from poleward of 60g gN indicate that there was asymmetric cooling of-1.54 and-0.61gg C for the Atlantic Arctic and the Pacific Arctic during the Holocene, respectively. We also present a similar consistent cooling pattern from an accelerated transient Holocene climate simulation based on the Community Earth System Model. Our results indicate that the asymmetric Holocene Arctic cooling trend is dominated by the winter temperature variability, with-0.67gg C cooling for the Atlantic Arctic and 0.09gg C warming for the Pacific Arctic, which is particularly pronounced at the proxy sites. Our findings indicate that sea ice in the North Atlantic expanded significantly during the late Holocene, while a sea ice retreat is seen in the North Pacific, amplifying the cooling in the Atlantic Arctic by the sea ice feedback. The positive Arctic dipole pattern, which promotes warm southerly winds to the North Pacific, offsets parts of the cooling trend in the Pacific Arctic. The Arctic dipole pattern also causes sea ice expansion in the North Atlantic, further amplifying the cooling asymmetry. We found that the temperature asymmetry is more pronounced in a simulation driven only by orbital forcing. The accelerated simulations lead to a partial delay in the feedback of climate processes. Therefore, we confirm the occurrence of the asymmetry of the Arctic temperature changes in un-Accelerated simulations using ECBilt-CLIO, IPSL, and in TraCE-21k.
37.	Zhang, Qiong, et al. (författare) Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-Earth3-LR 2021 Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 14:2, s. 1147-1169 Tidskriftsartikel (refereegranskat)abstract As global warming is proceeding due to rising greenhouse gas concentrations, the Earth system moves towards climate states that challenge adaptation. Past Earth system states are offering possible modelling systems for the global warming of the coming decades. These include the climate of the mid-Pliocene (similar to 3 Ma), the last interglacial (similar to 129-116 ka) and the mid-Holocene (similar to 6 ka). The simulations for these past warm periods are the key experiments in the Paleoclimate Model Intercomparison Project (PMIP) phase 4, contributing to phase 6 of the Coupled Model Intercomparison Project (CMIP6). Paleoclimate modelling has long been regarded as a robust out-of-sample test bed of the climate models used to project future climate changes. Here, we document the model setup for PMIP4 experiments with EC-Earth3-LR and present the large-scale features from the simulations for the mid-Holocene, the last interglacial and the mid-Pliocene. Using the pre-industrial climate as a reference state, we show global temperature changes, large-scale Hadley circulation and Walker circulation, polar warming, global monsoons and the climate variability modes - El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). EC-Earth3-LR simulates reasonable climate responses during past warm periods, as shown in the other PMIP4-CMIP6 model ensemble. The systematic comparison of these climate changes in past three warm periods in an individual model demonstrates the model's ability to capture the climate response under different climate forcings, providing potential implications for confidence in future projections with the EC-Earth model.
38.	Zhang, Qian, 1977-, et al. (författare) Unpacking spatial tensions : An interdisciplinary analysis of large-scale solar farm effects in drylands 2022 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract The last decades have seen rapid growth of renewable energy globally for accommodating the urgent need of mitigating climate change. Large-scale projects like solar farms are actively financed by transnational investors to get established in drylands like Sahara. The Earth-system model simulations on large-scale solar-farm scenarios show an increased regional rainfall and vegetation cover, analogue to a “green Sahara” that happened in the past. It will not only induce local climate and ecosystem changes but also prompt remote impacts globally through atmospheric teleconnections and ocean dynamics. This suggests that spatial tensions are inherent to climate change mitigation measures, where action in one place at a particular time impacts not only this place and the short time but place at distance and time in the future. Meanwhile, case studies in social sciences seem to suggest common unintended social consequences of the ongoing projects but no systematic assessment across these projects has been done. This study thus aims to pilot an interdisciplinary investigation of the multi-dimensional effects of large-scale renewable energy projects, mainly solar farms in drylands. Our literature review of the social effects across solar farms and other major types of renewable energy projects shows that, local host communities widely bear adverse social consequences from these projects despite there are benefits at regional, national, and transnational levels. Economic redistribution and social differentiation rapidly occur through land acquisition, livelihoods, compensation, and development programs, further dividing local communities and amplifying inequalities. These social effects could be further complicated by the likely local climate and ecosystem changes as shown by our Earth-system model simulations. Based on this combined analysis, we conclude that spatial tensions in the current climate change mitigation measures challenge the assumption of global common goods and the reach of global justices. We urge interdisciplinary research to combine their different expertise for developing integrated conceptual and methodological models, for better understanding the intersected effects of renewable energy projects on drylands, and for advising fair and just climate mitigation policy and measures.
