Glacial cycles of the late Quaternary are controlled by the asymmetrically varying mass balance of continental ice sheets in the Northern Hemisphere. Surface mass balance is governed by processes of ablation and accumulation. Here two ablation schemes, the positive-degree-day (PDD) method and the surface energy balance (SEB) approach, are compared in transient simulations of the last glacial cycle with the Earth system model of intermediate complexity CLIMBER-2. The standard version of the CLIMBER-2 model incorporates the SEB approach and simulates ice volume variations in reasonable agreement with paleoclimate reconstructions during the entire last glacial cycle. Using results from the standard CLIMBER-2 model version, we simulated ablation with the PDD method in offline mode by applying different combinations of three empirical parameters of the PDD scheme. We found that none of the parameter combinations allow us to simulate a surface mass balance of the American and European ice sheets that is similar to that obtained with the standard SEB method. The use of constant values for the empirical PDD parameters led either to too much ablation during the first phase of the last glacial cycle or too little ablation during the final phase. We then substituted the standard SEB scheme in CLIMBER-2 with the PDD scheme and performed a suite of fully interactive (online) simulations of the last glacial cycle with different combinations of PDD parameters. The results of these simulations confirmed the results of the offline simulations: no combination of PDD parameters realistically simulates the evolution of the ice sheets during the entire glacial cycle. The use of constant parameter values in the online simulations leads either to a buildup of too much ice volume at the end of glacial cycle or too little ice volume at the beginning. Even when the model correctly simulates global ice volume at the last glacial maximum (21 ka), it is unable to simulate complete deglaciation during the Holocene. According to our simulations, the SEB approach proves superior for simulations of glacial cycles.

Possible feedback effects between aeolian dust, climate and ice sheets are studied for the first time with an Earth system model of intermediate complexity over the late Pleistocene period. Correlations between climate and dust deposition records suggest that aeolian dust potentially plays an important role for the evolution of glacial cycles. Here climatic effects from the dust direct radiative forcing (DRF) caused by absorption and scattering of solar radiation are investigated. Key elements controlling the dust DRF are the atmospheric dust distribution and the absorption-scattering efficiency of dust aerosols. Effective physical parameters in the description of these elements are varied within uncertainty ranges known from available data and detailed model studies. Although the parameters can be reasonably constrained, the simulated dust DRF spans a~wide uncertainty range related to the strong nonlinearity of the Earth system. In our simulations, the dust DRF is highly localized. Medium-range parameters result in negative DRF of several watts per square metre in regions close to major dust sources and negligible values elsewhere. In the case of high absorption efficiency, the local dust DRF can reach positive values and the global mean DRF can be insignificantly small. In the case of low absorption efficiency, the dust DRF can produce a significant global cooling in glacial periods, which leads to a doubling of the maximum glacial ice volume relative to the case with small dust DRF. DRF-induced temperature and precipitation changes can either be attenuated or amplified through a feedback loop involving the dust cycle. The sensitivity experiments suggest that depending on dust optical parameters, dust DRF has the potential to either damp or reinforce glacial–interglacial climate changes.

An integral aeolian dust cycle model for glacial–interglacial climate simulations with the Earth system model of intermediate complexity CLIMBER-2 is introduced, to study the role of aeolian dust for the evolution of glacial cycles. Sources, transport and deposition of dust are modeled for the first time over the past four glacial cycles using internally consistent characteristics of the atmosphere, the terrestrial vegetation coupled to the carbon cycle, and the ice sheets. The space–time variability in the dust deposition reconstructed from ice cores, and marine and terrigenous sediments is reasonably reproduced on the coarse spatial grid of the CLIMBER-2 model for time scales from seasons to hundreds of millennia. Of prime importance for reproducing the variability in dust deposition are changes in dust source areas and dust emission, while changes in dust transport and dust deposition are of secondary importance. The dependence of the dust sources on characteristics of the biosphere and the cryosphere is pertinent for obtaining the strong 100-kyr cyclicity in aeolian dust.

