Climate Model Of Intermediate Complexity Research Articles

Assessing the effects of ocean diffusivity and climate sensitivity on the rate of global climate change

ABSTRACTThe range in the projections of future climate warming can be attributed to the inherent uncertainty in the representation of climate model parameters and processes. In this study, we assess the effect of uncertainty in climate sensitivity and ocean heat uptake on the rate of future climate change. We apply a range of values for climate sensitivity and ocean diapycnal diffusivity in an ensemble of simulations using an intermediate-complexity climate model. We further use probability density functions to estimate the likelihood of each model outcome; using this framework, we calculate a range of likely rates of temperature change in response to a given future CO2 emissions scenario. From this analysis, the most probable maximum rate of temperature change lies between 0.3 and 0.5 °C/decade, with a most likely value of 0.36 °C/decade, which is more than twice the observed rate in the late twentieth century. We show that changes in ocean diffusivity have a significant effect on the rate of transient climate change for high values of climate sensitivity, while they have little influence when climate sensitivity is low. The highest rates of warming occur with high values of climate sensitivity and low values of ocean diffusivity. Such high rates of change could adversely affect the adaptive capacity of healthy functional ecosystems.

Uncertainty of climate response to natural and anthropogenic forcings due to different land use scenarios

The A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS) climate model (CM) of intermediate complexity is extended by a spatially explicit terrestrial carbon cycle module. Numerical experiments with the IAP RAS CM are performed forced by the reconstructions of anthropogenic and natural forcings for the 16th to the 20th centuries and by combined SRES (Special Report on Emission Scenarios) A2-LUH (Land Use Harmonization) anthropogenic scenarios for the 21st century. Hereby, the impact of uncertainty in land-use scenarios on results of simulations with a coupled climate-carbon cycle model is tested. The simulations of the model realistically reproduced historical changes in carbon cycle characteristics. In the IAP RAS CM, climate warming reproduced in the 20th and 21st centuries enhanced terrestrial net primary production but terrestrial carbon uptake was suppressed due to an overcompensating increase in soil respiration. Around year 2100, the simulations the model forced by different land use scenarios diverged markedly, by about 70 Pg (C) in terms of biomass and soil carbon stock but they differed only by about 10 in terms of atmospheric carbon dioxide content.

The medieval climate anomaly in Europe: Comparison of the summer and annual mean signals in two reconstructions and in simulations with data assimilation

The spatial pattern and potential dynamical origin of the Medieval Climate Anomaly (MCA, around 1000AD) in Europe are assessed with two recent reconstructions and simulations constrained to follow those reconstructions by means of paleoclimate data assimilation. The simulations employ a climate model of intermediate complexity (LOVECLIM). The data assimilation technique is based on a particle filter using an ensemble of 96 simulations. The peak winter (and annual mean) warming during the MCA, in our analyses, is found to be strongest at high latitudes, associated with strengthened mid-latitude westerlies. Summer warmth, by contrast, is found to be greatest in southern Europe and the Mediterranean Sea, associated with reduced westerlies and strengthened southerly winds off North Africa. The results of our analysis thus underscore the complexity of the spatial and seasonal structure of the MCA in Europe.

Ice sheet collapse affects ocean circulation

As Earth's climate warms and ice melts, freshwater input to oceans could weaken the large‐scale Atlantic meridional overturning circulation, which acts as an important conveyor of heat and has significant effects on climate. Green et al. used an intermediate complexity climate model to study how freshwater input to oceans can affect the meridional overturning circulation. They applied their model to the collapse of the Barents ice sheet about 140,000 years ago—the first study of this kind for the time period—which resulted in a huge influx of freshwater to the North Atlantic Ocean as large icebergs calved off of the ice sheet. (Paleoceanography, doi:10.1029/2010PA002088, 2011)

Simulating the impact of freshwater inputs and deep-draft icebergs formed during a MIS 6 Barents Ice Sheet collapse

