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El Niño‐Southern Oscillation (ENSO) flavors have been defined to characterize ENSO events and their teleconnections. Studying El Niño flavor evolution during the Holocene period can provide valuable insights into changes over long time scales. We investigated ENSO flavor evolution using simulations spanning the last 6,000 years and present‐day observations. Two approaches to computing ENSO flavors, in agreement in the present, lead to opposite trends in the last 6,000 years. The methods also differ significantly in their representation of ENSO flavor patterns. However, incorporating the sensitivity of the methods to calibration periods and mean state changes yields similar interpretations of ENSO variability changes. Both methods suggest an increase in El Niño variability spreading to the west and east tropical Pacific over the past 6,000 years. Standardizing El Niño flavor definitions is necessary for meaningful comparisons between studies and robust climate variability analysis. Plain Language Summary: El Niño events are the dominant mode of interannual variability. Looking at El Niño events in the past, during the Holocene period, could shed light on the linkages between the characteristics of El Niño patterns and changes in the climate mean state. Different indicators are used to characterize El Niño pattern diversity depending on the available data. While equivalent under present‐day conditions, these indicators lead to opposite conclusions on the evolution of El Niño patterns over the last 6,000 years. An in‐depth analysis indicates that all the methods suggest that El Niño‐related variance has spread to the west and east tropical Pacific over time when accounting for sensitivity to calibration periods and changes in the mean state during the Holocene. Agreeing on a standard definition for El Niño flavors is essential for accurate comparison between studies and for avoiding misleading conclusions. Key Points: Two equivalent indicators of El Niño‐Southern Oscillation (ENSO) flavors in the present give opposite evolution during the HoloceneResults show that the conflict between methods comes from the representation of ENSO flavor patterns on short and long timescalesA proper assessment must account for the indicator's patterns and calibration, as well as the climate mean‐state change over time [ABSTRACT FROM AUTHOR]

A single‐column model approach conducted in the context of the Madden–Julian Oscillation through the CINDY2011/Dynamics of the Madden–Julian Oscillation field campaign is used to disentangle the respective role of the parameterizations of surface turbulent fluxes and of model atmospheric physics in controlling the surface latent heat flux. The major differences between the models used in this study occur during the suppressed phases of deep convection. They are attributed to differences in model atmospheric physics which is shown to control the near‐surface relative humidity and thereby the surface latent heat flux. In contrast, during active phases of deep convection, turbulent air‐sea flux parameterizations impact the latent heat flux through the drag coefficient and can represent two thirds of the divergence caused by the different atmospheric physics. The combined effects need to be accounted for to improve both the representation of latent heat flux and the atmospheric variables used to compute it. Key Points: A single‐column model approach is used in the context of the CINDY2011/DYNAMO field campaign to compare models results during a MJO eventNear‐surface relative humidity is a key constraint for the surface latent heat flux and the way it is simulated by different modelsCompared to atmospheric physics, air‐sea flux parameterization has a relatively large impact only during active convective phases [ABSTRACT FROM AUTHOR]

This paper is the first of a series of four GMD papers on the PMIP4-CMIP6 experiments. Part 2 (Otto-Bliesner et al., 2017) gives details about the two PMIP4-CMIP6 interglacial experiments, Part 3 (Jungclaus et al., 2017) about the last millennium experiment, and Part 4 (Kageyama et al., 2017) about the Last Glacial Maximum experiment. The mid-Pliocene Warm Period experiment is part of the Pliocene Model Intercomparison Project (PlioMIP) - Phase 2, detailed in Haywood et al. (2016). The goal of the Paleoclimate Modelling Intercomparison Project (PMIP) is to understand the response of the climate system to different climate forcings for documented climatic states very different from the present and historical climates. Through comparison with observations of the environmental impact of these climate changes, or with climate reconstructions based on physical, chemical, or biological records, PMIP also addresses the issue of how well state-of-the-art numerical models simulate climate change. Climate models are usually developed using the present and historical climates as references, but climate projections show that future climates will lie well outside these conditions. Palaeoclimates very different from these reference states therefore provide stringent tests for state-of-the-art models and a way to assess whether their sensitivity to forcings is compatible with palaeoclimatic evidence. Simulations of five different periods have been designed to address the objectives of the sixth phase of the Coupled Model Intercomparison Project (CMIP6): the millennium prior to the industrial epoch (CMIP6 name: past1000); the mid-Holocene, 6000 years ago (midHolocene); the Last Glacial Maximum, 21 000 years ago (lgm); the Last Interglacial, 127 000 years ago (lig127k); and the mid-Pliocene Warm Period, 3.2 million years ago (midPliocene-eoi400). These climatic periods are well documented by palaeoclimatic and palaeoenvironmental records, with climate and environmental changes relevant for the study and projection of future climate changes. This paper describes the motivation for the choice of these periods and the design of the numerical experiments and database requests, with a focus on their novel features compared to the experiments performed in previous phases of PMIP and CMIP. It also outlines the analysis plan that takes advantage of the comparisons of the results across periods and across CMIP6 in collaboration with other MIPs. [ABSTRACT FROM AUTHOR]

Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127 000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern ones. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land–sea contrast and highlatitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional sensitivity experiments, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically. [ABSTRACT FROM AUTHOR]

Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for Tier 1 simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127,000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land-sea contrast and high-latitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional CMIP6 Tier 2 and Tier 3 sensitivity experiments of PMIP4, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically. [ABSTRACT FROM AUTHOR]

The goal of the Palaeoclimate Modelling Intercomparison Project (PMIP) is to understand the response of the climate system to changes in different climate forcings and to feedbacks. Through comparison with observations of the environmental impacts of these climate changes, or with climate reconstructions based on physical, chemical or biological records, PMIP also addresses the issue of how well state-of-the-art models simulate climate changes. Palaeoclimate states are radically different from those of the recent past documented by the instrumental record and thus provide an out-of-sample test of the models used for future climate projections and a way to assess whether they have the correct sensitivity to forcings and feedbacks. Five distinctly different periods have been selected as focus for the core palaeoclimate experiments that are designed to contribute to the objectives of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This manuscript describes the motivation for the choice of these periods and the design of the numerical experiments, with a focus upon their novel features compared to the experiments performed in previous phases of PMIP and CMIP as well as the benefits of common analyses of the models across multiple climate states. It also describes the information needed to document each experiment and the model outputs required for analysis and benchmarking. [ABSTRACT FROM AUTHOR]

