Earth System Model Of Intermediate Complexity Research Articles

The faint young Sun problem revisited with a 3-D climate–carbon model – Part 1

Abstract. During the Archaean, the Sun's luminosity was 18 to 25% lower than the present day. One-dimensional radiative convective models (RCM) generally infer that high concentrations of greenhouse gases (CO2, CH4) are required to prevent the early Earth's surface temperature from dropping below the freezing point of liquid water and satisfying the faint young Sun paradox (FYSP, an Earth temperature at least as warm as today). Using a one-dimensional (1-D) model, it was proposed in 2010 that the association of a reduced albedo and less reflective clouds may have been responsible for the maintenance of a warm climate during the Archaean without requiring high concentrations of atmospheric CO2 (pCO2). More recently, 3-D climate simulations have been performed using atmospheric general circulation models (AGCM) and Earth system models of intermediate complexity (EMIC). These studies were able to solve the FYSP through a large range of carbon dioxide concentrations, from 0.6 bar with an EMIC to several millibars with AGCMs. To better understand this wide range in pCO2, we investigated the early Earth climate using an atmospheric GCM coupled to a slab ocean. Our simulations include the ice-albedo feedback and specific Archaean climatic factors such as a faster Earth rotation rate, high atmospheric concentrations of CO2 and/or CH4, a reduced continental surface, a saltier ocean, and different cloudiness. We estimated full glaciation thresholds for the early Archaean and quantified positive radiative forcing required to solve the FYSP. We also demonstrated why RCM and EMIC tend to overestimate greenhouse gas concentrations required to avoid full glaciations or solve the FYSP. Carbon cycle–climate interplays and conditions for sustaining pCO2 will be discussed in a companion paper.

Socioeconomic Analysis of Various Emission Pathways Considering the Uncertainties of Earth System Models

Although much uncertainty exists in simulating future climate profiles with earth system models (ESMs), it has not been evaluated in relation to socioeconomic aspects. In this study, we analyze the socioeconomic impact of three emission pathways, all of which possibly achieve 4.5 W/m2 of radiative forcing in the year 2100 within uncertainties estimated by an ESM of intermediate complexity tuned for full ESMs using a computable general equilibrium model. The results indicate that the impacts are larger in the lower bound emission pathway to achieve 4.5 W/m2 as expected, although the economy and energy demand increase continuously in this century. However, the global primary energy demand in 2100 in the lower bound case is slightly larger than in the mean case; this can be interpreted because biomass energy with carbon capture and storage technology is enhanced to achieve very low carbon dioxide emissions in the lower bound case. In a comparison between the upper bound and lower bound emission pathways, the carbon price of the latter is approximately three times higher in 2100. The gross domestic product (GDP) in the latter is 4.1% smaller than that in the former in 2100, which is equivalent to only a 0.042% decrease in the annual GDP growth rate. Thus, the socioeconomic impacts caused by ESM uncertainties, here evaluated by GDP and energy demand, are not insignificant but are smaller than the differences in the emission pathways to achieve 4.5 W/m2.

CO2 radiative forcing and Intertropical Convergence Zone influences on western Pacific warm pool climate over the past 400 ka

The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation and influences climate conditions worldwide. Estimating WPWP sensitivity to past radiative forcing perturbations is important for understanding the magnitudes and patterns of current and projected tropical climate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction over the past 400ka from the Bismarck Sea, off Papua New Guinea, along with benthic foraminiferal δ18O records and a transient intermediate complexity earth system model simulation. The Mg/Ca-SST record exhibits a close similarity with atmospheric CO2 content for the whole study period. Our model analysis demonstrates that greenhouse gas forcing is the primary driver for glacial/interglacial SST changes in the entire WPWP region. Mg/Ca-SST in the Bismarck Sea also includes a weaker precessional component, which covaries with reconstructed and simulated local precipitation, and simulated surface currents. We propose that orbitally driven latitudinal shifts of the Intertropical Convergence Zone and oceanic heat advection are responsible for this residual SST variability. On glacial timescales the reconstructed WPWP surface temperature changes over the past 400ka are highly correlated with East Antarctic air temperature variations. The strong effect of greenhouse gas forcings on both records and on global mean temperature variability allows us to determine a scaling factor of 1–1.5 between reconstructed WPWP temperature anomalies and estimates of the global mean temperature.

Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger events is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-D intermediate complexity earth system model covering the period 50 to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing northern hemispheric ice volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our results are consistent with the idea that variations in ice-sheet calving and subsequent changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of glacial millennial-scale variability seen in paleorecords across the globe.