39.	Zhao, Debo, et al. (författare) Delayed Collapse of the North Pacific Intermediate Water After the Glacial Termination 2021 Ingår i: Geophysical Research Letters. - 0094-8276. ; 48:13 Tidskriftsartikel (refereegranskat)abstract Carbon release from the North Pacific in glacial-interglacial cycles has been mainly linked to the North Pacific Intermediate Water (NPIW) formation and associated carbon/nutrient water upwelling and biological productivity changes. However, relationship between NPIW and atmospheric CO2 change in the early interglacial remains unclear. Here we report a high-resolution sediment record of NPIW evolution based on paleo-redox changes in the Western North Pacific during the last 400 ka. Our proxy and model results reveal a delayed collapse of NPIW after the glacial termination was coeval with decreased salinity of intermediate water and increased net rainfall in the North Pacific. Such weakened NPIW formation in the North Pacific probably make a contribution to maintain high atmospheric CO2 concentrations through weakened intermediate-to-deep ocean stratification and reduced subsurface biological pump net efficiency, countering the return to more stratified conditions in the Southern Ocean, which should drive down atmospheric CO2 during the early interglacial.
40.	Zhao, Debo, et al. (författare) Quaternary rainfall variability is governed by insolation in northern China and ice-sheet forcing in the South 2023 Ingår i: Communications Earth and Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 4:1 Tidskriftsartikel (refereegranskat)abstract Quaternary Asian low-latitude hydroclimate cyclicity has long been attributed to insolation forcing, in contrast to the dominant ice-sheet and CO2 controls identified in mid-high-latitude regions. However, debates exist regarding these rainfall variations and forcings due to inconsistent reconstructions and simulations. Here, by combining rainfall proxy records with multi-model simulations, dominant 23 ka rainfall cycle in northern China and 100 ka rainfall cycle in southern China and Southeast Asia were found. We propose that rainfall mainly occurs in summer in the north, primarily driven by insolation. Rainfall in the south is largely forced by high-latitude ice sheets, with enhanced spring and autumn rainfall in southern China and weakened rainfall in western Maritime Continent during glacial periods. This study highlights the seasonal contributions to orbital-scale rainfall changes, and sheds light on the Asian hydroclimate conditions associated with high-low-latitude climate interactions.
41.	Zhao, Debo, et al. (författare) Response of heterogeneous rainfall variability in East Asia to Hadley circulation reorganization during the late Quaternary 2020 Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 247 Tidskriftsartikel (refereegranskat)abstract The Quaternary East Asian summer rainfall evolution reconstructed with the Chinese speleothem δ18O records shows the consistency throughout the region of East Asia, and has long been considered equivalent to the monsoon intensity. Its variation and teleconnection with global climate were usually interpreted by the meridional shift of the intertropical convergence zone (ITCZ). However, many other proxy records, climate simulations and meteorological observations suggest the inconsistent rainfall patterns in the different regions of East Asia on multi-timescales. Such spatial heterogeneity in Quaternary East Asian summer rainfall indicates that the hydroclimate in this region is not fully paced by the migration of the ITCZ. Here, we report a sediment record of rainfall evolution during the last 400 ka in the northern East China Sea, and this record, in combination with a transient climate model simulation, indicates an out-of-phase relationship between rainfall over middle-southeastern East Asia and northern and southwestern East Asia on the precession band, with high boreal summer insolation corresponding to the increased rainfall intensity in the northern and southwestern East Asia, however, decreased rainfall intensity in the middle East Asia. We attribute this regional heterogeneity in East Asian rainfall to the reorganization of the Hadley circulation, including shifts in the ascending branch (ITCZ) and descending branch (subtropical westerly jet), in response to changes in the hemispheric meridional temperature gradient. Our results highlight the crucial role of the Hadley circulation in the East Asian hydroclimate and have important implications for future climate projections.