Simulations of the West African monsoon (WAM) for the present-day climate (0 ka) and the mid-Holocene (6 ka) using the coupled models from the Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) are assessed in this study. The authors first compare the ensemble simulations with modern observations and proxy estimates of past precipitation, showing that the PMIP2 model median captures the basic features of the WAM for 0 ka and the changes at 6 ka, despite systematic biases in the preindustrial (PI) simulations and underestimates of the northward extent and intensity of precipitation changes. The model spread is then discussed based on a classification of the monsoonal convective regimes for a subset of seven coupled models. Two major categories of model are defined based on their differences in simulating deep and moderate convective regimes in the PI simulations. Changes in precipitation at 6 ka are dominated by changes in the large-scale dynamics for most of the PMIP2 models and are characterized by a shift in the monsoonal circulation toward deeper convective regimes. Consequently, changes in the total precipitation at 6 ka depend on the changes in convective regimes and the characteristics of these regimes in the PI simulations. The results indicate that systematic model biases in simulating the radiation and heat fluxes could explain the damping of the meridional temperature gradient over West Africa and thereby the underestimation of precipitation in the Sahel-Sahara region. [ABSTRACT FROM AUTHOR]

The too diverse representation of ENSO in a coupled GCM limits one’s ability to describe future change of its properties. Several studies pointed to the key role of atmosphere feedbacks in contributing to this diversity. These feedbacks are analyzed here in two simulations of a coupled GCM that differ only by the parameterization of deep atmospheric convection and the associated clouds. Using the Kerry–Emanuel (KE) scheme in the L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL CM4; KE simulation), ENSO has about the right amplitude, whereas it is almost suppressed when using the Tiedke (TI) scheme. Quantifying both the dynamical Bjerknes feedback and the heat flux feedback in KE, TI, and the corresponding Atmospheric Model Intercomparison Project (AMIP) atmosphere-only simulations, it is shown that the suppression of ENSO in TI is due to a doubling of the damping via heat flux feedback. Because the Bjerknes positive feedback is weak in both simulations, the KE simulation exhibits the right ENSO amplitude owing to an error compensation between a too weak heat flux feedback and a too weak Bjerknes feedback. In TI, the heat flux feedback strength is closer to estimates from observations and reanalysis, leading to ENSO suppression. The shortwave heat flux and, to a lesser extent, the latent heat flux feedbacks are the dominant contributors to the change between TI and KE. The shortwave heat flux feedback differences are traced back to a modified distribution of the large-scale regimes of deep convection (negative feedback) and subsidence (positive feedback) in the east Pacific. These are further associated with the model systematic errors. It is argued that a systematic and detailed evaluation of atmosphere feedbacks during ENSO is a necessary step to fully understand its simulation in coupled GCMs. [ABSTRACT FROM AUTHOR]

Abstract: The outputs of the climate simulated by two General Circulation Models (GCMs), (IPSL and UGAMP) have been used to force a vegetation model (LPJ-GUESS) to analyze the Holocene African humid period (AHP) and related vegetation changes over the 18°W-35°E, 5°S-25°N region. At the continental scale, simulations with the two models confirm the intensified African monsoon during the Holocene as compared to now, and the early but gradual termination of the AHP in eastern regions as compared to western regions. At the regional scale, the two GCMs results present important differences in the timing of the AHP, its spatial extent and the summer rainfall amplitude. Consequently, the vegetation model simulates changes that are globally in agreement with pollen data, but with large differences according to the region and the model considered. During the AHP, the IPSL climate induced proper vegetation changes in the eastern Sahara and in the Sahel, whereas the UGAMP climate induced correct changes in the western Sahara and in the equatorial zone. [Copyright &y& Elsevier]

Abstract: The role of ocean feedback on monsoon variations at 6 and 9.5kyr Before Present (BP) compared to present-day is investigated by using sets of simulations computed with the IPSL–CM4 ocean–atmosphere coupled model and simulations with the atmospheric model only with the SST prescribed to the present-day simulation for the coupled model. This work is complementary to the study by Marzin and Braconnot (2009) who have analyzed in detail the response of Indian and African monsoons to changes in insolation at 6 and 9.5kyr BP using the IPSL–CM4 coupled model. The monsoon rainfall was intensified at 6 and 9.5kyr BP compared to 0kyr BP as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. In this paper, the impact of the ocean feedback is analysed for the Indian, East-Asian and African monsoons. The response of the ocean to the 6 and 9.5kyr BP insolation forcing shares similarities between the two periods, but we highlight local differences and a delay in the response of the surface ocean between 6 and 9.5kyr BP. The ocean feedback is shown to be positive for the early stage of the African monsoon. A dipole of SST in the tropical Atlantic favouring the earlier build-up of the monsoon in the 6 and 9.5kyr BP coupled simulations. However, it is strongly negative for the Indian and East Asian monsoons, and of stronger amplitude at 9.5 than at 6kyr BP over India. In these Asian regions, the convection is more active over the ocean than over the continent during the late monsoon season due to the ocean feedback. The results are consistent with previous studies about 6kyr BP climate. In addition, it is shown that the ocean feedback is not sufficient to explain the relative amplifications of the different monsoon systems within the three periods of the Holocene, but that the mechanisms such as the effect of the precession on the seasonal cycle of monsoons as discussed in Marzin and Braconnot (2009) are more plausible. [Copyright &y& Elsevier]

This paper briefly surveys areas of paleoclimate modeling notable for recent progress. New ideas, including hypotheses giving a pivotal role to sea ice, have revitalized the low-order models used to simulate the time evolution of glacial cycles through the Pleistocene, a prohibitive length of time for comprehensive general circulation models (GCMs). In a recent breakthrough, however, GCMs have succeeded in simulating the onset of glaciations. This occurs at times (most recently, 115 kyr b.p.) when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm, enhancing the moisture source. More generally, the improvement in models has allowed simulations of key periods such as the Last Glacial Maximum and the mid-Holocene that compare more favorably and in more detail with paleoproxy data. These models now simulate ENSO cycles, and some of them have been shown to reproduce the reduction of ENSO activity observed in the early to middle Holocene. Modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time. An urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs (i.e., Dansgaard–Oescher cycles, Heinrich events, and the Younger Dryas). Efforts have begun to simulate the last millennium. Over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output. Simulations of these natural variations test the models relied on for future climate change projections. They provide better estimates of the internal and naturally forced variability at centennial time scales, elucidating how unusual the recent global temperature trends are. [ABSTRACT FROM AUTHOR]