Recent studies have drawn attention to differences in the seasonal impact of the 8.2 ka event, with longer cooler summers and shorter cooler/drier winters. However, there are no data available on the simultaneity or the rate of onset of the seasonal changes in Europe. Based on the microfacies and geochemical analyses of seasonally laminated varved sediments from Holzmaar, we present evidence of differences in duration and onset time of changes in summer temperature and winter rainfall during the 8.2 ka event. Since both summer and winter climate signals are co-registered within a single varve, there can be no ambiguity about the phasing and duration of the signals. Our data show that the onset and withdrawal of the 8.2 ka summer cooling occurred within a year, and that summer rains were reduced or absent during the investigated period. The onset of cooler summers preceded the onset of winter dryness by ca. 28 years. In view of the differences in nature and duration of the impact of the 8.2 ka event we suggest that a clearer definition of the 8.2 ka event (summer cooling or winter cooling/dryness) needs to be developed. Based on regional comparison and available modelling studies we also discuss the roles of solar variability, changes in North Atlantic Thermohaline circulation, and North Atlantic Circulation (NAO) during the period under consideration. Wavelet analyses of seasonal laminae indicates that the longer NAO cycles, linked to changes in the N. Atlantic temperatures, were more frequent during the drier periods. (C) 2009 Elsevier B.V. All rights reserved.

Within a stochastic version of the model it has been possible to mimic the 8.2 ka event within a plausible experimental setting and with relatively good accuracy considering the timing of the event in comparison to other modeling exercises [1]. The simulation of the centennial cold event is effectively determined by the oceanic cooling rate which depends largely on the ocean diffusivity described by diffusion coefficients of relatively wide uncertainty ranges. The idea now is to discriminate between the different values of diffusivities according to their likelihood to rightly represent the duration of the 8.2 ka event and thus to exploit the paleo data to constrain uncertainty in model parameters in analogue to [2]. Implementing this inverse Bayesian Analysis with this model the technical difficulty arises to establish the related likelihood numerically in addition to the uncertain model parameters: While mainstream uncertainty analyses can assume a quasi-Gaussian shape of likelihood, with weather fluctuating around a long term mean, the 8.2 ka event as a highly nonlinear effect precludes such an a priori assumption. As a result of this study [3] the Bayesian Analysis showed a reduction of uncertainty in vertical ocean diffusivity parameters of factor 2 compared to prior knowledge. This learning effect on the model parameters is propagated to other model outputs of interest; e.g. the inverse ocean heat capacity, which is important for the dominant time scale of climate response to anthropogenic forcing which, in combination with climate sensitivity, strongly influences the climate systems reaction for the nearand medium-term future.

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Greenland ice-core data containing the 8.2 ka event are utilized by a model-data intercomparison within the Earth system model of intermediate complexity, CLIMBER-2.3 to investigate their potential for constraining the range of uncertain ocean diffusivity properties. Within a stochastic version of the model (Bauer et al. in Paleoceanography 19:PA3014, 2004) it has been possible to mimic the pronounced cooling of the 8.2 ka event with relatively good accuracy considering the timing of the event in comparison to other modelling exercises. When statistically inferring from the 8.2 ka event on diffusivity the technical difficulty arises to establish the related likelihood numerically per realisation of the uncertain model parameters: while mainstream uncertainty analyses can assume a quasi-Gaussian shape of likelihood, with weather fluctuating around a long term mean, the 8.2 ka event as a highly nonlinear effect precludes such an a priori assumption. As a result of this study the Bayesian Analysis leads to a sharp single-mode likelihood for ocean diffusivity parameters within CLIMBER-2.3. Depending on the prior distribution this likelihood leads to a reduction of uncertainty in ocean diffusivity parameters (e.g. for flat prior uncertainty in the vertical ocean diffusivity parameter is reduced by factor 2). These results highlight the potential of paleo data to constrain uncertain system properties and strongly suggest to make further steps with more complex models and richer data sets to harvest this potential.