Scheme for calculation of multi-layer cloudiness and precipitation for climate models of intermediate complexity

Abstract. In this study we present a scheme for calculating the characteristics of multi-layer cloudiness and precipitation for Earth system models of intermediate complexity (EMICs). This scheme considers three-layer stratiform cloudiness and single-column convective clouds. It distinguishes between ice and droplet clouds as well. Precipitation is calculated by using cloud lifetime, which depends on cloud type and phase as well as on statistics of synoptic and convective disturbances. The scheme is tuned to observations by using an ensemble simulation forced by the ERA-40-derived climatology for 1979–2001. Upon calibration, the scheme realistically reproduces basic features of fields of cloud fractions, cloud water path, and precipitation. The simulated globally and annually averaged total cloud fraction is 0.59, and the simulated globally averaged annual precipitation is 100 cm yr−1. Both values agree with empirically derived values. The simulated cloud water path is too small, probably because the simulated vertical extent of stratiform clouds is too small. Geographical distribution and seasonal changes of calculated cloud fraction and precipitation are broadly realistic as well. However, some important regional biases still remain in the scheme, e.g. too little precipitation in the tropics. We discuss possibilities for future improvements in the scheme.

An optimally tuned ensemble of the "eb_go_gs" configuration of GENIE: parameter sensitivity and bifurcations in the Atlantic overturning circulation

Abstract. The key physical parameters for the "eb_go_gs" configuration of version 2.7.4 of GENIE, an Earth system model of intermediate complexity (EMIC), are tuned using a multi-objective genetic algorithm. An ensemble of 90 parameter sets is tuned using two ocean and two atmospheric state variables as targets. These are "Pareto-optimal", representing a range of trade-offs between the four tuning targets. For the leading five parameter sets, simulations are evaluated alongside a simulation with untuned "default" parameters, comparing selected variables and diagnostics that describe the state of the atmosphere, ocean and sea ice. Further experiments are undertaken with these selected parameter sets to compare equilibrium climate sensitivities and transient climate responses. The pattern of warming under doubled CO2 is strongly shaped by changes in the Atlantic meridional overturning circulation (AMOC), while the pattern and rate of warming under rising CO2 is closely linked to changing sea ice extent. One of the five tuned parameter sets is identified as marginally optimal, and the objective function (error) landscape is further analysed in the vicinity of the tuned values of this parameter set. "Cliffs" along some dimensions motivate closer inspection of corresponding variations in the AMOC. This reveals that bifurcations in the AMOC are highly sensitive to parameters that are not typically associated with MOC stability. Specifically, the state of the AMOC is sensitive to parameters governing the wind-driven circulation and atmospheric heat transport. For the GENIE configuration presented here, the marginally optimal parameter set is recommended for single simulations, although the leading five parameter sets may be used in ensemble mode to admit a constrained degree of parametric uncertainty in climate prediction.

Long-Term Climate Change Commitment and Reversibility: An EMIC Intercomparison

AbstractThis paper summarizes the results of an intercomparison project with Earth System Models of Intermediate Complexity (EMICs) undertaken in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). The focus is on long-term climate projections designed to 1) quantify the climate change commitment of different radiative forcing trajectories and 2) explore the extent to which climate change is reversible on human time scales. All commitment simulations follow the four representative concentration pathways (RCPs) and their extensions to year 2300. Most EMICs simulate substantial surface air temperature and thermosteric sea level rise commitment following stabilization of the atmospheric composition at year-2300 levels. The meridional overturning circulation (MOC) is weakened temporarily and recovers to near-preindustrial values in most models for RCPs 2.6–6.0. The MOC weakening is more persistent for RCP8.5. Elimination of anthropogenic CO2 emissions after 2300 results in slowly decreasing atmospheric CO2 concentrations. At year 3000 atmospheric CO2 is still at more than half its year-2300 level in all EMICs for RCPs 4.5–8.5. Surface air temperature remains constant or decreases slightly and thermosteric sea level rise continues for centuries after elimination of CO2 emissions in all EMICs. Restoration of atmospheric CO2 from RCP to preindustrial levels over 100–1000 years requires large artificial removal of CO2 from the atmosphere and does not result in the simultaneous return to preindustrial climate conditions, as surface air temperature and sea level response exhibit a substantial time lag relative to atmospheric CO2.