Past climate conditions represent a many faceted challenge for our understanding of the natural variability of the climate system and of the major interactions between its various components. This paper presents a short review of the evolution of past climate simulations using three-dimensional models for the Last Glacial Maximum and the mid-Holocene climates. Its shows how the increase in model complexity allows to test new ideas and mechanisms, using results from the Paleoclimate Modeling Intercomparison Project (PMIP) and from simulations where the atmospheric model is coupled to an ocean or a vegetation model. The two periods are respectively representative of the response of the climate system to an annual mean forcing and a seasonal forcing. To cite this article: P. Braconnot, C. R. Geoscience 336 (2004). [Copyright &y& Elsevier]

State-of-the-art Earth system models, like the ones used in the Coupled Model Intercomparison Project Phase 6 (CMIP6), suffer from temporal inconsistencies at the ocean–atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on the Schwarz iterative method. Despite its large computational cost compared to standard coupling methods, which makes the algorithm implementation impractical for production runs, the Schwarz method is useful to evaluate some of the errors made in state-of-the-art ocean–atmosphere coupled models (e.g., in the representation of the processes related to diurnal cycle), as illustrated by the present study. IPSL-CM6-SW-VLR is a low-resolution version of the IPSL-CM6 coupled model with a simplified land surface model, implementing a Schwarz iterative coupling scheme. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme (referred to as the parallel algorithm) show large differences after sunrise and before sunset, when the external forcing (insolation at the top of the atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100 % of the solution, even with a small coupling period, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method, which leads to a doubling of the computing cost. Besides the parallel algorithm used in IPSL-CM6, we also test a so-called sequential atmosphere-first algorithm, which is used in some coupled ocean–atmosphere models. We show that the sequential algorithm improves the numerical results compared to the parallel one at the expanse of a loss of parallelism. The present study focuses on the ocean–atmosphere interface with no sea ice. The problem with three components (ocean–sea ice–atmosphere) remains to be investigated. [ABSTRACT FROM AUTHOR]

The Indian Summer Monsoon Rainfall (ISMR) varies on a wide range of timescales, from interannual to multi-centennial. Slight variations of seasonal rainfall amounts strongly affect livelihood, which mostly relies on rain-fed agriculture. Improving our understanding of multi-scale variations of ISMR is thus critical to advance its potential predictability and reduce society vulnerability to climate change.Here, we explore such multi-scale variations by analyzing a multi-complexity ensemble of transient simulations ran with the IPSL Earth system model from mid- to late Holocene (6000 years BP to 1950). First, we show that orbital forcing induces low-frequency changes in the ISMR mean state, with near-linear drying from mid- to late Holocene. The relative long term response between land and ocean is compared to available observations from pollen and lakes over land and ocean sediments to assess the realism of the simulation over the broad Indian sector. Second, we show that interannual-to-decadal variability increases along the period despite the drying trend of the ISMR mean state. We isolated a relationship with a concomitant increase in SST variance in the tropical Pacific, especially in its eastern equatorial part. Is the increased variability in precipitation and SST part of the same large-scale response of local variability to long term insolation? Or is the increased variability in precipitation induced by the strengthening of ENSO variability? In addition, we'll use high resolution paleo records from coral and shell over the Indo-Pacific sector and from speleothems and tree rings over land to assess the realism of the simulated changes in long term teleconnections. [ABSTRACT FROM AUTHOR]

The turbulent fluxes across the ocean/atmosphere interface, which represent one of the main drivers of the global atmospheric and oceanic circulations, are parameterized in climate models. To improve the representation of turbulent fluxes, a new parameterization has been developed in the IPSL model. It takes into account the gustiness effects which are calculated thanks to the coupling between the bulk formula (used to compute the surface turbulent fluxes) and the thermal and the convection schemes (that provide the deep and free convection effect on the subgrid variability of surface winds).Here we investigate how this new effect modulates the ocean-atmosphere interactions and feedbacks. We consider both atmosphere only and coupled ocean-atmosphere simulations ran with the IPSL climate model. We focus on the thermodynamical and the dynamical adjustments of the atmospheric and oceanic circulation induced by the coupling between gustiness and turbulent fluxes.In atmospheric stand-alone simulations, the gustiness has a direct effect on turbulent fluxes through the increase of low-level wind used for their calculation. This increases wind stress and leads to indirect effect by reducing the low-level wind. In addition, gustiness significantly impacts latent heat flux in weak wind regions. This modifies the zonal humidity gradient in the Pacific Ocean, which affects the Walker circulation. In coupled ocean-atmosphere simulations, part of the latent heat flux effect is offset by the ocean adjustment through ocean heat storage and sea surface temperature changes. A feedback loop between the wind stress and ocean circulation also affects the overall ocean and atmospheric circulations. The relative amplitude of these feedbacks will be discussed, considering the role of changes in surface turbulent fluxes on ocean and atmospheric circulations, which control heat transports and inter-basin teleconnections. [ABSTRACT FROM AUTHOR]

The Indian and West African monsoons vary on a large variety of timescales, with alternation of extremely wet and dry years having huge impacts on local economy and society. Some of these extreme monsoon events result from the chaotic nature of climate. They are also favored by teleconnection with various modes of climate variability ranging from interannual (e.g., El Nino Southern Oscillation and the Indian Ocean dipole) to decadal (e.g., Atlantic dipole and Atlantic meridional oscillation) timescales. However the relationships between extreme monsoon years and these multi-time scales of varability are not fully understood. The last 6000 years offers a wide range of test cases to analyze these relationships and the diversity of events.The mid-Holocene, 6000 years BP, is characterized by increased monsoon precipitation in West Africa and India compared to modern conditions. This is caused by the Earth's orbital configuration. The seasonal cycle of incoming solar radiation was enhanced in the northern Hemisphere and reduced it in the southern Hemisphere, thereby enhancing the monsoon flow between ocean and land. The retreat of the two monsoon systems back to present day conditions has been punctuated by periods of rapid transitions and periods of mega droughts that had significant imprints on the environment.We consider extreme monsoon events in a multi-complexity ensemble of transient simulations of the last 6000 years ran with the IPSL Earth system model. These simulations allow us to analyze the monsoon response to Earth's orbit and trace gazes, and how multiscale interactions affect extreme African and Indian monsoon events. First results indicate that interannual to decadal variability in Indian precipitation increases with time when its mean amount decreases. This likely relates to increased ENSO variability in the model, the latter being consistent with paleo-reconstructions from coral and bivalves in the Pacific. By sorting the results in different frequency band or type of extreme events, we also explore the relative changes in variability between the Indian and African monsoon, the characteristics of period with high monsoon or prolonged droughts as well as their relationship with modes of climate variability. [ABSTRACT FROM AUTHOR]