42.	Zheng, Minjie, et al. (författare) Modeling Atmospheric Transport of Cosmogenic Radionuclide 10Be Using GEOS-Chem 14.1.1 and ECHAM6.3-HAM2.3 : Implications for Solar and Geomagnetic Reconstructions 2024 Ingår i: Geophysical Research Letters. - 0094-8276. ; 51:2 Tidskriftsartikel (refereegranskat)abstract A prerequisite to applying 10Be in natural archives for solar and geomagnetic reconstructions is to know how 10Be deposition reflects atmospheric production changes. However, this relationship remains debated. To address this, we use two state-of-the-art global models GEOS-Chem and ECHAM6.3-HAM2.3 with the latest beryllium production model. During solar modulation, both models suggest that 10Be deposition reacts proportionally to global production changes, with minor latitudinal deposition biases (<5%). During geomagnetic modulation, however, 10Be deposition changes are enhanced by ∼15% in the tropics and attenuated by 20%–35% in subtropical and polar regions compared to global production changes. Such changes are also hemispherically asymmetric, attributed to asymmetric production between hemispheres. For the solar proton event in 774/5 CE, 10Be shows a 15% higher deposition increase in polar regions than in tropics. This study highlights the importance of atmospheric mixing when comparing 10Be from different locations or to independent geomagnetic field records.
43.	Zheng, Minjie, et al. (författare) Simulations of7Be and10Be with the GEOS-Chem global model v14.0.2 using state-of-The-Art production rates 2023 Ingår i: Geoscientific Model Development. - 1991-959X. ; 16:23, s. 7037-7057 Tidskriftsartikel (refereegranskat)abstract The cosmogenic radionuclides 7Be and 10Be are useful tracers for atmospheric transport studies. Combining 7Be and 10Be measurements with an atmospheric transport model can not only improve our understanding of the radionuclide transport and deposition processes but also provide an evaluation of the transport process in the model. To simulate these aerosol tracers, it is critical to evaluate the influence of radionuclide production uncertainties on simulations. Here we use the GEOS-Chem chemical transport model driven by the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis to simulate 7Be and 10Be with the state-of-The-Art production rate from the CRAC:Be (Cosmic Ray Atmospheric Cascade: Beryllium) model considering realistic spatial geomagnetic cutoff rigidities (denoted as P16spa). We also perform two sensitivity simulations: one with the default production rate in GEOS-Chem based on an empirical approach (denoted as LP67) and the other with the production rate from the CRAC:Be but considering only geomagnetic cutoff rigidities for a geocentric axial dipole (denoted as P16). The model results are comprehensively evaluated with a large number of measurements including surface air concentrations and deposition fluxes. The simulation with the P16spa production can reproduce the absolute values and temporal variability of 7Be and 10Be surface concentrations and deposition fluxes on annual and sub-Annual scales, as well as the vertical profiles of air concentrations. The simulation with the LP67 production tends to overestimate the absolute values of 7Be and 10Be concentrations. The P16 simulation suggests less than 10% differences compared to P16spa but a significant positive bias (g1/418%) in the 7Be deposition fluxes over East Asia. We find that the deposition fluxes are more sensitive to the production in the troposphere and downward transport from the stratosphere. Independent of the production models, surface air concentrations and deposition fluxes from all simulations show similar seasonal variations, suggesting a dominant meteorological influence. The model can also reasonably simulate the stratosphere-Troposphere exchange process of 7Be and 10Be by producing stratospheric contribution and 10Be/7Be ratio values that agree with measurements. Finally, we illustrate the importance of including the time-varying solar modulations in the production calculation, which significantly improve the agreement between model results and measurements, especially at mid-latitudes and high latitudes. Reduced uncertainties in the production rates, as demonstrated in this study, improve the utility of 7Be and 10Be as aerosol tracers for evaluating and testing transport and scavenging processes in global models. For future GEOS-Chem simulations of 7Be and 10Be, we recommend using the P16spa (versus default LP67) production rate.
44.	Zheng, Minjie, et al. (författare) Solar, Atmospheric, and Volcanic Impacts on 10Be Depositions in Greenland and Antarctica During the Last 100 Years 2023 Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 128:16 Tidskriftsartikel (refereegranskat)abstract Cosmogenic radionuclides (e.g., 10Be) from ice cores are a powerful tool for solar reconstructions back in time. However, superimposed on the solar signal, other factors like weather/climate and volcanic influences on 10Be can complicate the interpretation of 10Be data. A comprehensive study of 10Be records over the recent period, when atmospheric 10Be production and meteorological conditions are relatively well-known, can improve our interpretation of 10Be records. Here we conduct a systematic study of the production and climate/volcanic signals in Antarctica and Greenland 10Be records, including a new 10Be record from the East GReenland Ice-core Project site. Greenland and Antarctica records show significant decreasing trends (5%–6.5%/decade) for 1900–1950, which is comparable with the expected production rate inferred from sunspot observations. By comparing 10Be records with reanalysis data and atmospheric circulation patterns, 10Be records from Southern/Southeastern Greenland are significantly correlated with the Scandinavia pattern. Stacking 10Be records from different locations can enhance the production signal. However, this approach is not always straightforward as uncertainties in some records can lead to a weaker solar signal. A strategy can be employed to select records for the bipolar stack by comparing Greenland records with Antarctica records, assuming the shared signal is a production signal. Finally, we observe significant increases (36%–64%) in 10Be depositions in Greenland related to the Agung eruption. This large increase in Greenland 10Be records after the Agung eruption, could be partly explained by the enhanced air mass transport from mid-latitudes coinciding with the decreased precipitation en-route.