Allowable carbon emissions lowered by multiple climate targets

Climate targets are designed to inform policies that would limit the magnitude and impacts of climate change caused by anthropogenic emissions of greenhouse gases and other substances. The target that is currently recognized by most world governments places a limit of two degrees Celsius on the global mean warming since preindustrial times. This would require large sustained reductions in carbon dioxide emissions during the twenty-first century and beyond. Such a global temperature target, however, is not sufficient to control many other quantities, such as transient sea level rise, ocean acidification and net primary production on land. Here, using an Earth system model of intermediate complexity (EMIC) in an observation-informed Bayesian approach, we show that allowable carbon emissions are substantially reduced when multiple climate targets are set. We take into account uncertainties in physical and carbon cycle model parameters, radiative efficiencies, climate sensitivity and carbon cycle feedbacks along with a large set of observational constraints. Within this framework, we explore a broad range of economically feasible greenhouse gas scenarios from the integrated assessment community to determine the likelihood of meeting a combination of specific global and regional targets under various assumptions. For any given likelihood of meeting a set of such targets, the allowable cumulative emissions are greatly reduced from those inferred from the temperature target alone. Therefore, temperature targets alone are unable to comprehensively limit the risks from anthropogenic emissions.

Historical and idealized climate model experiments: an intercomparison of Earth system models of intermediate complexity

Abstract. Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO2, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions.

Global Paleofire Syntheses: Lessons from the Global Charcoal Database and members of the Global Paleofire Working Group

The last 6000 years are of particular interest to the understanding of the Earth System because the boundary conditions of the climate system did not change dramatically (in comparison to larger glacial–interglacial changes), and because abundant, detailed regional palaeoclimatic proxy records cover this period. We use selected proxy-based reconstructions of different climate variables, together with state-of-the-art time series of natural forcings (orbital variations, solar activity variations, large tropical volcanic eruptions, land cover and greenhouse gases), underpinned by results from General Circulation Models (GCMs) and Earth System Models of Intermediate Complexity (EMICs), to establish a comprehensive explanatory framework for climate changes from the Mid-Holocene (MH) to pre-industrial time. The redistribution of solar energy, due to orbital forcing on a millennial timescale, was the cause of a progressive southward shift of the Northern Hemisphere (NH) summer position of the Intertropical Convergence Zone (ITCZ). This was accompanied by a pronounced weakening of the monsoon systems in Africa and Asia and increasing dryness and desertification on both continents. The associated summertime cooling of the NH, combined with changing temperature gradients in the world oceans, likely led to an increasing amplitude of the El Niño Southern Oscillation (ENSO) and, possibly, increasingly negative North Atlantic Oscillation (NAO) indices up to the beginning of the last millennium. On decadal to multi-century timescales, a worldwide coincidence between solar irradiance minima, tropical volcanic eruptions and decadal to multi-century scale cooling events was not found. However, reconstructions show that widespread decadal to multi-century scale cooling events, accompanied by advances of mountain glaciers, occurred in the NH (e.g., in Scandinavia and the European Alps). This occurred namely during the Little Ice Age (LIA) between AD ∼1350 and 1850, when the lower summer insolation in the NH, due to orbital forcing, coincided with solar activity minima and several strong tropical volcanic eruptions. The role of orbital forcing in the NH cooling, the southward ITCZ shift and the desertification of the Sahara are supported by numerous model simulations. Other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics, but there is also evidence that changes in the ocean heat transport took place during the LIA at high northern latitudes, with possible additional implications for climates of the Southern Hemisphere (SH).

Stability of the Atlantic meridional overturning circulation: A model intercomparison

The evolution of the Atlantic Meridional Overturning Circulation (MOC) in 30 models of varying complexity is examined under four distinct Representative Concentration Pathways. The models include 25 Atmosphere‐Ocean General Circulation Models (AOGCMs) or Earth System Models (ESMs) that submitted simulations in support of the 5th phase of the Coupled Model Intercomparison Project (CMIP5) and 5 Earth System Models of Intermediate Complexity (EMICs). While none of the models incorporated the additional effects of ice sheet melting, they all projected very similar behaviour during the 21st century. Over this period the strength of MOC reduced by a best estimate of 22% (18%–25%; 5%–95% confidence limits) for RCP2.6, 26% (23%–30%) for RCP4.5, 29% (23%–35%) for RCP6.0 and 40% (36%–44%) for RCP8.5. Two of the models eventually realized a slow shutdown of the MOC under RCP8.5, although no model exhibited an abrupt change of the MOC. Through analysis of the freshwater flux across 30°–32°S into the Atlantic, it was found that 56% of the CMIP5 models were in a bistable regime of the MOC for at least part of their RCP integrations. The results support previous assessments that it is very unlikely that the MOC will undergo an abrupt change to an off state as a consequence of global warming.