State-of-the-art Earth System models, like the ones used in the CMIP6 intercomparison project, suffer from temporal inconsistencies at the ocean-atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere, and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on Schwarz methods. Despite the fact that the computational cost is large compared to standard coupling methods, which makes the method impractical as is for production runs, Schwarz algorithms are useful to evaluate some of the errors made in state-of-the-art ocean-atmosphere coupled models (e.g. in the representation of the processes related to diurnal cycle), as illustrated by the present study. A new coupling scheme based on such iterative method has been implemented in the IPSL coupled model. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme show large differences at sunrise and sunset, when the external forcing (insolation at top of atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100% of the solution, even with a small coupling time step, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method which leads to a doubling of the computing cost. A thorough design of the first guess to initialize the iterative process may yield a solution that reduces the error with only one iteration. The present study focuses on the ocean-atmosphere interface, with no sea-ice. The problem with three components (ocean/sea-ice/atmosphere) remains to be investigated. [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid‐to‐late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid‐ and late‐Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is a deep ocean circulation system that is both important for climate and vulnerable to climate changes. Here we use a set of multiple climate models to look at how the AMOC responded to changes in climate drivers over the past few thousand years. The changes are only small in all of the models, and do not agree in their direction. The AMOC naturally varies on decadal timescales, but we do not see any strong trends in its variability either. We consider these simulations to indicate that the overall AMOC has not changed over the past 6,000 years, which fits with recent data reconstructions. Key Points: A multi‐model ensemble of Holocene transient simulations by general circulation models has been assembledAlthough some models show changes, no consistent trend in overall Atlantic Meridional Overturning Circulation (AMOC) strength during the mid‐to‐late Holocene emerges from the ensembleWe interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions [ABSTRACT FROM AUTHOR]

The Last Glacial Maximum (LGM, 21 000 years ago) is one of the suite of paleoclimate simulations included in the current phase of the Coupled Model Intercomparison Project (CMIP6). It is an interval when insolation was similar to the present, but global ice volume was at a maximum, eustatic sea level was at or close to a minimum, greenhouse gas concentrations were lower, atmospheric aerosol loadings were higher than today, and vegetation and landsurface characteristics were different from today. The LGM has been a focus for the Paleoclimate Modelling Intercomparison Project (PMIP) since its inception, and thus many of the problems that might be associated with simulating such a radically different climate are well documented. The LGM state provides an ideal case study for evaluating climate model performance because the changes in forcing and temperature between the LGM and pre-industrial are of the same order of magnitude as those projected for the end of the 21st century. Thus, the CMIP6 LGM experiment could provide additional information that can be used to constrain estimates of climate sensitivity. The design of the Tier 1 LGM experiment (lgm) includes an assessment of uncertainties in boundary conditions, in particular through the use of different reconstructions of the ice sheets and of the change in dust forcing. Additional (Tier 2) sensitivity experiments have been designed to quantify feedbacks associated with landsurface changes and aerosol loadings, and to isolate the role of individual forcings. Model analysis and evaluation will capitalize on the relative abundance of paleoenvironmental observations and quantitative climate reconstructions already available for the LGM. [ABSTRACT FROM AUTHOR]

The intraseasonal variability (ISV) is an intermittent phenomenon with variable perturbation patterns. To assess the robustness of the simulated ISV in climate models, it is thus interesting to consider the distribution of perturbation patterns rather than only one average pattern. To inspect this distribution, the authors first introduce a distance that measures the similarity between two patterns. The reproducibility (realism) of the simulated intraseasonal patterns is then defined as the distribution of distances between each pattern and the average simulated (observed) pattern. A good reproducibility is required to analyze the physical source of the simulated disturbances. The realism distribution is required to estimate the proportion of simulated events that have a perturbation pattern similar to observed patterns. The median value of this realism distribution is introduced as an ISV metric. The reproducibility and realism distributions are used to evaluate boreal summer ISV of precipitations over the Indian Ocean for 19 phase 3 of the Coupled Model Intercomparison Project (CMIP3) models. The 19 models are classified in increasing ISV metric order. In agreement with previous studies, the four best ISV metrics are obtained for models having a convective closure totally or partly based on the moisture convergence. Models with high metric values (poorly realistic) tend to give (i) poorly reproducible intraseasonal patterns, (ii) rainfall perturbations poorly organized at large scales, (iii) small day-to-day variability with overly red temporal spectra, and (iv) less accurate summer monsoon rainfall distribution. This confirms that the ISV is an important link in the seamless system that connects weather and climate. [ABSTRACT FROM AUTHOR]

The response of climate to freshwater input in the North Atlantic (NA) has raised a lot of concern about the issue of climate stability since the discovery of abrupt coolings during the last glacial period. Such coolings have usually been related to a weakening of the Atlantic meridional overturning circulation (AMOC), probably associated with massive iceberg surges or meltwater pulses. Additionally, the recent increase in greenhouse gases in the atmosphere has also raised the possibility of a melting of the Greenland ice sheet, which may impact the future AMOC, and thereby the climate. In this study, the extent to which the mean climate influences the freshwater release linked to ice sheet melting in the NA and the associated climatic response is explored. For this purpose the simulations of several climatic states [last interglacial, Last Glacial Maximum, mid-Holocene, preindustrial, and future (2 × CO2)] are considered, and the climatic response to a freshwater input computed interactively according to a surface heat flux budget over the ice sheets is analyzed. It is shown that the AMOC response is not linear with the freshwater input and depends on the mean climate state. The climatic responses to these different AMOC changes share qualitative similarities for the general picture, notably a cooling in the Northern Hemisphere and a southward shift of the intertropical convergence zone (ITCZ) in the Atlantic and across the Panama Isthmus. The cooling in the Northern Hemisphere is related to the sea ice cover response, which strongly depends on the responses of the atmospheric circulation, the local oceanic heat transport, and the density threshold of the oceanic convection sites. These feedbacks and the magnitude of temperature and precipitation changes outside the North Atlantic depend on the mean climate. [ABSTRACT FROM AUTHOR]