45.	Zhengyao, Lu, et al. (författare) Dynamic Vegetation Simulations of the Mid-Holocene Green Sahara 2018 Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 45:16, s. 8294-8303 Tidskriftsartikel (refereegranskat)abstract The Green Sahara is a period when North Africa was characterized by vegetation cover and wetlands. To qualitatively identify the orbital-climatic causation of the Green Sahara regime, we performed dynamic vegetation model (LPJ-GUESS) simulations, driven by climate forcings from coupled general circulation model (EC-Earth) simulations for the mid-Holocene, in which the vegetation is prescribed to be either modern desert or artificially vegetated with a reduced dust load. LPJ-GUESS simulates a vegetated Sahara covered by both herbaceous and woody vegetation types consistent with proxy reconstructions only in the latter scenario. Sensitivity experiments identify interactions required to capture the northward extension of vegetation. Increased precipitation is the main driver of the vegetation extent changes, and the temperature anomalies determine the plant functional types mainly through altered fire disturbance. Furthermore, the simulated vegetation composition also depends on the correct representation of soil texture in a humid environment like Green Sahara. Plain Language Summary The Sahara Desert experienced wet and vegetated conditions in the past. The vegetation-atmosphere feedbacks play an important role in sustaining vegetation cover in that region. Here we perform dynamic vegetation model simulations to reproduce herbaceous and woody vegetation types in North Africa 6,000 years ago. We further investigate separately the relative importance of various climate forcings (precipitation, temperature, radiation, and soil temperature) in inducing the Green Sahara. We conclude that vegetation extent is mainly determined by precipitation, while vegetation composition is mainly determined by temperature, and the correct representation of soil texture is also important. Future modeling work considering dynamic vegetation-atmosphere feedbacks could be valuable for providing analogues to Sahara/Sahel climate and vegetation regimes in the past and future.
46.	Zhong, Yi, et al. (författare) Coupled Impacts of Atmospheric Circulation and Sea-Ice on Late Pleistocene Terrigenous Sediment Dynamics in the Subarctic Pacific Ocean 2021 Ingår i: Geophysical Research Letters. - 0094-8276. ; 48:19 Tidskriftsartikel (refereegranskat)abstract Processes controlling environmental change in the subarctic Pacific Ocean on millennial to orbital timescales are not well understood. Here we use a 230-kyr sedimentary record from the northwest Pacific Ocean to assess the response of late Pleistocene sediment dynamics to orbital forcing. Combining a source-to-sink perspective based on sedimentological records with climate model reanalysis, we reveal that fluctuations in sediment provenance were closely linked to obliquity-forced changes in atmospheric circulation modes. Specifically, the position of the Aleutian Low controlled sediment transport from the Bering Sea and Aleutian Arc sources. Furthermore, a distinct shift in North Pacific ocean circulation during the Last Glacial Maximum may have been related to a strengthened Siberian High. The coincidence of atmospheric mode switches with changes in sea-ice extent and North Pacific Intermediate Water formation in the marginal seas suggests that this coupled ocean-atmosphere system may have acted as a regional amplifier of global climate variability.
47.	Zhong, Yi, et al. (författare) Humidification of Central Asia and equatorward shifts of westerly winds since the late Pliocene 2022 Ingår i: Communications Earth and Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 3:1 Tidskriftsartikel (refereegranskat)abstract The production, transport, and deposition of mineral dust exert major influences on climate change and Earth’s biogeochemical cycles. Furthermore, their imprint, as recorded in pelagic sediments, provides an avenue for determining past changes in terrestrial aridity and atmospheric circulation patterns in response to global climate change. Here, by examining geochemical and magnetic data obtained from a ferromanganese crust in the western Pacific Ocean, we investigate the eolian dust source-region conditions and dust transport mechanisms from the Asian interior to the Pacific Ocean since the Pliocene. We identify a gradual provenance change in the dust source regions, from a dominant Gobi Desert source during the early Pliocene to a mixed Gobi-Taklimakan Desert source during the late Pliocene and Pleistocene, alongside increasing chemical weathering in those source areas. Climate model simulations suggest that these changes were related to an equatorward shift of the westerly jet and humidification of Central Asia during the gradual transition from a warm Pliocene climate to the cool Pleistocene.
