Claudia Kubatzki

Simulation of an abrupt change in Saharan vegetation in the Mid‐Holocene

Climate variability during the present inter- glacial, the Holocene, has been rather smooth in compar- ison with the last glacial. Nevertheless, there were some rather abrupt climate changes. One of these changes, the desertication 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 de- sertication was triggered by subtle variations in the Earths orbit which were strongly amplied by atmosphere- vegeta- tion feedbacks in the subtropics. The timing of this tran- sition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.

CLIMBER-2: A climate system model of intermediate complexity. Part I: Model description and performance for present climate

Abstract 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 fluxes of momentum, energy, water and carbon. The model has a coarse spatial resolution, nevertheless capturing the major features of the Earths 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 flux adjustments and has a fast turnaround time, it can be used to study climates significantly different 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.

Carbon cycle, vegetation, and climate dynamics in the Holocene: Experiments with the CLIMBER-2 model

Holocene was accompanied by significant changes in vegetation cover and an increase inatmosphericCO2concentration.Theessentialquestioniswhetheritispossibletoexplain thesechangesinaconsistentway,accounting fortheorbitalparametersasthemainexternal 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 CO2 record requires an additional assumption of excessive CaCO3sedimentation in the ocean. The modeled decrease in the carbonate ion concentration in the deep ocean is similar to that inferred from CaCO3 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

Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model

Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called Dansgaard–Oeschger events, with a quasi-periodicity of ∼1,470 years for the late glacial period. 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. But whereas pronounced solar cycles of ∼87 and ∼210 years are well known, a ∼1,470-year solar cycle has not been detected. 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.

Climate change in northern Africa: The past is not the future

By using a climate system model of intermediate complexity, we have simulated long-term natural climate changes occurring over the last 9000 years. The paleo-simulations in which the model is driven by orbital forcing only, i.e., by changes in insolation caused by changes in the Earths orbit, are compared with sensitivity simulations in which various scenarios of increasing atmospheric CO2 concentration are prescribed. Focussing on climate and vegetation change in northern Africa, we recapture the strong greening of the Sahara in the early and mid-Holocene (some 9000–6000 years ago), and we show that some expansion of grasslandinto the Sahara is theoretically possible, if the atmospheric CO2 concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO2 increase, vegetation migration into the Sahara can be rapid, up to 1/10th of the Saharan area per decade, but could not exceed a coverage of 45%. In ourmodel, vegetation expansion into todays Sahara is triggered by an increase in summer precipitation which is amplified by a positive feedback between vegetation and precipitation. This is valid for simulations with orbital forcing and greenhouse-gas forcing. However, we argue that the mid-Holocene climate optimum some 9000 to 6000 years ago with its marked reduction of deserts in northern Africa is not a direct analogue for future greenhouse-gas induced climate change, as previously hypothesized. Not only does the global pattern of climate change differ between the mid-Holocene model experiments and the greenhouse-gas sensitivity experiments, but the relative role of mechanisms which lead to a reduction of the Sahara also changes. Moreover, the amplitude of simulated vegetation cover changes in northern Africa is less than is estimated for mid-Holocene climate.

STABILITY ANALYSIS OF THE CLIMATE-VEGETATION SYSTEM IN THE NORTHERN HIGH LATITUDES

The stability of the climate-vegetation system in the northern high latitudesis analysed with three climate system models of different complexity: A comprehensive 3-dimensional model of the climate system, GENESIS-IBIS, and two Earth system models of intermediate complexity (EMICs), CLIMBER-2 andMoBidiC. The biogeophysical feedback in the latitudinal belt 60–70° N, although positive, is not strong enough to support multiple steady states: A unique equilibriumin the climate-vegetation system is simulated by all the models on a zonal scale for present-day climate and doubled CO2 climate.EMIC simulations with decreased insolation also reveal a unique steady state. However, the climate sensitivity to tree cover,Δ TF, exhibits non-linear behaviour within the models. For GENESIS-IBIS and CLIMBER-2, Δ TF islower for doubled CO2 climate than for present-day climate due to a shorter snow season and increased relative significance ofthe hydrological effect of forest cover. For the EMICs, Δ TF is higher for low tree fraction than for high treefraction, mainly due to a time shift in spring snow melt in response to changes in tree cover. The climate sensitivity to tree coveris reduced when thermohaline circulation feedbacks are accounted for in the EMIC simulations. Simpler parameterizations of oceanic processes have opposite effects onΔ TF: Δ TF is lower in simulations with fixed SSTs and higher in simulations with mixed layer oceans. Experiments with transient CO2 forcing show climate and vegetation not in equilibrium in the northern high latitudes at the end of the 20thcentury. The delayed response of vegetation and accelerated global warming lead to rather abrupt changes in northern vegetation cover in the first halfof the 21st century, when vegetation cover changes at double the present day rate.