The temperature response to a greenhouse gas (GHG) concentration change is studied in an ocean–atmosphere coupled model—L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL-CM4)—for both a glacial and an interglacial context. The response to a GHG concentration changing from Last Glacial Maximum (LGM) to preindustrial values is similar for both climatic contexts in terms of temperature pattern, but the magnitude is greater under modern ones. The model simulates the classical amplification of the temperature response in the northern high latitudes compared to lower latitudes and over the land surfaces compared to the ocean. The physical reasons for the differential warming according to the latitude and to the surface type are studied through an analysis of the energy flux changes, which are decomposed to consider and quantify many different physical processes. The results highlight the role of many different factors in the thermal response to a GHG forcing for different regions, and stress, for instance, the large effect of increased water vapor concentration in the atmosphere. Concerning the land–sea warming ratio, several fluxes contribute to the final value of the ratio, with latent flux having the greatest influence. The different contributions are quantified. The comparison of the flux changes between the interglacial and glacial contexts shows that the differences are more than a simple effect of different surface emissions of the base state. It suggests that the climatic context is particularly important for the cloud and oceanic advection responses to the forcing, along with albedo effects. [ABSTRACT FROM AUTHOR]

The Sahara was significantly wetter and greener than today during the mid‐Holocene (∼6,000 years before present), and those conditions were likely maintained by feedbacks from evaporating wetlands and riparian zones. A lack of spatially continuous wetland reconstruction is the major obstacle to investigating their impacts on climate and vegetation during that epoch. Here, we estimate high‐resolution gridded wetland distribution up to 15″ in the mid‐Holocene North Africa obtained with three statistical and hydrological modeling approaches forced by enhanced and calibrated precipitation from climate models. These wetland models have good performance for present‐day conditions and reproduce mid‐Holocene hydrological elements evaluated by 297 paleo‐records. Simulation results show that 18.9 ± 4.0% of land surface in North Africa was covered by wetlands during the mid‐Holocene. Our results highlight the impact of natural climate change on wetland areas and provide a data set for modeling studies to include wetland feedbacks. Plain Language Summary: Several lines of evidence show that northern Africa was considerably wetter and greener than today at ∼6,000 years ago, which is known as the mid‐Holocene Green Sahara (GS). However, most current models could not reproduce climate in the GS. The importance of wetland feedbacks on sustaining a wetter climate has partially been recognized while large uncertainties in wetland coverage make it difficult to examine wetland feedbacks in climate models. We trained several wetland models under present climate and applied them to wetland reconstructions of GS. The produced wetland maps could capture dense wetlands indicated by 297 paleo‐records. The total wetland fraction in the mid‐Holocene North Africa is 18.9 ± 4.0%, which is more than five times the area today (∼2.8%). The relationship between wetland fraction and precipitation is examined in our models. This work improves our understanding of the GS enigma, and has implications for potential greening and wetting of Sahel and Sahara in the future. Key Points: We present high‐resolution reconstructions of wetland distribution up to 15″ for the mid‐Holocene North AfricaDuring the mid‐Holocene, 18.9 ± 4.0% of land surface in North Africa was covered by wetlandsThe relationship between precipitation and wetland fraction in the mid‐Holocene Sahara is explored [ABSTRACT FROM AUTHOR]

Many models in the framework of the Paleoclimate Modelling Intercomparison Project have undertaken simulations of the mid-Holocene (6 kyr ago) climate change. Analysis of the results have mainly focused on the North African summer monsoon that was enhanced 6 kyr ago, in all models, in response to the prescribed enhanced summer insolation. The magnitude of the simulated increase in total rainfall is very different, however, among the models, and so is the prescribed mean hot desert albedo, which varies from 19% to 38%. The appropriate prescription of hot desert's brightness, in the simulation of present-day climate, is known to be a key parameter since the work of Charney, which has been confirmed by many subsequent studies. There is yet no consensus, however, on the albedo climatological values to be used by climate modelers. Here, it is questioned whether changes in the prescription of hot desert albedo may also affect the simulated intensity of climate change. Using the Laboratoire de Météorologie Dynamique atmospheric general circulation model, two sets of simulations, with a mean hot desert albedo of respectively 35% and 28%, have been carried out. The simulated mid-Holocene summer monsoon change in northern Africa is significantly larger when the background hot desert albedo is the lowest (i.e., 28%). The associated increased northward penetration of monsoon rains allows a greater reduction of hot desert area that is in better agreement with paleodata. At least three good reasons have been found to explain these changes, one of them being that when hot desert albedo is relatively low, the atmosphere above is more unstable and the same increase in solar forcing leads to larger changes in precipitable water. The implication of such a study is that differences in models' responses to any external forcing (insolation, increased atmospheric CO[sub 2] , etc.) may be partly explained by differences in the prescription of land surface properties. Th... [ABSTRACT FROM AUTHOR]

Orbital forcing alone is not sufficient to explain the massive northward penetration of monsoon rains in Africa shown by data during the mid-Holocene (6000 yr ago). Feedbacks associated with changes in SSTs and land surface cover may be necessary to produce a sufficient increase in the monsoon. A step toward a better un-derstanding of the respective role of oceans and land surfaces is to design sensitivity studies with prescribed forcings, inferred from observations. In the first study, SSTs are lowered in the upwelling regions offshore of West Africa and Somalia, and increased in the Bay of Bengal and South China Sea. In the second simulation, the modern Sahara desert is replaced by a combination of xerophytic woods/scrub and grassland. In both cases the amount of water vapor advected from oceanic sources is increased north of 108N in Africa in response to the increased land--sea temperature contrast, thereby enhancing rainfall. But the magnitude of the simulated changes is much larger when land surface is modified. The lower albedo (compared to desert) increases the amount of radiation absorbed by the surface in northern Africa and warms it up, and the larger roughness length increases both the sensible and latent heat fluxes. Moreover, vegetation is more efficient in recycling water than a bare soil, and the release of latent heat in the atmosphere increases convection, which in turn helps maintain the onshore oceanic advection. The monsoon season is then lengthened by 1--2 months compared to all other simulations reported in the paper. The intensity of monsoon rains is also modified in Asia in both sensitivity experiments. Warmer SSTs in the Bay of Bengal and South China Sea reduce the land--sea contrast and therefore the inland penetration of monsoon rains. Changes in the position of the main large-scale convergence area in the case of a green Sahara enhances the precipitation in India. Changes are also discussed in terms of atmospheric... [ABSTRACT FROM AUTHOR]