[1] An intermediate complexity climate model is used to simulate the collapse of the Barents Ice Sheet during Marine Isotope Stage 6 (MIS 6; 140 ka B.P) with the purpose of investigating whether a mass input of freshwater from the collapse could have affected the convection and deep water formation in the North Atlantic Ocean. Further experiments used a coupled dynamic and thermodynamic iceberg model to determine the effects of deep-draft icebergs, rather than freshwater alone, on the ocean circulation. The results predict that the collapse of the Barents Ice Sheet had a significant impact on the meridional overturning circulation in both the Atlantic and Pacific oceans. Freshwater fluxes have more of an impact on the Atlantic overturning circulation during the actual release period compared to icebergs, but the bergs induce effects over longer time scales even after the pulse is removed. Freshwater fluxes of 0.15 sverdrup (Sv) and iceberg surges of 0.1 Sv trigger significant changes in the global patterns, particularly in the North Pacific where there is strengthening of the overturning circulation at the expense of that in the North Atlantic, and associated increases in Pacific sea surface temperatures. These results highlight the importance of simulating not only the correct flux but also the form of the freshwater input from ice sheet collapses appropriately.

Late Eemian warming in the Nordic Seas as seen in proxy data and climate models

We analyze a transient simulation of the last glacial inception in a climate model of intermediate complexity, focusing on sea ice–ocean circulation dynamics in the North Atlantic and Nordic Seas. As northern high‐latitude summer insolation decreases toward the end of the Eemian interglacial, Arctic sea ice export to the North Atlantic increases. This surface fresh water transport weakens deep water formation in the North Atlantic and the near‐surface circulation of the subpolar gyre. As a consequence, the relative contribution of subpolar gyre waters to the Atlantic inflow into the Nordic Seas is reduced, giving way to more warm and saline subtropical waters from the North Atlantic Current. We thus find an episode of relatively high heat and salt transport into the Nordic Seas during the last glacial inception between 119,000 and 115,000 years before present. This stabilizes deep ocean convection in the region and warms Scandinavia during a phase of low insolation. These findings are in good agreement with proxy data from the Nordic Seas and North Atlantic. At the end of the warm interval, sea surface temperature drops by about 3°C, marking the onset of large‐scale glacier growth over Scandinavia.

The Impact of Wind Stress Feedback on the Stability of the Atlantic Meridional Overturning Circulation

Abstract Recent results based on models using prescribed surface wind stress forcing have suggested that the net freshwater transport Σ by the Atlantic meridional overturning circulation (MOC) into the Atlantic basin is a good indicator of the multiple-equilibria regime. By means of a coupled climate model of intermediate complexity, this study shows that this scalar Σ cannot capture the connection between the properties of the steady state and the impact of the wind stress feedback on the evolution of perturbations. This implies that, when interpreting the observed value of Σ, the position of the present-day climate is systematically biased toward the multiple-equilibria regime. The results show, however, that the stabilizing influence of the wind stress feedback on the MOC is restricted to a narrow window of freshwater fluxes, located in the vicinity of the state characterized by a zero freshwater flux divergence over the Atlantic basin. If the position of the present-day climate is farther away from this state, then wind stress feedbacks are unable to exert a persistent effect on the modern MOC. This is because the stabilizing influence of the shallow reverse cell situated south of the equator during the off state rapidly dominates over the destabilizing influence of the wind stress feedback when the freshwater forcing gets stronger. Under glacial climate conditions by contrast, a weaker sensitivity with an opposite effect is found. This is ultimately due to the relatively large sea ice extent of the glacial climate, which implies that, during the off state, the horizontal redistribution of fresh waters by the subpolar gyre does not favor the development of a thermally direct MOC as opposed to the modern case.

Estimates of climate changes in the 20th-21st centuries based on the version of the IAP RAS climate model including the model of general ocean circulation