Plant-driven variation in decomposition rates improves projections of global litter stock distribution

Abstract. Plant litter stocks are critical, regionally for their role in fueling fire regimes and controlling soil fertility, and globally through their feedback to atmospheric CO2 and climate. Here we employ two global databases linking plant functional types to decomposition rates of wood and leaf litter (Cornwell et al., 2008; Weedon et al., 2009) to improve future projections of climate and carbon cycle using an intermediate complexity Earth System model. Implementing separate wood and leaf litter decomposabilities and their temperature sensitivities for a range of plant functional types yielded a more realistic distribution of litter stocks in all present biomes with the exception of boreal forests and projects a strong increase in global litter stocks by 35 Gt C and a concomitant small decrease in atmospheric CO2 by 3 ppm by the end of this century. Despite a relatively strong increase in litter stocks, the modified parameterization results in less elevated wildfire emissions because of a litter redistribution towards more humid regions.

Development of a system emulating the global carbon cycle in Earth system models

Abstract. Recent studies have indicated that the uncertainty in the global carbon cycle may have a significant impact on the climate. Since state of the art models are too computationally expensive for it to be possible to explore their parametric uncertainty in anything approaching a comprehensive fashion, we have developed a simplified system for investigating this problem. By combining the strong points of general circulation models (GCMs), which contain detailed and complex processes, and Earth system models of intermediate complexity (EMICs), which are quick and capable of large ensembles, we have developed a loosely coupled model (LCM) which can represent the outputs of a GCM-based Earth system model, using much smaller computational resources. We address the problem of relatively poor representation of precipitation within our EMIC, which prevents us from directly coupling it to a vegetation model, by coupling it to a precomputed transient simulation using a full GCM. The LCM consists of three components: an EMIC (MIROC-lite) which consists of a 2-D energy balance atmosphere coupled to a low resolution 3-D GCM ocean (COCO) including an ocean carbon cycle (an NPZD-type marine ecosystem model); a state of the art vegetation model (Sim-CYCLE); and a database of daily temperature, precipitation, and other necessary climatic fields to drive Sim-CYCLE from a precomputed transient simulation from a state of the art AOGCM. The transient warming of the climate system is calculated from MIROC-lite, with the global temperature anomaly used to select the most appropriate annual climatic field from the pre-computed AOGCM simulation which, in this case, is a 1% pa increasing CO2 concentration scenario. By adjusting the effective climate sensitivity (equivalent to the equilibrium climate sensitivity for an energy balance model) of MIROC-lite, the transient warming of the LCM could be adjusted to closely follow the low sensitivity (with an equilibrium climate sensitivity of 4.0 K) version of MIROC3.2. By tuning of the physical and biogeochemical parameters it was possible to reasonably reproduce the bulk physical and biogeochemical properties of previously published CO2 stabilisation scenarios for that model. As an example of an application of the LCM, the behavior of the high sensitivity version of MIROC3.2 (with a 6.3 K equilibrium climate sensitivity) is also demonstrated. Given the highly adjustable nature of the model, we believe that the LCM should be a very useful tool for studying uncertainty in global climate change, and we have named the model, JUMP-LCM, after the name of our research group (Japan Uncertainty Modelling Project).

The utility of Earth system Models of Intermediate Complexity (EMICs)

AbstractIntermediate‐complexity models are models which describe the dynamics of the atmosphere and/or ocean in less detail than conventional General Circulation Models (GCMs). At the same time, they go beyond the approach taken by atmospheric Energy Balance Models (EBMs) or ocean box models by using sophisticated parameterizations of the unresolved flow or by explicitly resolving the equations of geophysical fluid dynamics albeit at coarse spatial resolution. Being computationally fast, intermediate‐complexity models have the capability to treat slow climate variations. Hence, they often include components of the climate system that are associated with long‐term feedbacks like ice sheets, vegetation and biogeochemical cycles. Here again they differ from conventional GCM‐type models that feature only atmosphere and ocean/sea‐ice components. Many different approaches exist in building such a reduced model, resulting in a ‘spectrum of Earth system Models of Intermediate Complexity closing the gap between EBMs and GCMs’. Copyright © 2010 John Wiley & Sons, Inc.This article is categorized under: Climate Models and Modeling > Earth System Models