We investigate hydrology during a past climate slightly warmer than the present: the last interglacial (LIG). With daily output of preindustrial and LIG simulations from eight new climate models we force hydrological model PCR‐GLOBWB and in turn hydrodynamic model CaMa‐Flood. Compared to preindustrial, annual mean LIG runoff, discharge, and 100‐yr flood volume are considerably larger in the Northern Hemisphere, by 14%, 25%, and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due to the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, and Pearl and lower for the Mississippi, Saint Lawrence, Amazon, Paraná, Orange, Zambesi, Danube, and Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible. Plain Language Summary: It is still uncertain how the water cycle will respond to a warmer climate in the coming decades. To increase our understanding of the relationships between climate and hydrology, we study the past climate of the last interglacial, which was slightly warmer than the present. We present the results of a modeling approach, showing that while Northern Hemisphere precipitation was higher during the last interglacial, discharge of rivers was even higher, and floods were even larger. On the contrary, in the Southern Hemisphere precipitation, discharge and floods were lower. We show that, for some regions, precipitation, discharge, and floods do not have the same direction of change. The seasonal timing of discharge also changes for some large basins of the Northern Hemisphere. Finally, for 23 sites, we compare our results to geological evidence. These results form a useful term of comparison to both projections of the future and geological studies of past hydrology. Key Points: We present the first modeling of hydrology and floods for the last interglacialBoreal precipitation and runoff are higher than preindustrial, and boreal river discharge and flood volume are (in %) even higherMost of the discharge occurs later in the year in large river basins of the Northern Hemisphere [ABSTRACT FROM AUTHOR]

This study presents the global climate model IPSL‐CM6A‐LR developed at Institut Pierre‐Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone [ITCZ], frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation [ENSO] dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed. Plain Language Summary: Climate models are unique tools to investigate the characteristics and behavior of the climate system. While climate models and their components are developed gradually over the years, the sixth phase of the Coupled Model Intercomparison Project (CMIP6) has been the opportunity for the Institut Pierre‐Simon Laplace to develop, test, and evaluate a new configuration of its climate model called IPSL‐CM6A‐LR. The characteristics and emerging properties of this new model are presented in this study. The model climatology, as assessed from a range of metrics, is strongly improved, although a number of biases common to many models do persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. Key Points: The IPSL‐CM6A‐LR model climatology is much improved over the previous version, although some systematic biases and shortcomings persistA long preindustrial control and a large number of historical and scenario simulations have been performed as part of CMIP6The effective climate sensitivity of the IPSL model increases from 4.1 to 4.8 K between IPSL‐CM5A‐LR and IPSL‐CM6A‐LR [ABSTRACT FROM AUTHOR]

Based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5)-generation previous Institut Pierre Simon Laplace (IPSL) Earth system model, we designed a new version, IPSL-CM5A2, aiming at running multi-millennial simulations typical of deep-time paleoclimate studies. Three priorities were followed during the setup of the model: (1) improving the overall model computing performance, (2) overcoming a persistent cold bias depicted in the previous model generation and (3) making the model able to handle the specific continental configurations of the geological past. These developments include the integration of hybrid parallelization Message Passing Interface – Open Multi-Processing (MPI-OpenMP) in the atmospheric model of the Laboratoire de Météorologie Dynamique (LMDZ), the use of a new library to perform parallel asynchronous input/output by using computing cores as "I/O servers" and the use of a parallel coupling library between the ocean and the atmospheric components. The model, which runs with an atmospheric resolution of 3.75∘×1.875∘ and 2 to 0.5 ∘ in the ocean, can now simulate ∼100 years per day, opening new possibilities towards the production of multi-millennial simulations with a full Earth system model. The tuning strategy employed to overcome a persistent cold bias is detailed. The confrontation of a historical simulation to climatological observations shows overall improved ocean meridional overturning circulation, marine productivity and latitudinal position of zonal wind patterns. We also present the numerous steps required to run IPSL-CM5A2 for deep-time paleoclimates through a preliminary case study for the Cretaceous. Namely, specific work on the ocean model grid was required to run the model for specific continental configurations in which continents are relocated according to past paleogeographic reconstructions. By briefly discussing the spin-up of such a simulation, we elaborate on the requirements and challenges awaiting paleoclimate modeling in the next years, namely finding the best trade-off between the level of description of the processes and the computing cost on supercomputers. [ABSTRACT FROM AUTHOR]

The implementation of boundary conditions is a key aspect of climate simulations. We describe here how the Climate Model Intercomparison Project Phase 6 (CMIP6) forcing data sets have been processed and implemented in Version 6 of the Institut Pierre‐Simon Laplace (IPSL) climate model (IPSL‐CM6A‐LR) as used for CMIP6. Details peculiar to some of the Model Intercomparison Projects are also described. IPSL‐CM6A‐LR is run without interactive chemistry; thus, tropospheric and stratospheric aerosols as well as ozone have to be prescribed. We improved the aerosol interpolation procedure and highlight a new methodology to adjust the ozone vertical profile in a way that is consistent with the model dynamical state at the time step level. The corresponding instantaneous and effective radiative forcings have been estimated and are being presented where possible. Plain Language Summary: Climate Model Intercomparison Project Phase 6 is an international project to compare the results from climate model simulations performed according to a common protocol. Such simulations require boundary conditions (called "climate forcings"), which are fed to the models in order to represent, for example, long‐lived greenhouse gases, ozone, atmospheric aerosols, or land surface properties. The same forcing data sets are used by the different modeling groups who carry out the Climate Model Intercomparison Project Phase 6 simulations; however, their implementation may differ as it depends on the model structure. This article gives details of how these forcing data were implemented in the IPSL‐CM6A‐LR model. Some of the forcing data are common to all types all simulations, whereas others depend on the runs considered. Radiative forcings, as estimated in the model, are presented for some of the forcing mechanisms. Key Points: We present how the CMIP6 forcing data were implemented in the IPSL‐CM6A‐LR climate model for the realization of the CMIP6 set of climate simulationsAn improved conservative interpolation procedure for emissions is detailed and illustrated to compute tropospheric aerosolsWe present a new methodology to adjust the prescribed ozone vertical profile to match the model atmospheric dynamical state around the tropopause [ABSTRACT FROM AUTHOR]

Anthropogenic changes in land use and land cover (LULC) during the pre-industrial Holocene could have affected regional and global climate. Existing scenarios of LULC changes during the Holocene are based on relatively simple assumptions and highly uncertain estimates of population changes through time. Archaeological and palaeoenvironmental reconstructions have the potential to refine these assumptions and estimates. The Past Global Changes (PAGES) LandCover6k initiative is working towards improved reconstructions of LULC globally. In this paper, we document the types of archaeological data that are being collated and how they will be used to improve LULC reconstructions. Given the large methodological uncertainties involved, both in reconstructing LULC from the archaeological data and in implementing these reconstructions into global scenarios of LULC, we propose a protocol to evaluate the revised scenarios using independent pollen-based reconstructions of land cover and climate. Further evaluation of the revised scenarios involves carbon cycle model simulations to determine whether the LULC reconstructions are consistent with constraints provided by ice core records of CO2 evolution and modern-day LULC. Finally, the protocol outlines how the improved LULC reconstructions will be used in palaeoclimate simulations in the Palaeoclimate Modelling Intercomparison Project to quantify the magnitude of anthropogenic impacts on climate through time and ultimately to improve the realism of Holocene climate simulations. [ABSTRACT FROM AUTHOR]

