Progress in paleoclimate modeling

International audienceThis paper briefly surveys areas of paleoclimate modeling notable for recent progress. New ideas, including hypotheses giving a pivotal role to sea ice, have revitalized the low-order models used to simulate the time evolution of glacial cycles through the Pleistocene, a prohibitive length of time for comprehensive general circulation models (GCMs). In a recent breakthrough, however, GCMs have succeeded in simulating the onset of glaciations. This occurs at times (most recently, 115 kyr B.P.) when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm, enhancing the moisture source. More generally, the improvement in models has allowed simulations of key periods such as the Last Glacial Maximum and the mid-Holocene that compare more favorably and in more detail with paleoproxy data. These models now simulate ENSO cycles, and some of them have been shown to reproduce the reduction of ENSO activity observed in the early to middle Holocene. Modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time. An urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs (i.e., Dansgaard-Oescher cycles, Heinrich events, and the Younger Dryas). Efforts have begun to simulate the last millennium. Over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output. Simulations of these natural variations test the models relied on for future climate change projections. They provide better estimates of the internal and naturally forced variability at centennial time scales, elucidating how unusual the recent global temperature trends are

The computational and energy cost of simulation and storage for climate science: lessons from CMIP6

The Coupled Model Intercomparison Project (CMIP) is one of the biggest international efforts aimed at better understanding the past, present, and future of climate changes in a multi-model context. A total of 21 model intercomparison projects (MIPs) were endorsed in its sixth phase (CMIP6), which included 190 different experiments that were used to simulate 40 000 years and produced around 40 PB of data in total. This paper presents the main findings obtained from the CPMIP (the Computational Performance Model Intercomparison Project), a collection of a common set of metrics, specifically designed for assessing climate model performance. These metrics were exclusively collected from the production runs of experiments used in CMIP6 and primarily from institutions within the IS-ENES3 consortium. The document presents the full set of CPMIP metrics per institution and experiment, including a detailed analysis and discussion of each of the measurements. During the analysis, we found a positive correlation between the core hours needed, the complexity of the models, and the resolution used. Likewise, we show that between 5 %–15 % of the execution cost is spent in the coupling between independent components, and it only gets worse by increasing the number of resources. From the data, it is clear that queue times have a great impact on the actual speed achieved and have a huge variability across different institutions, ranging from none to up to 78 % execution overhead. Furthermore, our evaluation shows that the estimated carbon footprint of running such big simulations within the IS-ENES3 consortium is 1692 t of CO2 equivalent. As a result of the collection, we contribute to the creation of a comprehensive database for future community reference, establishing a benchmark for evaluation and facilitating the multi-model, multi-platform comparisons crucial for understanding climate modelling performance. Given the diverse range of applications, configurations, and hardware utilised, further work is required for the standardisation and formulation of general rules. The paper concludes with recommendations for future exercises aimed at addressing the encountered challenges which will facilitate more collections of a similar nature

Modeling extreme climates of the past 20.000 years with general circulation models

International audienc

15. Fonctionnement du climat à différentes échelles de temps

Le climat varie à différentes échelles de temps, des années aux milliards d’années de l’histoire de la Terre (cf. I-8). Ces variations peuvent résulter des interactions entre les composantes du système climatique* – atmosphère, océans, cryosphère*, biosphère* et lithosphère* (cf. I-3) – et de facteurs externes au système climatique qui modifient le bilan radiatif de la Terre. On qualifie généralement ces facteurs externes de « forçages » dans la mesure où ils induisent des variations du clima..

L’Insu au milieu des années 2000 : atouts et changements

Directrice du département (SDU) et de l’Institut national des sciences de l’Univers (Insu) de 2003 à 2006, à la suite de Philippe Gillet (2001-2003), Sylvie Joussaume a été confrontée à plusieurs enjeux de taille pendant une période de changements majeurs pour le CNRS. Elle revient sur les nombreux projets qui ont marqué ces trois années de l’histoire de l’Institut.Director of the Department (SDU) and of the National Institute for Earth Sciences and Astronomy (Insu) from 2003 to 2006, after Philippe Gillet (2001-2003), Sylvie Joussaume had to face high challenges during a period of major changes for the CNRS. She presents the many projects that have marked these three years of the history of the Institute