A new one-dimensional simple energy balance and carbon cycle coupled model for global warming simulation

Global warming and accompanying climate change may be caused by an increase in atmospheric greenhouse gasses generated by anthropogenic activities. In order to supply such a mechanism of global warming with a quantitative underpinning, we need to understand the multifaceted roles of the Earth's energy balance and material cycles. In this study, we propose a new one-dimensional simple Earth system model. The model consists of carbon and energy balance submodels with a north–south zonal structure. The two submodels are coupled by interactive feedback processes such as CO2 fertilization of net primary production (NPP) and temperature dependencies of NPP, soil respiration, and ocean surface chemistry. The most important characteristics of the model are not only that the model requires a relatively short calculation time for carbon and energy simulation compared with a General Circulation Model (GCM) and an Earth system Model of Intermediate Complexity (EMIC), but also that the model can simulate average latitudinal variations. In order to analyze the response of the Earth system due to increasing greenhouse gasses, several simulations were conducted in one dimension from the years 1750 to 2000. Evaluating terrestrial and oceanic carbon uptake output of the model in the meridional direction through comparison with observations and satellite data, we analyzed the time variation patterns of air temperature in low- and middle-latitude belts. The model successfully reproduced the temporal variation in each latitude belt and the latitudinal distribution pattern of carbon uptake. Therefore, this model could more accurately demonstrate a difference in the latitudinal response of air temperature than existing models. As a result of the model evaluations, we concluded that this new one-dimensional simple Earth system model is a good tool for conducting global warming simulations. From future projections using various emission scenarios, we showed that the spatial distribution of terrestrial carbon uptake may vary greatly, not only among models used for climate change simulations, but also amongst emission scenarios.

Contributions of carbon cycle uncertainty to future climate projection spread

We have characterized the relative contributions to uncertainty in predictions of global warming amount by year 2100 in the C4MIP model ensemble (Friedlingstein et al., 2006) due to both carbon cycle process uncertainty and uncertainty in the physical climate properties of the Earth system. We find carbon cycle uncertainty to be important. On average the spread in transient climate response is around 40% of that due to the more frequently debated uncertainties in equilibrium climate sensitivity and global heat capacity. This result is derived by characterizing the influence of different parameters in a global climate-carbon cycle ‘box’ model that has been calibrated against the 11 General Circulation models (GCMs) and Earth system Models of Intermediate Complexity (EMICs) in the C4MIP ensemble; a collection of current state-of-the-art climate models that include an explicit representation of the global carbon cycle. DOI: 10.1111/j.1600-0889.2009.00414.x

Two-way coupling of an ENSO model to the global climate model CLIMBER-3 [formula omitted

We present a model study that investigates to what extent it is possible to introduce ENSO variability to an Earth system Model of Intermediate Complexity (EMIC). The Zebiak–Cane ENSO model is dynamically coupled to the EMIC CLIMBER-3 α , which by itself exhibits no interannual or multidecadal variability. ENSO variability is introduced to CLIMBER-3 α by adding ENSO-related sea surface temperature anomalies to the upper layers of the model ocean. For the other coupling direction, changes in the mean CLIMBER-3 α climate on decadal time scales are used to change the background state of the ENSO model, achieving a two-way coupling. We compare typical ENSO-related patterns of a fully coupled pre-industrial model run to reanalysis data and point out the possibilities and limitations of this model configuration. Although introduced ENSO-related SST anomalies and other related variables like the Southern Oscillation Index are well reproduced by the EMIC in the forcing domain, teleconnections to other regions are damped, especially in meridional direction. The reason for this limitation is the atmospheric model, which does not sufficiently resolve the necessary transport mechanisms. Despite this limitation the presented coupling method may still be a useful tool in combination with higher resolution atmospheric models as being in development for the successor model CLIMBER-3 and possibly other EMICs.

Contributions of carbon cycle uncertainty to future climate projection spread

We have characterized the relative contributions to uncertainty in predictions of global warming amount by year 2100 in the C4MIP model ensemble (Friedlingstein et al., 2006) due to both carbon cycle process uncertainty and uncertainty in the physical climate properties of the Earth system. We find carbon cycle uncertainty to be important. On average the spread in transient climate response is around 40% of that due to the more frequently debated uncertainties in equilibrium climate sensitivity and global heat capacity.This result is derived by characterizing the influence of different parameters in a global climate-carbon cycle ‘box’ model that has been calibrated against the 11 General Circulation models (GCMs) and Earth system Models of Intermediate Complexity (EMICs) in the C4MIP ensemble; a collection of current state-of-the-art climate models that include an explicit representation of the global carbon cycle.