A new model for climate system analysis: Outline of the model and application to palaeoclimate simulations.

We present a new reduced-form model for climate system analysis. This model, called CLIMBER-2 (for CLIMate and BiosphERe, level 2), fills the current gap between simple, highly parameterized climate models and computationally expensive coupled models of global atmospheric and oceanic circulation. We outline the basic assumptions implicit in CLIMBER-2 and we present examples of climate system analysis including a study of atmosphere–ocean interaction during the last glacial maximum, an analysis of synergism between various components of the climate system during the mid-Holocene around 6000 years ago, and a transient simulation of climate change during the last 8000 years. These studies demonstrate the feasibility of a computationally efficient analysis of climate system dynamics which is a prerequisite for future climate impact research and, more generally, Earth system analysis, i.e., the analysis of feedbacks between our environment and human activities.

Mechanisms and time scales of glacial inception simulated with an earth system model of intermediate complexity

Abstract. We investigate glacial inception and glacial thresholds in the climate-cryosphere system utilising the Earth system model of intermediate complexity CLIMBER-2, which includes modules for atmosphere, terrestrial vegetation, ocean and interactive ice sheets. The latter are described by the three-dimensional polythermal ice-sheet model SICOPOLIS. A bifurcation which represents glacial inception is analysed with two different model setups: one setup with dynamical ice-sheet model and another setup without it. The respective glacial thresholds differ in terms of maximum boreal summer insolation at 65° N (hereafter referred as Milankovitch forcing (MF)). The glacial threshold of the configuration without ice-sheet dynamics corresponds to a much lower value of MF compared to the full model. If MF attains values only slightly below the aforementioned threshold there is fast transient response. Depending on the value of MF relative to the glacial threshold, the transient response time of inland-ice volume in the model configuration with ice-sheet dynamics ranges from 10 000 to 100 000 years. Due to these long response times, a glacial threshold obtained in an equilibrium simulation is not directly applicable to the transient response of the climate-cryosphere system to time-dependent orbital forcing. It is demonstrated that in transient simulations just crossing of the glacial threshold does not imply large-scale glaciation of the Northern Hemisphere. We found that in transient simulations MF has to drop well below the glacial threshold determined in an equilibrium simulation to initiate glacial inception. Finally, we show that the asynchronous coupling between climate and inland-ice components allows one sufficient realistic simulation of glacial inception and, at the same time, a considerable reduction of computational costs.

Modelling the end of an interglacial (MIS 1, 5, 7, 9, 11)

Abstract With the CLIMBER-2 model, the last glacial inception is simulated as a rapid ice-sheet expansion over northern North America, with only little ice sheets in Scandinavia. We present sensitivity studies that more deeply investigate the climatic feedbacks at the end of an interglacial. (i) An ice-free Greenland during the Eemian appears as a second stable state in the model. The initial size of the Greenland ice sheet, however, has only little effect on the subsequent glacial inception. (ii) North American glaciation can be reduced or even suppressed using preindustrial or Eemian vegetation and/or ocean surface conditions. (iii) Timing and amplitude of the last glacial inception cannot be estimated by the use of time-slice simulations. (iv) Changes in precession (perihelion) are crucial for the ice-sheet growth, obliquity and CO 2 merely act as amplifiers. (v) Cold events at the end of the interglacial can be reproduced by the introduction of freshwater disturbances into the North Atlantic. (vi) The model is able to simulate earlier glacial inceptions as well, although difficulties exist with respect to the strength of the glaciation. Future glacial inception in the model happens only in about 50 000 years from now.

Modelers and geologists join forces at workshop

The use of geological evidence for climate model evaluation is inevitable. This is clearly reflected by the status of past climatic changes in the latest IPCC Assessment Report (www ipcc.ch/). Earth System Models of Intermediate Complexity (EMICs) [Claussen et al., 2002] are designed to bridge the gap between simple, conceptual climate models and comprehensive General Circulation Models (GCMs). The big advantage of EMICs lies in their computational efficiency, which allows for long-term climate simulations over several tens of thousands of years. As such, they are reasonable tools for simulating past glacial-interglacial climatic changes. The large number of processes described in the EMICs, even though in a reduced form, enables the user to investigate interactions and feedbacks within the climate system in a broad range of sensitivity experiments.