Gilles Ramstein

Effect of orogeny, plate motion and land–sea distribution on Eurasian climate change over the past 30 million years

The Eurasian climates of today, 10 million and 3O million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions. The resulting climates compare well with various palaeoclimate records. The retreat of the Paratethys–an epicontinental sea–shifts the central Asian climate from temperate to continental conditions, and plays as important a role as uplift of the Himalayan/Tibetan plateau in driving the Asian monsoon changes.

Tectonic Uplift and Eastern Africa Aridification

The history of Eastern African hominids has been linked to a progressive increase of open grassland during the past 8 million years. This trend was explained by global climatic processes, which do not account for the massive uplift of eastern African topography that occurred during this period. Atmosphere and biosphere simulations quantify the role played by these tectonic events. The reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation. Our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation, engendering the strong aridification and paleoenvironmental changes suggested by the data.

Simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation

According to Milankovitch theory, the lower summer insolation at high latitudes about 115,000 years ago allowed winter snow to persist throughout summer, leading to ice-sheet build-up and glaciation. But attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only. These results point towards the importance of feedback effects—for example, through changes in vegetation or the ocean circulation—for the amplification of solar forcing. Here we present a fully coupled ocean–atmosphere model of the last glaciation that produces a build-up of perennial snow cover at known locations of ice sheets during this period. We show that ocean feedbacks lead to a cooling of the high northern latitudes, along with an increase in atmospheric moisture transport from the Equator to the poles. These changes agree with available geological data and, together, they lead to an increased delivery of snow to high northern latitudes. The mechanism we present explains the onset of glaciation—which would be amplified by changes in vegetation—in response to weak orbital forcing.

Northern Hemisphere storm-tracks in present day and last glacial maximum climate simulations: a comparison of the European PMIP models

Abstract Extratropical weather systems are an essential feature of the midlatitude climate and global circulation. At the last glacial maximum (LGM), the formation of regions of high transient activity, referred to as “storm tracks,” is strongly affected by the presence of large ice sheets over northern America and Scandinavia and by differences in sea surface temperature (SST) distributions. In the framework of the Palaeoclimate Modelling Intercomparison Project, simulations of the LGM climate have been run with a wide range of atmospheric general circulation models (AGCMs) using the same set of boundary conditions, allowing a valuable comparison between simulations of a climate very different from the present one. In this study, the authors focus on the storm track representation in the models and its relationship with the surface temperatures, the mean flow, and the precipitation. Storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three-dimensional ...

The Sturtian 'snowball' glaciation: fire and ice

Abstract The Sturtian ‘snowball’ glaciation (730 Ma) is contemporary with the dislocation of the Rodinia supercontinent. This dislocation is heralded and accompanied by intense magmatic events, including the onset of large basaltic provinces between 825 and 755 Ma. Among these magmatic events, the most important one is the onset of a Laurentian magmatic province at 780 Ma around a latitude of 30°N. The presence of these fresh basaltic provinces increases the weatherability of the continental surfaces, resulting in an enhanced consumption of atmospheric CO 2 through weathering, inducing a global long-term climatic cooling. Based on recent weathering laws for basaltic lithology and on climatic model results, we show that the weathering of a 6×10 6 km 2 basaltic province located within the equatorial region (where weathering of the province and consumption of CO 2 are boosted by optimal climatic conditions) is sufficient to trigger a snowball glaciation, assuming a pre-perturbation PCO 2 value of 280 ppmv. We show that the Laurentian magmatic province might be the main culprit for the initiation of the Sturtian ‘snowball’ glaciation, since the Laurentian magmatic province had drifted within the equatorial region by the time of the glaciation.

Sea Surface Temperature of the mid-Piacenzian Ocean: A Data-Model Comparison

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

Mid-Holocene and Last Glacial Maximum African monsoon changes as simulated within the Paleoclimate Modelling Intercomparison Project

As part of the Paleoclimate Modelling Intercomparison Project (PMIP), several atmospheric general circulation models have performed climate simulations of the mid-Holocene (6 ka BP) and of the Last Glacial Maximum (LGM) climates (21 ka BP) using the same experimental design for boundary conditions, insolation and CO2 forcing. PMIP results are used in this study to investigate how the position of the intertropical convergence zone (ITCZ) changes over West Africa throughout the seasonal cycle in these two climates. The Mid-Holocene corresponds to a period of enhanced seasonal cycle with colder winters and warmer summers, leading to an increase of the African monsoon. During LGM, the presence of large ice sheets and lower CO2 give rise to a colder climate and a reduced summer monsoon. For the two time periods and all the models, it is found that over West Africa, both the atmospheric low level temperature gradient and the position of the maximum of surface temperature control the position of the ITCZ. The energetic changes associated with the change in the hydrological cycle are driven by latent heat release in the atmosphere within the rainbelt and by radiative fluxes north of the ITCZ. The sensible heat flux plays also a non-negligible role over the desert during mid-Holocene. Differences between the PMIP simulations reflect differences in model parameterization, including surface processes and clouds. However, for a given model, the partitioning between the changes in the different heat fluxes is quite similar for the mid-Holocene and the LGM climate change.