Mid- to Late Holocene climate change: an overview

The last 6000 years are of particular interest to the understanding of the Earth System because the boundary conditions of the climate system did not change dramatically (in comparison to larger glacial–interglacial changes), and because abundant, detailed regional palaeoclimatic proxy records cover this period. We use selected proxy-based reconstructions of different climate variables, together with state-of-the-art time series of natural forcings (orbital variations, solar activity variations, large tropical volcanic eruptions, land cover and greenhouse gases), underpinned by results from General Circulation Models (GCMs) and Earth System Models of Intermediate Complexity (EMICs), to establish a comprehensive explanatory framework for climate changes from the Mid-Holocene (MH) to pre-industrial time. The redistribution of solar energy, due to orbital forcing on a millennial timescale, was the cause of a progressive southward shift of the Northern Hemisphere (NH) summer position of the Intertropical Convergence Zone (ITCZ). This was accompanied by a pronounced weakening of the monsoon systems in Africa and Asia and increasing dryness and desertification on both continents. The associated summertime cooling of the NH, combined with changing temperature gradients in the world oceans, likely led to an increasing amplitude of the El Niño Southern Oscillation (ENSO) and, possibly, increasingly negative North Atlantic Oscillation (NAO) indices up to the beginning of the last millennium. On decadal to multi-century timescales, a worldwide coincidence between solar irradiance minima, tropical volcanic eruptions and decadal to multi-century scale cooling events was not found. However, reconstructions show that widespread decadal to multi-century scale cooling events, accompanied by advances of mountain glaciers, occurred in the NH (e.g., in Scandinavia and the European Alps). This occurred namely during the Little Ice Age (LIA) between AD ∼1350 and 1850, when the lower summer insolation in the NH, due to orbital forcing, coincided with solar activity minima and several strong tropical volcanic eruptions. The role of orbital forcing in the NH cooling, the southward ITCZ shift and the desertification of the Sahara are supported by numerous model simulations. Other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics, but there is also evidence that changes in the ocean heat transport took place during the LIA at high northern latitudes, with possible additional implications for climates of the Southern Hemisphere (SH).

Glacial inceptions: Past and future†

The realistic simulation of northern hemisphere glacial inceptions, which occurred during the Quaternary period, has challenged scores of climate theoreticians and modellers. After reviewing the Milankovitch theory of glaciation, a number of earlier modelling studies of the last glacial inception (LGI) which have employed either high‐resolution General Circulation Models (GCMs) or Earth system Models of Intermediate Complexity (EMICs) are described. The latter class of models has been developed over the past two decades in order to investigate the many interactions and feedbacks among the geophysical and biospheric components of the Earth system that take place over long time scales. Following a description of the McGill Paleoclimate Model (MPM) and other EMICs, some recent McGill simulations of the LGI in response to orbital (Milankovitch) and radiative (atmospheric CO2) forcings are presented. Special attention is given to determining the relative roles of the ocean's thermohaline circulation, freshwater fluxes into the ocean, orography, cryospheric processes and vegetation dynamics during the inception phase. In particular, it is shown that with the vegetation‐albedo feedback included in the model, the buildup of ice sheets over North America is larger than over Eurasia, in agreement with observations. This paper concludes with a discussion on the (possible) occurrence of the next glacial period. To address this issue, which has been inspired by recent publications of Berger and Loutre, MPM simulations of the climate for the next 100 kyr, forced by various prescribed atmospheric CO2 levels, as well as the future insolation changes as calculated by the Berger algorithm, are presented. The influence of a near‐term global warming scenario on glacial inception is also examined. If it is assumed that after such a warming scenario the concentration of CO2in the atmosphere returns to pre‐industrial levels (in the range of 280–290 ppm), then the MPM predicts that the next glacial would start at around 50 kyr after present, which is consistent with the results of Berger and Loutre. Finally, recent simulations of future glacial inceptions using the Potsdam EMIC which includes an atmosphere‐ocean carbon cycle component are described. From one of these simulations in which 5000 GtC are released into the atmosphere due to human activities, it is concluded that the current interglacial will last for at least another half‐million years because of the limited ability of the oceans to absorb such a large carbon release to the atmosphere.