Based on the CMIP5-generation previous IPSL earth system model, we designed a new version, IPSL-CM5A2, aiming at running multi-millennial simulations typical of deep-time paleoclimates studies. Three priorities were followed during the set-up of the model: (1) improving the overall model computing performance, (2) overcoming a persistent cold bias depicted in the previous model generation, and (3) making the model able to handle the specific continental configurations of the geological past. Technical developments have been performed on separate components and on the coupling system to speed up the whole coupled model. These developments include the integration of hybrid parallelization MPI-OpenMP in LMDz atmospheric component, the use of a new library to perform parallel asynchronous input/output by using computing cores as "IO servers", the use of a parallel coupling library between the ocean and the atmospheric components. The model can now simulate ~100 years per day, opening new possibilities towards the production of multi-millennial simulations with a full earth system model. The tuning strategy employed to overcome a persistent cold bias is detailed. The confrontation of an historical simulation to climatological observations shows overall improved ocean meridional overturning circulation, marine productivity and latitudinal position of zonal wind patterns. We also present the numerous steps required to run the IPSL-CM5A2 for deep-time paleoclimates through a preliminary case-study for the Cretaceous. Namely, a specific work on the ocean model grid was required to run the model for specific continental configurations in which continents are relocated according to past paleogeographic reconstructions. By briefly discussing the spin-up of such a simulation, we elaborate on the requirements and challenges awaiting paleoclimate modelling in the next years, namely finding the best trade-off between the level of description of the processes and the computing cost on supercomputers. [ABSTRACT FROM AUTHOR]

Earth’s climate is a complex dynamical system. Its underlying components interact with eachother in linear or nonlinear ways on multiple several spatial and time scales creating possiblepositive and negative feedback loops. Understanding and studying such a system is a difficultchallenge with crucial implications on our society but requires data mining through anextremely large and complex amount of data. In particular, the investigation ofchanges in dynamical and statistical properties in the climate of the past represents aninvaluable opportunity to understand interactions between external forcing (e.g., orbitalparameters) and the internal variability of the climate system. Given a simulatedclimate field embedded on a two dimensional grid, we propose a new methodology to(i) quantify nonlinear changes in variability in simulated climate fields by meansof an information entropy quantifier, (ii) identify regions that undergo changes inentropy in a homogeneous way and (iii) investigate the time dependency of theconnectivity patterns between these regions. To address these points, we analyze twoglobal transient simulations ran with the IPSL Earth system model from mid- to lateHolocene (6000 years BP to 1950). These simulations differ according to theirhorizontal resolution (3.75ox 1.89ovs. 2.5ox 1.27o), their hydrological component(2 vs. 11 layers) and the way vegetation interacts with climate (prescribed versusdynamic). Specifically, we focus on the variability and connectivity of two fields: seasurface temperature (SST) and precipitation. Time resolution is monthly. For agiven field, the input of the algorithm is a two dimensional grid with detrendedanomalies embedded in it. We consider time windows of 100 years of data every20 years and for each time window we compute the entropy of every time seriesembedded in the grid. The entropy serves as a measure of complexity of time series andits computation depends on recurrence plots, an advanced nonlinear data analysistechnique. The product of this first step is then a spatio-temporal entropy field.We then use δ-MAPS, a dimensionality reduction methodology recently proposedby the authors, to reduce the dimensionality of the entropy field by identifyingsemi-autonomous components, referred to as "domains". Domains are defined as spatiallycontiguous, possibly overlapping, structures that undergo changes in entropy in ahomogeneous way. This allows to study both past climate regime shifts and their spatialheterogeneity. Finally, given two sets of domains (in the SST and precipitation fields),for each time window, we compute their signals and infer a functional network ofnetworks (NoN) between them. The NoN is a weighted and potentially directed graphwhich, in a single framework, quantify the interactions between domains in the samefield and the dependency among domains of different fields. A first analysis of thetime dependency of domains’ complexity and of their NoN topology is presented. [ABSTRACT FROM AUTHOR]

Anthropogenic changes in land use and land cover (LULC) during the pre-industrial Holocene could have affected regional and global climate. Current LULC scenarios are based on relatively simple assumptions and highly uncertain estimates of population changes through time. Archaeological and palaeoenvironmental reconstructions have the potential to refine these assumptions and estimates. The Past Global Changes (PAGES) LandCover6k initiative is working towards improved reconstructions of LULC globally. In this paper, we document the types of archaeological data that are being collated and how they will be used to improve LULC reconstructions. Given the large methodological uncertainties involved, we propose methods to evaluate the revised scenarios by using independent pollen-based reconstructions of land cover and of climate. A further test involves carbon-cycle simulations to determine whether the LULC reconstructions are consistent with constraints provided by ice-core records of CO2 evolution and modern-day LULC. Finally, we outline a protocol for using the improved LULC reconstructions in palaeoclimate simulations within the framework of the Palaeoclimate Modelling Intercomparison Project in order to quantify the magnitude of anthropogenic impacts on climate through time and ultimately to improve the realism of Holocene climate simulations. [ABSTRACT FROM AUTHOR]

The pre-industrial millennium is among the periods selected by the Paleoclimate Model Intercomparison Project (PMIP) for experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and the fourth phase of the PMIP (PMIP4). The past1000 transient simulations serve to investigate the response to (mainly) natural forcing under background conditions not too different from today, and to discriminate between forced and internally generated variability on interannual to centennial timescales. This paper describes the motivation and the experimental set-ups for the PMIP4-CMIP6 past1000 simulations, and discusses the forcing agents orbital, solar, volcanic, and land use/land cover changes, and variations in greenhouse gas concentrations. The past1000 simulations covering the pre-industrial millennium from 850 Common Era (CE) to 1849 CE have to be complemented by historical simulations (1850 to 2014 CE) following the CMIP6 protocol. The external forcings for the past1000 experiments have been adapted to provide a seamless transition across these time periods. Protocols for the past1000 simulations have been divided into three tiers. A default forcing data set has been defined for the Tier 1 (the CMIP6 past1000) experiment. However, the PMIP community has maintained the flexibility to conduct coordinated sensitivity experiments to explore uncertainty in forcing reconstructions as well as parameter uncertainty in dedicated Tier 2 simulations. Additional experiments (Tier 3) are defined to foster collaborative model experiments focusing on the early instrumental period and to extend the temporal range and the scope of the simulations. This paper outlines current and future research foci and common analyses for collaborative work between the PMIP and the observational communities (reconstructions, instrumental data). [ABSTRACT FROM AUTHOR]

