A 2.5-dimensional climate system model of intermediate complexity CLIMBER-2 and its performance for present climate conditions are presented. The model consists of modules describing atmosphere, ocean, sea ice, land surface processes, terrestrial vegetation cover, and global carbon cycle. The modules interact through the¯uxes of momentum, energy, water and carbon. The model has a coarse spatial resolution, nevertheless capturing the major features of the Earth's geography. The model describes temporal variability of the system on seasonal and longer time scales. Due to the fact that the model does not employ¯ux adjustments and has a fast turnaround time, it can be used to study climates signi®cantly dierent from the present one and to perform long-term (multimillennia) simulations. The comparison of the model results with present climate data show that the model successfully describes the seasonal variability of a large set of characteristics of the climate system, including radiative balance, temperature, precipitation, ocean circulation and cryosphere.

[1] Multiple proxy data reveal that the early to middle Holocene (ca. 8-6 kyr B.P.) was warmer than the preindustrial period in most regions of the Northern Hemisphere. This warming is presumably explained by the higher summer insolation in the Northern Hemisphere, owing to changes in the orbital parameters. Subsequent cooling in the late Holocene was accompanied by significant changes in vegetation cover and an increase in atmospheric CO 2 concentration. The essential question is whether it is possible to explain these changes in a consistent way, accounting for the orbital parameters as the main external forcing for the climate system. We investigate this problem using the computationally efficient model of climate system, CLIMBER-2, which includes models for oceanic and terrestrial biogeochemistry. We found that changes in climate and vegetation cover in the northern subtropical and circumpolar regions can be attributed to the changes in the orbital forcing. Explanation of the atmospheric CO 2 record requires an additional assumption of excessive CaCO 3 sedimentation in the ocean. The modeled decrease in the carbonate ion concentration in the deep ocean is similar to that inferred from CaCO 3 sediment data [Broecker et al., 1999]. For 8 kyr B.P., the model estimates the terrestrial carbon pool ca. 90 Pg higher than its preindustrial value. Simulated atmospheric d 13 C declines during the course of the Holocene, similar to d 13 C data from the Taylor Dome ice core [Indermühle et al., 1999]. Amplitude of simulated changes in d 13 C is smaller than in the data, while a difference between the model and the data is comparable with the range of data uncertainty.

Abstract.Climate variability during the present interglacial, the Holocene, has been rather smooth in comparison with the last glacial. Nevertheless, there were some rather abrupt climate changes. One of these changes, the desertification of the Saharan and Arabian region some 4 -6 thousand years ago, was presumably quite important for human society. It could have been the stimulus leading to the foundation of civilizations along the Nile, Euphrat and Tigris rivers. Here we argue that Saharan and Arabian desertification was triggered by subtle variations in the Earth's orbit which were strongly amplified by atmosphere-vegetation feedbacks in the subtropics. The timing of this transition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.

surement precisions (0.5%) (20) in the travel times themselves. 20. Single-crystal samples of MgO (purity Ͼ99.9%) were oriented with x-ray diffraction method described in (13) to [100] and [110] orientations (better than 1°); the samples were cored, cut, and polished to cylinders of about 1.8 mm in diameter and 1.2 to 1.5 mm in length. The original sample lengths were measured to micrometers, that is, Ϯ 1 m. Measurements of the sample travel time were achieved by overlapping the glass buffer rod echo and the sample echo and recording the amplitudes of the interference signal as a function of the frequency (from 20 to 70 MHz). Precision in the travel time measurements is about 2 ns or 0.5% for the sample used in this study. We used Pt (20 m thick) and Au (2 m thick) foils to couple the acoustic wave from the glass buffer rod to the sample. These metal foils alleviate the problems of the compressibility and thermal expansivity mismatches between the buffer and the sample, but they also introduce uncertainties in the acoustic travel time [ (25) after correcting for the effect of the bonds (Pt or Au foils), the sample travel times agree within the measurement uncertainties (2 ns). 21. In extracting the cross pressure-temperature derivatives of the elastic moduli ‫ץ(‬ 2 C ij /‫ץ‬P‫ץ‬T ), our modulus data at elevated P and T are fit by polynomials [with the constraints of fixing the pressure derivatives (‫ץ‬C ij /‫ץ‬P) at ambient temperature (from our high pressure-ambient temperature data) and the temperature derivatives (‫ץ‬C ij /‫ץ‬T ) at ambient pressure [from averaging the rectangular parallelopiped resonance (RPR) data (5)]. Such fits are mathematically robust because the boundary conditions (along the pressure and temperature axes) are well defined: the RPR method provides data if very high precision along the temperature axis, and the acoustic data along the pressure axis also have high precision [in light of their good agreement with the results obtained from the gas pressure vessel (Fig. 3) Simulations with a synchronously coupled atmosphere-ocean-vegetation model show that changes in vegetation cover during the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly (through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the atmosphere-vegetation feedback is most important. Moreover, a reduction of the thermohaline circulation in the Atlantic Ocean leads to a warming of the Southern Hemisphere.Numerous paleodata suggest that the climate of the mid-Holocene around 6 thousand years ago (ka) was quite different from that of today. Generally, the summer in many mid-to high-latitude regions of the Northern Hemisphere was warmer, and paleobotanic da...

Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called DansgaardOeschger events, with a quasi-periodicity of ,1,470 years for the late glacial period [1][2][3][4][5][6] . Various hypotheses have been suggested to explain these rapid temperature shifts, including internal oscillations in the climate system and external forcing, possibly from the Sun 7 . But whereas pronounced solar cycles of ,87 and ,210 years are well known [8][9][10][11][12] , a ,1,470-year solar cycle has not been detected 8 . Here we show that an intermediate-complexity climate model with glacial climate conditions simulates rapid climate shifts similar to the Dansgaard-Oeschger events with a spacing of 1,470 years when forced by periodic freshwater input into the North Atlantic Ocean in cycles of ,87 and ,210 years. We attribute the robust 1,470-year response time to the superposition of the two shorter cycles, together with strongly nonlinear dynamics and the long characteristic timescale of the thermohaline circulation. For Holocene conditions, similar events do not occur. We conclude that the glacial 1,470-year climate cycles could have been triggered by solar forcing despite the absence of a 1,470-year solar cycle.The onset of successive Dansgaard-Oeschger (DO) events, as documented in Greenland ice-cores 1,2 for example, is typically spaced by ,1,470 years or integer multiples thereof 13,14 . Because deviations from this cyclicity are small, often less than 100-200 years 15 , external forcing (solar or orbital) was suggested to trigger DO events 6,15,16 . However, neither orbital nor solar forcing shows a 1,470-year frequency. Spectral analysis performed on records of cosmogenic nuclides [8][9][10][11] , which are commonly used as proxies for solar variability 12 , indicates the possible existence of pronounced and stable 10,11 centennial-scale solar cycles (the DeVries-Suess and Gleissberg cycles with periods near 210 and 87 years 10,11 ) but does not reveal a 1,470-year cycle 8 . However, the DeVries and Gleissberg cycles are close to prime factors of 1,470 years (1,470/7 ¼ 210; 1,470/17 < 86.5). The superposition of two such frequencies could result in variability that repeats with a 1,470-year period.Here we propose that these two solar frequencies could have synchronized the glacial 1,470-year climate cycle. Support for the idea that a multi-century climate cycle might be linked with centuryscale solar variability comes from Holocene data: a multi-centennial drift-ice cycle in the North Atlantic was reported 17 to coincide with "rapid (100-to 200-year), conspicuously large-amplitude variations" in the production rates of the cosmogenic isotopes 14 C and 10 Be. To test our hypothesis, we force the coupled climate system model CLIMBER-2 (version 3) with the two solar frequencies. Earlier simulations with this model showed that, when forced by periodic and/or stochastic variations in the freshwater flux into the northern Atlantic, abrupt glacial warming events are triggere...

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 arid 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 farming 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.

A set of sensitivity experiments with the climate system model of intermediate complexity CLIMBER-2 was performed to compare its sensitivity to changes in dierent types of forcings and boundary conditions with the results of comprehensive models (GCMs). We investigated the climate system response to changes in freshwater¯ux into the Northern Atlantic, CO 2 concentration, solar insolation, and vegetation cover in the boreal zone and in the tropics. All these experiments were compared with the results of corresponding experiments performed with dierent GCMs. Qualitative, and in many respects, quantitative agreement between the results of CLIMBER-2 and GCMs demonstrate the ability of our climate system model of intermediate complexity to address diverse aspects of the climate change problem. In addition, we used our model for a series of experiments to assess the impact of some climate feedbacks and uncertainties in model parameters on the model sensitivity to dierent forcings. We studied the role of freshwater feedback and vertical ocean diffusivity for the stability properties of the thermohaline ocean circulation. We show that freshwater feedback plays a minor role, while changes of vertical diusivity in the ocean considerably aect the circulation stability. In global warming experiments we analysed the impact of hydrological sensitivity and vertical diusivity on the long-term evolution of the thermohaline circulation. In the boreal and tropical deforestation experiments we assessed the role of an interactive ocean and showed that for both types of deforestation scenarios, an interactive ocean leads to an additional cooling due to albedo and water vapour feedbacks.

Summary In order to estimate the effect of historical land cover change (deforestation) on climate, we perform a set of experiments with a climate system model of intermediate complexity – CLIMBER‐2. We focus on the biophysical effect of the land cover change on climate and do not explicitly account for the biogeochemical effect. A dynamic scenario of deforestation during the last millennium is formulated based on the rates of land conversion to agriculture. The deforestation scenario causes a global cooling of 0.35 °C with a more notable cooling of the northern hemisphere (0.5 °C). The cooling is most pronounced in the northern middle and high latitudes, especially during the spring season. To compare the effect of deforestation on climate with other forcings, climate responses to the changing atmospheric CO2 concentration and solar irradiance are also analysed. When all three factors are taken into account, dynamics of northern hemisphere temperature during the last 300 years within the model are generally in agreement with the observed (reconstructed) temperature trend. We conclude that the impact of historical land cover changes on climate is comparable with the impact of the other climate forcings and that land cover forcing is important for reproducing historical climate change.