We use a coupled climate–carbon cycle model of intermediate complexity to investigate scenarios of stratospheric sulfur injections as a measure to compensate for CO 2 -induced global warming. The baseline scenario includes the burning of 5,000 GtC of fossil fuels. A full compensation of CO 2 -induced warming requires a load of about 13 MtS in the stratosphere at the peak of atmospheric CO 2 concentration. Keeping global warming below 2°C reduces this load to 9 MtS. Compensation of CO 2 forcing by stratospheric aerosols leads to a global reduction in precipitation, warmer winters in the high northern latitudes and cooler summers over northern hemisphere landmasses. The average surface ocean pH decreases by 0.7, reducing the calcifying ability of marine organisms. Because of the millennial persistence of the fossil fuel CO 2 in the atmosphere, high levels of stratospheric aerosol loading would have to continue for thousands of years until CO 2 was removed from the atmosphere. A termination of stratospheric aerosol loading results in abrupt global warming of up to 5°C within several decades, a vulnerability of the Earth system to technological failure.

The Earth system model CLIMBER-2 is extended by a scheme for calculating the climatic response to anthropogenic sulphur dioxide emissions. The scheme calculates the direct radiative forcing, the first indirect cloud albedo effect, and the second indirect cloud lifetime effect induced by geographically resolved sulphate aerosol burden. The simulated anthropogenic sulphate aerosol burden in the year 2000 amounts to 0.47 TgS. The best guesses for the radiative forcing due to the direct effect are -0.4 Wm-2 and for the decrease in short-wave radiation due to all aerosol effects -0.8 Wm-2. The simulated global warming by 1 K from 1850 to 2000 caused by anthropogenic greenhouse gases reduces to 0.6 K when the sulphate aerosol effects are included. The model’s hydrological sensitivity of 4%/K is decreased by the second indirect effect to 0.8%/K. The quality of the geographically distributed climatic response to the historic emissions of sulphur dioxide and greenhouse gases makes the extended model relevant to computational efficient investigations of future climate change scenarios.

We explore the hypothesis that the abrupt drainage of Laurentide lakes and associated rapid switch of the North Atlantic thermohaline circulation 8200 yr ago had a catastrophic influence on Neolithic civilisation in large parts of southeastern Europe, Anatolia, Cyprus, and the Near East. The event at 8200 cal yr BP is observed in a large number of high-resolution climate proxies in the Northern Hemisphere, and in many cases corresponds to markedly cold and and conditions. We identify the relevant archaeological levels of major Neolithic settlements in Central Anatolia, Cyprus, Greece and Bulgaria, and examine published stratigraphic, architectural, cultural and geoarchaeological studies for these sites. The specific archaeological events and processes we observe at a number of these sites during the study interval 8400-8000 cal yr BP lead us to refine some previously established Neolithisation models. The introduction of fanning to South-East Europe occurs in all study regions (Thrace, Macedonia, Thessaly, Bulgaria) near 8200 cal yr BP. We observe major disruptions of Neolithic cultures in the Levant, North Syria, South-East Anatolia, Central Anatolia and Cyprus, at the same time. We conclude that the 8200 cal yr BP aridity event triggered the spread of early farmers, by different routes, out of West Asia and the Near East into Greece and Bulgaria. (c) 2006 University of Washington. All rights reserved.

Temperature observations from the Northern Hemisphere reveal a warming since 1861 which is larger in winter than in summer. Possible explanations for a decline in seasonal spread are discussed using the Earth system model CLIMBER‐2. Simulations forced by natural and anthropogenic factors (Milankovitch forcing, solar variability, volcanism, atmospheric CO2 concentration, deforestation) generate specific seasonal responses. While the Milankovitch forcing increased the millennial seasonal spread, and solar variability and volcanism proved ancillary in reducing the spread on the centennial timescale, the anthropogenic factors appear the primary agents to attenuate the seasonal spread. The climatic effect of the anthropogenic factors is amplified by seasonally varying feedbacks related to the albedo of changing sea‐ice and snow cover.