The pre-industrial millennium is among the periods selected by the Paleoclimate Model Intercomparison Project (PMIP) for experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and the fourth phase of PMIP (PMIP4). The past1000 transient simulations serve to investigate the response to (mainly) natural forcing under background conditions not too different from today, and to discriminate between forced and internally generated variability on interannual to centennial time scales. This manuscript describes the motivation and the experimental set-ups for the PMIP4-CMIP6 past1000 simulations, and discusses the forcing agents: orbital, solar, volcanic, land-use/land-cover changes, and variations in greenhouse gas concentrations. The past1000 simulations covering the pre-industrial millennium from 850 Common Era (CE) to 1849 CE have to be complemented by historical simulations (1850 to 2014 CE) following the CMIP6 protocol. The external forcings for the past1000 experiments have been adapted to provide a seamless transition across these time periods. Protocols for the past1000 simulations have been divided into three tiers. A default forcing data set has been defined for the "tier-1" (the CMIP6 past1000) experiment. However, the PMIP community has maintained the flexibility to conduct coordinated sensitivity experiments to explore uncertainty in forcing reconstructions as well as parameter uncertainty in dedicated "tier-2" simulations. Additional experiments ("tier-3") are defined to foster collaborative model experiments focusing on the early instrumental period and to extend the temporal range and the scope of the simulations. This manuscript outlines current and future research foci and common analyses for collaborative work between the PMIP and the observational communities (reconstructions, instrumental data). [ABSTRACT FROM AUTHOR]

Global climate change will increase the frequency of hot temperatures, impairing health and productivity for millions of working people and raising labor costs. In mainland China, high-temperature subsidies (HTSs) are allocated to employees for each working day in extremely hot environments, but the potential heat-related increase in labor cost has not been evaluated so far. Here, we estimate the potential HTS cost in current and future climates under different scenarios of socioeconomic development and radiative forcing (Representative Concentration Pathway), taking uncertainties from the climate model structure and bias correction into account. On average, the total HTS in China is estimated at 38.6 billion yuan/y (US $6.22 billion/y) over the 1979-2005 period, which is equivalent to 0.2% of the gross domestic product (GDP). Assuming that the HTS standards (per employee per hot day) remain unchanged throughout the 21st century, the total HTS may reach 250 billion yuan/y in the 2030s and 1,000 billion yuan/y in 2100. We further show that, without specific adaptation, the increased HTS cost is mainly determined by population growth until the 2030s and climate change after the mid-21st century because of increasingly frequent hot weather. Accounting for the likely possibility that HTS standards follow the wages, the share of GDP devoted to HTS could become as high as 3% at the end of 21st century. [ABSTRACT FROM AUTHOR]

The causes and timing of tropical glacier fluctuations during the Holocene epoch (10,000 years ago to present) are poorly understood. Yet constraining their sensitivity to changes in climate is important, as these glaciers are both sensitive indicators of climate change and serve as water reservoirs for highland regions. Studies have so far documented extra-tropical glacier fluctuations, but in the tropics, glacier-climate relationships are insufficiently understood. Here we present a 10Be chronology for the past 11,000?years (11?kyr), using 57 moraines from the Bolivian Telata glacier (in the Cordillera Real mountain range). This chronology indicates that Telata glacier retreated irregularly. A rapid and strong melting from the maximum extent occurred from 10.8?±?0.9 to 8.5?±?0.4?kyr ago, followed by a slower retreat until the Little Ice Age, about 200 years ago. A dramatic increase in the rate of retreat occurred over the twentieth century. A glacier-climate model indicates that, relative to modern climate, annual mean temperature for the Telata glacier region was ?3.3?±?0.8 °C cooler at 11?kyr ago and remained ?2.1?±?0.8 °C cooler until the end of the Little Ice Age. We suggest that long-term warming of the eastern tropical Pacific and increased atmospheric temperature in response to enhanced austral summer insolation were the main drivers for the long-term Holocene retreat of glaciers in the southern tropics. [ABSTRACT FROM AUTHOR]

The mid-Holocene period (MH) has long been an ideal target for the validation of Global Circulation Model (GCM) results against reconstructions gathered in global datasets. These studies aimed to test the GCM sensitivity mainly to the seasonal changes induced by the orbital parameters (precession). Despite widespread agreement between model results and data on the MH climate, some important differences still exist. There is no consensus on the continental size of the MH thermal climate response, which makes regional quantitative reconstruction critical to obtain a comprehensive understanding of the MH climate patterns. Here, we compare the annual and seasonal outputs from the most recent Paleoclimate Modelling Intercomparison Projects Phase 3 (PMIP3) models with an updated synthesis of climate reconstruction over China, including, for the first time, a seasonal cycle of temperature and precipitation. Most of the models provide a linear response driven by the seasonal forcing (warmer in summer, cooler in winter), which disagrees with the new seasonal data reconstruction over China. We show that to capture the seasonal pattern reconstructed by data, it is critical to assess surface processes. These results pinpoint the crucial importance of including the non-linear of the surface water and energy balance to vegetation changes. [ABSTRACT FROM AUTHOR]

The typical resolution used in CMIP-class Earth system models limits, among others, the ability to assess climate risks that are associated with smaller-scale weather phenomena such as tropical cyclones. We introduce higher-resolution atmosphere-only simulations following the HighResMIP Tier1 protocol. A baseline simulation (LMDZ-OR) is run with LMD-Z, the atmospheric component of the IPSL-CM Earth System model, with CMIP6 physics including the land-surface model ORCHIDEE, at a resolution equivalent to 50km at mid-latitudes. Large-scale parallel I/O with on-the-fly data processing is enabled by the XIOS I/O server. In two additional simulations (ICO-LMDZ-OR), the LMD-Z latitude-longitude dynamical core is replaced by the more scalable DYNAMICO dynamical core, using icosahedral-hexagonal meshes with quasi-uniform resolution of 50km and 25km.We provide a description and preliminary analysis of these simulations. We present the scalability and throughput of LMDZ-OR and ICO-LMDZ-OR. Some differences between LMDZ-OR and ICO-LMDZ-OR with respect to the processing of input datasets, especially orography, are discussed. The climates in LMDZ-OR and ICO-LMD-OR simulations at similar resolutions are compared. Dependence on resolution of key climate features is examined. [ABSTRACT FROM AUTHOR]