Numerical experiments are analyzed for 1860–2100 with the version of the climate model of intermediate complexity of the Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences (IAP RAS) including the model of general ocean circulation as the oceanic module (CM IAP RAS-GOC) taking account of concentration variations of anthropogenic greenhouse gases and tropospheric sulfate aerosols from the data of observations and reconstructions for the second half of the 19th and for the 20th century and, according to SRES scenarios, in the 21st century. In the 20th century, the model simulates realistically the variations of surface atmospheric temperature, characteristics of heat absorption by the ocean, and oceanic meridional heat transport. The linear trend of global surface atmospheric temperature in the 20th century (in its last 30 years) in this version of the model amounts to 0.5 ± 0.1 K/100 years (0.22 ± 0.05 K/10 years) that is agreed with the observational data. In the 21st century, the global increase in the surface temperature amounts to 2.5 K (3.5 and 4.1 K) for SRES B1 scenario (for SRES A1B and SRES A2 scenarios, respectively). The increase in the surface temperature is the most significant in high latitudes, especially in the Northern Hemisphere and it is higher, on the whole, over the land than over the ocean. The warming near the surface is larger in winter than in summer. The maximum warming is observed in the Arctic and over the land of subpolar latitudes of the Northern Hemisphere reaching 6–10 K by the end of the 21st century in these regions as compared with the end of the 20th century depending on the anthropogenic impact scenario. At the increase in surface temperature in the 20th–21st centuries, the increase in the heat flow to the ocean and the weakening of the heat transport by the ocean from the tropics to the polar area by 1.5–2 times are registered, on the whole. At the warming, the CM IAP RAS-GOC gives the general increase in the annual precipitation amount which is especially appreciable in the tropics and in the storm-track regions. At the global averaging, the precipitation in the 21st century increase by 20–25%.

Deep draft icebergs from the Barents Ice Sheet during MIS 6 are consistent with erosional evidence from the Lomonosov Ridge, central Arctic

Iceberg scour marks on the Lomonosov Ridge in the central Arctic imply that bergs with drafts exceeding 850 m once existed in the Arctic, whereas similar erosion patterns observed on the Yermak Plateau suggest that some of these deep‐keeled icebergs exited the Arctic through the Fram Strait into the Nordic Seas. An intermediate complexity climate model with a dynamic and thermodynamic iceberg model is used to simulate the collapse of the Barents Ice Sheet during MIS 6 with the purpose of investigating if deep‐draft “mega”‐icebergs can reach and ground on the Lomonosov Ridge. A number of simulations with different iceberg sizes show that deep‐draft bergs seeded from near the Franz Victoria Trough could indeed have reached the Ridge, whereas other bergs crossed the Yermak Plateau and exited through the Fram Strait. The draft of the bergs mainly determines the trajectory, with only minor influence from the mass, and length and width of the bergs. This work is consistent with previous speculation that these icebergs originated from the disintegration of the Barents Ice Sheet during Marine Isotope Stage 6 (MIS 6; 140 ka BP), but suggests their origin was more likely to be from the western, rather than eastern, major ice stream of that ice sheet. The long‐term impact on the overturning circulation from melting icebergs is more severe than that from a freshwater pulse of equivalent magnitude, and a correct representation of icebergs in paleoclimate models is therefore of importance.

Precalibrating an intermediate complexity climate model

Credible climate predictions require a rational quantification of uncertainty, but full Bayesian calibration requires detailed estimates of prior probability distributions and covariances, which are difficult to obtain in practice. We describe a simplified procedure, termed precalibration, which provides an approximate quantification of uncertainty in climate prediction, and requires only that uncontroversially implausible values of certain inputs and outputs are identified. The method is applied to intermediate-complexity model simulations of the Atlantic meridional overturning circulation (AMOC) and confirms the existence of a cliff-edge catastrophe in freshwater-forcing input space. When uncertainty in 14 further parameters is taken into account, an implausible, AMOC-off, region remains as a robust feature of the model dynamics, but its location is found to depend strongly on values of the other parameters.

Modelling glacial climatic millennial-scale variability related to changes in the Atlantic meridional overturning circulation: a review

Since the discovery of glacial abrupt variability in Greenland and the North Atlantic about two decades ago, its modelling remains a conceptual and technical challenge. While the global climatic signature of Heinrich events can reasonably be ascribed to the fresh water discharge related to the massive iceberg armadas released by the Laurentide ice-sheet in the North Atlantic, the existence of such a triggering mechanism is elusive as far as Dansgaard–Oeschger type variability is concerned. Even if other mechanisms have been proposed, the majority of modelling attempts to better understand glacial climate variability has assumed it to be linked to abrupt variations in the Atlantic Meridional Overturning Circulation (AMOC). In this review we introduce the rationale behind this approach, i.e. the possible existence of several deep ocean circulation equilibria associated with distinct climate signatures. We then review the published fresh water hosing experiments and the mechanisms at work to explain the climate differences related to the different AMOC states. Most of these works have been performed under pre-industrial boundary conditions. The few hosing experiments run under glacial conditions show that the sensitivity of the climate system to fresh water hosing is very dependent on the background climate state. In order to rigourously compare several types of experiments (equilibrium runs vs transient runs with pre-industrial vs glacial boundary conditions) we present experiments especially run for this review with two climate models of intermediate complexity: UVic and LOVECLIM. Although there are similarities between the responses simulated by these models, there are also many differences which would benefit being compared to reconstructions such as those presented in this special issue. However, a strict comparison will require to consider boundary conditions as realistic as possible for Marine Isotopic Stage 3. Modelling glacial variability to compare model results with paleoclimatic data remains a big challenge.