Fish tooth δ18O revising Late Cretaceous meridional upper ocean water temperature gradients

The oxygen isotope composition of fossil fi sh teeth, a paleo– upper ocean temperature proxy exceptionally resistant to diagenetic alteration, provides new insight on the evolution of the low- to middlelatitude thermal gradient between the middle Cretaceous climatic optimum and the cooler latest Cretaceous period. The new middle Cretaceous low to middle latitude thermal gradient agrees with that previously inferred from planktonic foraminifera δ 18O recovered from Deep Sea Drilling Project and Ocean Drilling Program drilling sites, although the isotopic temperatures derived from δ 18O of fish teeth are uniformly higher by ~3–4 °C. In contrast, our new latest Cretaceous thermal gradient is markedly steeper than those previously published for this period. Fish tooth δ18O data demonstrate that low- to middle-latitude thermal gradients of the middle Cretaceous climatic optimum and of the cooler latest Cretaceous are similar to the modern one, despite a cooling of 7 °C between the two periods. Our new results imply that no drastic changes in meridional heat transport are required to explain the Late Cretaceous climate. Based on climate models, such a cooling without any change in the low to middle latitude thermal gradient supports an atmospheric CO2 decrease as the primary driver of the climatic evolution recorded during the Late Cretaceous.

Is there a conflict between the Neoproterozoic glacial deposits and the snowball Earth interpretation: an improved understanding with numerical modeling

The behavior of the terrestrial glacial regime during the Neoproterozoic glaciations is still a matter of debate. Some papers claim that the glacial sequences cannot be explained with the snowball Earth scenario. Indeed, the near shutdown of the hydrological cycle simulated by climatic models, once the Earth is entirely glaciated, has been put in contrast with the need for active, wet-based continental ice sheets to produce the observed thick glacial deposits. A climate ice-sheet model is applied to the older extreme Neoproterozoic glaciation (around 750 Ma) with a realistic paleogeographic reconstruction of Rodinia. Our climate model shows that a small quantity of precipitation remains once the ocean is completely ice-covered, thanks to sublimation processes over the sea-ice at low latitudes acting as a water vapor source. After 10 ka of the ice-sheet model, the ice volume in the tropics is small and confined as separate ice caps on coastal areas where water vapor condenses. However, after 180 ka, large ice sheets can extend over most of the supercontinent Rodinia. Several areas of basal melting appear while ice sheets reach their ice-volume equilibrium state, at 400 ka, they are located either under the two single-domed ice sheets covering the Antarctica and the Laurentia cratons, or near the ice-sheet margins where fast flow occurs. Only the isolated and high-latitude cratons stay cold-based. Finally, among the simulated ice sheets, most have a dynamic behavior, in good agreement with the needs inferred by the preserved thick formations of diamictite, and share the features of the Antarctica present-day ice sheet. Therefore, our conclusion is that a global glaciation would not have hindered the formation of the typical glacial structures seen everywhere in the rock record of Neoproterozoic times. < 2003 Elsevier Science B.V. All rights reserved.

Simulations of Northern Hemisphere ice-sheet retreat:: sensitivity to physical mechanisms involved during the Last Deglaciation

A 3-dimensional thermomechanical ice-sheet model is used to simulate the evolution of the geometry of Northern Hemisphere ice sheets through the Last Deglaciation. The ice-sheet model is forced by a time-evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the LMD5.3 atmospheric general circulation model (AGCM) at different periods of the Last Deglaciation (21, 15, 9, 6 and 0 kyr BP). The AGCM is driven by insolation, atmospheric CO2 content, ice-sheet configuration and sea surface temperatures. Although our approach is able to produce the complete continental ice retreat, our simulated deglaciation presents a phase-lag with reconstructions based on observational evidences. This suggests that physical mechanisms related to climate forcing and/or ice-sheet internal dynamics are not properly represented. The influence of millennial-scale forcing, feedback mechanisms between ice-sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments. The rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature-mass balance relationships. Fennoscandia appears to be strongly sensitive to the small-scale ice-sheet instability. Both ice sheets are to some extent sensitive to an increased basal sliding.