48.	Zhong, Yi, et al. (författare) Orbital Controls on North Pacific Dust Flux During the Late Quaternary 2024 Ingår i: Geophysical Research Letters. - 0094-8276. ; 51:4 Tidskriftsartikel (refereegranskat)abstract Airborne mineral dust is sensitive to climatic changes, but its response to orbital forcing is still not fully understood. Here, we present a reconstruction of dust input to the Subarctic Pacific Ocean covering the past 190 kyr. The dust composition record is indicative of source moisture conditions, which were dominated by precessional variations. In contrast, the dust flux record is dominated by obliquity variations and displays an out-of-phase relationship with a dust record from the mid-latitude North Pacific Ocean. Climate model simulations suggest precession likely drove changes in the aridity and extent of dust source regions. Additionally, the obliquity variations in dust flux can be explained by meridional shifts in the North Pacific westerly jet, driven by changes in the meridional atmospheric temperature gradient. Overall, our findings suggest that North Pacific dust input was primarily modulated by orbital-controlled source aridity and the strength and position of the westerly winds.
49.	Zhong, Yi, et al. (författare) Paleoclimate evolution of the North Pacific Ocean during the late Quaternary : Progress and challenges 2023 Ingår i: Geosystems and Geoenvironment. - : Elsevier BV. - 2772-8838. ; 2:1 Forskningsöversikt (refereegranskat)abstract High- and low-latitude climatic processes in the North Pacific Ocean are important components of the global climate system. For example, the interplay among North Pacific atmospheric circulation, ocean circulation, and biological productivity affects atmospheric carbon dioxide levels and marine oxygen concentrations. Here we review recent research on the North Pacific paleoclimatic and paleoceanographic evolution during the late Quaternary and its response to external forcings such as orbital insolation, ice-sheet extent, and greenhouse gas concentrations. First, we summarize the principles and application of relative paleointensity as a critical chronological tool in North Pacific paleoclimate research. Second, we illustrate the latest discoveries on the interaction between North Pacific Intermediate Water formation and high-to-low latitude teleconnection processes. Third, recent progress in linking dust fluxes and marine productivity and their global significance for the carbon cycle are presented. Finally, several key scientific problems are highlighted for future research on ocean-atmosphere-climate interactions in the North Pacific, pointing to the importance of combining paleo-records and modeling simulations. Overall, this review also aims to provide a broad insight into possible future changes of ocean-atmosphere circulation in the North Pacific region under a rapidly warming climate.
50.	Zhou, Putian, et al. (författare) Simulating dust emissions and secondary organic aerosol formation over northern Africa during the mid-Holocene Green Sahara period 2023 Ingår i: Climate of the Past. - 1814-9324 .- 1814-9332. ; 19:12, s. 2445-2462 Tidskriftsartikel (refereegranskat)abstract Paleo-proxy data indicate that a “Green Sahara” thrived in northern Africa during the early- to mid-Holocene (MH; 11 000 to 5000 years before present), characterized by more vegetation cover and reduced dust emissions. Utilizing a state-of-the-art atmospheric chemical transport model, TM5-MP, we assessed the changes in biogenic volatile organic compound (BVOC) emissions, dust emissions and secondary organic aerosol (SOA) concentrations in northern Africa during this period relative to the pre-industrial (PI) period. Our simulations show that dust emissions reduced from 280.6 Tg a−1 in the PI to 26.8 Tg a−1 in the MH, agreeing with indications from eight marine sediment records in the Atlantic Ocean. The northward expansion in northern Africa resulted in an increase in annual emissions of isoprene and monoterpenes during the MH, around 4.3 and 3.5 times higher than that in the PI period, respectively, causing a 1.9-times increase in the SOA surface concentration. Concurrently, enhanced BVOC emissions consumed more hydroxyl radical (OH), resulting in less sulfate formation. This effect counteracted the enhanced SOA surface concentration, altogether leading to a 17 % increase in the cloud condensation nuclei at 0.2 % super saturation over northern Africa. Our simulations provide consistent emission datasets of BVOCs, dust and the SOA formation aligned with the northward shift of vegetation during the “Green Sahara” period, which could serve as a benchmark for MH aerosol input in future Earth system model simulation experiments.

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