Last glacial ice‐rafted debris off southwestern Europe: the role of the British–Irish Ice Sheet

AbstractIce‐rafted debris (IRD) seeded into the ocean from Northern Hemisphere ice sheets is found in ocean cores along the southwestern European margin through the last glacial period. It is known that the origin of this IRD, especially off Iberia, can vary between North America and western Europe during short‐lived episodes of greatly enhanced iceberg flux, known as Heinrich events, although in most Heinrich events the IRD has a North American source. During the longer times of much lower IRD fluxes between Heinrich events, use of an intermediate complexity climate model, coupled to an iceberg dynamic and thermodynamic model, shows that background levels of IRD most likely originate from western Europe, particularly the British–Irish Ice Sheet. Combining modelling with palaeoceanographic evidence supports reconstructions of a short‐lived, but substantial, Celtic and Irish Sea Ice Stream around 23 ka. Copyright © 2010 John Wiley & Sons, Ltd.

The Role of Oceanic Heat Transport and Wind Stress Forcing in Abrupt Millennial-Scale Climate Transitions

Abstract The last glacial period was punctuated by rapid climate shifts, known as Dansgaard–Oeschger events, with strong imprint in the North Atlantic sector suggesting that they were linked with the Atlantic meridional overturning circulation. Here an idealized single-hemisphere three-dimensional ocean–atmosphere–sea ice coupled model is used to explore the possible origin of the instability driving these abrupt events and to provide a plausible explanation for the relative stability of the Holocene. Focusing on the physics of noise-free millennial oscillations under steady external (solar) forcing, it was shown that cold climates become unstable, that is, exhibit abrupt millennial-scale transitions, for significantly lower freshwater fluxes than warm climates, in agreement with previous studies making use of zonally averaged coupled models. This fundamental difference is a direct consequence of the weaker stratification of the glacial ocean, mainly caused by upper-ocean cooling. Using a two-hemisphere configuration of a coupled climate model of intermediate complexity, it is shown that this result is robust to the added presence of a bottom water mass of southern origin. The analysis reveals that under particular conditions, a pronounced interdecadal variability develops during warm interstadials. While the nature of the instability driving the millennial oscillations is identical to that found in ocean models under mixed boundary conditions, these interstadial–interdecadal oscillations share the same characteristics as those previously found in ocean models forced by fixed surface fluxes. The wind stress forcing is shown to profoundly affect both the properties and bifurcation structure of thermohaline millennial oscillations across a wide range of variation of freshwater forcing. In particular, it is shown that the wind stress forcing favors the maintenance of thermally direct meridional overturning circulations during the cold stadial phases of Dansgaard–Oeschger cycles.

Atmosphere and ocean dynamics: contributors to the European Little Ice Age?

The role of a reduction in the Atlantic meridional overturning and that of a persistently negative North Atlantic Oscillation in explaining the coldness of the European Little Ice Age (LIA) has been assessed in two sets of numerical experiments. These experiments are performed using an intermediate complexity climate model and a full complexity GCM. The reduction in the Meridional Overturning Circulation (MOC) of ca. 25% is triggered by a conventional fresh-water hosing set-up. A persistently negative NAO winter circulation, at NAO-index value −0.5, is imposed using recently developed data-assimilation techniques applicable on paleoclimatic timescales. The hosing experiments lead to a reduction in oceanic meridional heat transport and cooler sea-surface temperatures. Next to a direct cooling effect on European climate, the change in ocean surface temperatures feedback on the atmospheric circulation modifying European climate significantly. The data-assimilation experiments showed a reduction of winter temperatures over parts of Europe, but there is little persistence into the summer season. The output of all model experiments are compared to reconstructions of winter and summer temperature based on the available temperature data for the LIA period. This demonstrates that the hypothesis of a persistently negative NAO as an explanation for the European LIA does not hold. The hosing experiments do not clearly support the hypothesis that a reduction in the MOC is the primary driver of LIA climate change. However, a reduction in the Atlantic overturning might have been a cause of the European LIA climate, depending on whether there is a strong enough feedback on the atmospheric circulation.