We assess the role of changing natural (volcanic, aerosol, insolation) and anthropogenic (CO2 emissions, land cover) forcings on the global climate system over the last 150 years using an earth system model of intermediate complexity, CLIMBER-2. We apply several datasets of historical land-use reconstructions: the cropland dataset by Ramankutty & Foley (1999) (R&F), the HYDE land cover dataset of Klein Goldewijk (2001), and the land-use emissions data from Houghton & Hackler (2002). Comparison between the simulated and observed temporal evolution of atmospheric CO2 and delta(13)CO(2) are used to evaluate these datasets. To check model uncertainty, CLIMBER-2 was coupled to the more complex Lund-Potsdam-Jena (LPJ) dynamic global vegetation model.In simulation with R&F dataset, biogeophysical mechanisms due to land cover changes tend to decrease global air temperature by 0.26degreesC, while biogeochemical mechanisms act to warm the climate by 0.18degreesC. The net effect on climate is negligible on a global scale, but pronounced over the land in the temperate and high northern latitudes where a cooling due to an increase in land surface albedo offsets the warming due to land-use CO2 emissions.Land cover changes led to estimated increases in atmospheric CO2 of between 22 and 43 ppmv. Over the entire period 1800-2000, simulated delta(13)CO(2) with HYDE compares most favourably with ice core during 1850-1950 and Cape Grim data, indicating preference of earlier land clearance in HYDE over R&F. In relative terms, land cover forcing corresponds to 25-49% of the observed growth in atmospheric CO2. This contribution declined from 36-60% during 1850-1960 to 4-35% during 1960-2000. CLIMBER-2-LPJ simulates the land cover contribution to atmospheric CO2 growth to decrease from 68% during 1900-1960 to 12% in the 1980s. Overall, our simulations show a decline in the relative role of land cover changes for atmospheric CO2 increase during the last 150 years.

During the last millennium, variations in solar luminosity, volcanic activity, anthropogenic land use change and greenhouse emissions are the major forcings of the Earth climate. Recent reconstructions of these forcings are employed in CLIMBER-2, a coupled atmosphere-ocean-biosphere model of intermediate complexity, to estimate their effect on the climate system. Solar-forced CLIMBER-2 simulations produce changes in the annual-mean surface temperature and in the North Atlantic meridional overturning in reasonable agreement with simulations by a comprehensive atmosphere-ocean general circulation model. The solar-forced simulation reproduces the centennial climate variability reconstructed from paleoclimatic data, for the relatively warm and wet 'Medieval Warm Period' compared to the cold and dry 'Little Ice Age'. However, in the southern European region the simulated precipitation increased during the Little Ice Age in consistence with reports on advances of glaciers and on flooding events. The rather low temperatures in the second half of the 19th century axe attributed mainly to deforestation and little to solar activity and volcanic activity. The subsequent warming in the 20th century can be simulated reasonably only when the effect from the growing emission of greenhouse gases is included.

The cold climate anomaly about 8200 years ago is investigated with CLIMBER‐2, a coupled atmosphere‐ocean‐biosphere model of intermediate complexity. This climate model simulates a cooling of about 3.6 K over the North Atlantic induced by a meltwater pulse from Lake Agassiz routed through the Hudson strait. The meltwater pulse is assumed to have a volume of 1.6 × 1014 m3 and a period of discharge of 2 years on the basis of glaciological modeling of the decay of the Laurentide Ice Sheet (LIS). We present a possible mechanism which can explain the centennial duration of the 8.2 ka cold event. The mechanism is related to the existence of an additional equilibrium climate state with reduced North Atlantic Deep Water (NADW) formation and a southward shift of the NADW formation area. Hints at the additional climate state were obtained from the largely varying duration of the pulse‐induced cold episode in response to overlaid random freshwater fluctuations in Monte Carlo simulations. The model equilibrium state was attained by releasing a weak multicentury freshwater flux through the St. Lawrence pathway completed by the meltwater pulse. The existence of such a climate mode appears essential for reproducing climate anomalies in close agreement with paleoclimatic reconstructions of the 8.2 ka event. The results furthermore suggest that the temporal evolution of the cold event was partly a matter of chance.