Characterizing post-industrial changes in the ocean carbon cycle in an Earth system model

Understanding the oceanic uptake of carbon from the atmosphere is essential for better constraining the global budget, as well as for predicting the air-borne fraction of CO2 emissions and thus degree of climate change. Gaining this understanding is difficult, because the ‘natural’ carbon cycle, the part of the global carbon cycle unaltered by CO2 emissions, also responds to climate change and ocean acidification. Using a global climate model of intermediate complexity, we assess the evolution of the natural carbon cycle over the next few centuries. We find that physical mechanisms, particularly Atlantic meridional overturning circulation and gas solubility, alter the natural carbon cycle the most and lead to a significant reduction in the overall oceanic carbon uptake. Important biological mechanisms include reduced organic carbon export production due to reduced nutrient supply, increased organic carbon production due to higher temperatures and reduced CaCO3 production due to increased ocean acidification. A large ensemble of model experiments indicates that the most important source of uncertainty in ocean uptake projections in the near term future are the upper ocean vertical diffusivity and gas exchange coefficient. By year 2300, the model’s climate sensitivity replaces these two and becomes the dominant factor as global warming continues.

Stratification-dependent mixing may increase sensitivity of a wind-driven Atlantic overturning to surface freshwater flux

[1] Stratification-dependent mixing is employed in a coupled climate model of intermediate complexity with a 3-dimensional ocean component. Oceanic vertical diffusivity is calculated as κ ∼ N−α, where N is the local buoyancy frequency. The sensitivity of the Atlantic meridional overturning circulation (AMOC) to freshwater forcing is tested for exponents 0 ≤ α ≤ 2 by first slowly increasing, then decreasing the freshwater forcing over the North Atlantic, keeping the model close to equilibrium. The surface fresh anomaly imposed between 20°N and 50°N in the Atlantic reaches the deep ocean by vertical diffusion, and by AMOC advection via the northern convection sites. The fresh anomaly leads to enhanced stratification and thereby reduces vertical mixing stronger for higher values of α. Consequently, the freshwater anomaly reaches the northern deep water formation regions less diluted, and reduces the AMOC more strongly compared to lower values of α. Our findings indicate that modeled changes in the AMOC depend critically on the details of the mixing parameterization employed in the model.

Mechanisms of interdecadal climate variability and the role of ocean–atmosphere coupling

Climate variability and mid-latitude mechanisms of ocean–atmosphere interactions are investigated with coupled and uncoupled integrations of a three-dimensional ocean–atmosphere–land–ice climate model of intermediate complexity. We focus on the decadal and interdecadal variability of the system and give a statistical and dynamical description of its oceanic and atmospheric signatures. In our coupled control integration, an oceanic oscillation of a period of around 20 years is found to be associated with variability of the meridional overturning circulation and is manifested by surface anomalies of temperature and salinity. On such timescales the oceanic oscillation is able to imprint itself on the atmosphere, which then covaries with the ocean at the oscillation period. The essentially slaved atmospheric pattern helps maintain the oceanic oscillation by providing large-scale anomalous heat fluxes, so catalyzing the oscillation. That is to say, because the atmosphere covaries with the ocean the damping felt by the ocean is less than what would be felt with a fixed atmosphere, so broadening the parameter regime over which such variability occurs. In addition to the presence of an atmosphere, the period and amplitude of the oscillation are found to be influenced both by the oceanic vertical diffusivity κ v , by geometrical factors, and by the presence of stochastic heat fluxes. In general, oscillations occur most readily for large values of κ v , when the mean state of the ocean is characterized by a strong meridional overturning circulation. If κ v is sufficiently strong, the ocean will oscillate even in the absence of a dynamical atmosphere. However, for more realistic values of κ v , the presence of an interacting atmosphere is required for significant oscillations. If the ocean is forced by imposed stochastic heat fluxes, instead of a fully interacting atmosphere, then decadal-scale oscillations can be produced suggestive of a damped oscillator. However, the parameter range over which oscillations occur is smaller than when the ocean is coupled to full atmosphere. More generically, the ability of comprehensive coupled ocean–atmosphere models to produce multi-decadal variability, realistic or otherwise, will depend on the oceanic mean state, and so on the diapycnal diffusivity of the modelled ocean, as well as on the ability of the atmosphere to reduce the damping felt by the ocean and so on the atmosphere’s ability to respond to persistent sea-surface temperature anomalies.

The 15th century Arctic warming in coupled model simulations with data assimilation

Abstract. An ensemble of simulations of the climate of the past millennium conducted with a three-dimensional climate model of intermediate complexity are constrained to follow temperature histories obtained from a recent compilation of well-calibrated surface temperature proxies using a simple data assimilation technique. Those simulations provide a reconstruction of the climate of the Arctic that is compatible with the model physics, the forcing applied and the proxy records. Available observational data, proxy-based reconstructions and our model results suggest that the Arctic climate is characterized by substantial variations in surface temperature over the past millennium. Though the most recent decades are likely to be the warmest of the past millennium, we find evidence for substantial past warming episodes in the Arctic. In particular, our model reconstructions show a prominent warm event during the period 1470–1520. This warm period is likely related to the internal variability of the climate system, that is the variability present in the absence of any change in external forcing. We examine the roles of competing mechanisms that could potentially produce this anomaly. This study leads us to conclude that changes in atmospheric circulation, through enhanced southwesterly winds towards northern Europe, Siberia and Canada, are likely the main cause of the late 15th/early 16th century Arctic warming.

An Intermediate Complexity Climate Model (ICCMp1) based on the GFDL flexible modelling system

Abstract. An intermediate complexity coupled ocean-atmosphere-land-ice model, based on the Geophysical Fluid Dynamics Laboratory (GFDL) Flexible Modelling System (FMS), has been developed to study mechanisms of ocean-atmosphere interactions and natural climate variability at interannual to interdecadal and longer time scales. The model uses the three-dimensional primitive equations for both ocean and atmosphere but is simplified from a "state of the art" coupled model by using simplified atmospheric physics and parameterisation schemes. These simplifications provide considerable savings in computational expense and, perhaps more importantly, allow mechanisms to be investigated more cleanly and thoroughly than with a more elaborate model. For example, the model allows integrations of several millennia as well as broad parameter studies. For the ocean, the model uses the free surface primitive equations Modular Ocean Model (MOM) and the GFDL/FMS sea-ice model (SIS) is coupled to the oceanic grid. The atmospheric component consists of the FMS B-grid moist primitive equations atmospheric dynamical core with highly simplified physical parameterisations. A simple bucket model is implemented for our idealised land following the GFDL/FMS Land model. The model is supported within the standard MOM releases as one of its many test cases and the source code is thus freely available. Here we describe the model components and present a climatology of coupled simulations achieved with two different geometrical configurations. Throughout the paper, we give a flavour of the potential for this model to be a powerful tool for the climate modelling community by mentioning a wide range of studies that are currently being explored.

Investigating the evolution of major Northern Hemisphere ice sheets during the last glacial-interglacial cycle

Abstract. A 2.5-dimensional climate model of intermediate complexity, CLIMBER-2, fully coupled with the GREMLINS 3-D thermo-mechanical ice sheet model is used to simulate the evolution of major Northern Hemisphere ice sheets during the last glacial-interglacial cycle and to investigate the ice sheets responses to both insolation and atmospheric CO2 concentration. This model reproduces the main phases of advance and retreat of Northern Hemisphere ice sheets during the last glacial cycle, although the amplitude of these variations is less pronounced than those based on sea level reconstructions. At the last glacial maximum, the simulated ice volume is 52.5×1015 m3 and the spatial distribution of both the American and Eurasian ice complexes is in reasonable agreement with observations, with the exception of the marine parts of these former ice sheets. A set of sensitivity studies has also been performed to assess the sensitivity of the Northern Hemisphere ice sheets to both insolation and atmospheric CO2. Our results suggest that the decrease of summer insolation is the main factor responsible for the early build up of the North American ice sheet around 120 kyr BP, in agreement with benthic foraminifera δ18O signals. In contrast, low insolation and low atmospheric CO2 concentration are both necessary to trigger a long-lasting glaciation over Eurasia.