Research project CG/DD1/13 (Research action CG)
Context
Detailed three-dimensional general circulation models of the atmosphere and the oceans (GCM) were developed over the last 25 years. These models based on physical principles are showing some skill in reproducing the main features of the general circulation of the atmosphere and the oceans. However they need huge computer resources and therefore the length of these GCM simulations are still limited to decades, century or a millennium in the most favourable case. Simpler models, which are less time consuming, must therefore be used to simulate long-term climatic changes and climate variability at the decadal to millennia time scales.
Objectives
The primary goal of this project is to simulate the climate and its variability over the next two thousand years. Climate history and different factors influencing the climate, such as CO2 atmospheric concentration, solar variability or volcanic eruptions, are better known over the last 2000 years than over any other period. However this period did not experience climate very different from the present-day one. Thus older epochs centred around 6, 11 and 21 kyr BP were selected in addition to the last 2000 years.
MoBidiC (Modèle Bidimensionnel du Climat) is a 2-D sectorially averaged climate model. The major components of the climate system (atmosphere, ocean, sea ice, ice sheet) and their interactions are represented. The resolution is 5° in latitude. The first step of this work was the improvement and the validation of this new version of the model. The main points to be mentioned are the extension to a global representation, the inclusion of a dynamical ocean model and a thermodynamic representation of the sea ice.
The MoBidiC model makes possible to perform snapshot experiments in order to simulate very different climates in equilibrium with the forcings, for example the present-day conditions, the Last Glacial Maximum or intermediate climatic situations. A first test to evaluate the ability of
the model to simulate climate is to compare the MoBidiC results to the GCM's ones.
However the main purpose of this tool is to simulate the climate over a few thousand years. The ability of the model to reproduce the past climate changes was tested and the model results were compared to observations. Palaeoclimatic records from corals, tree rings, ice cores, marine and continental sediments can be used to extend the recent observational and satellite data panel.
This climate model is also a rapid and simple tool for the recognition of threshold climatic behaviour. Indeed a large number of experiments can be carried on in order to test the response of the climate system to small variations of either a forcing or a process during climatic change episodes.
Deliverables and expected results
This research is giving first hand information on the past, present and future climate. The climate model is able to simulate the behaviour of different global variables, such as the ice volume, the surface temperature, the intensity of the ocean circulation, which characterise the climate. The time and space behaviour of these variables must be analysed to identify the climatic variability. Moreover the origin of this variability can be traced through the comparison of different simulations using slightly different parameterisations or forcings. Finally scenarios for the next 2000 years will be designed and the transient response of the climate system to these forcing analysed. The natural as well as different anthropogenic influences will be considered in these simulations.
Scientific collaboration
International research programmes such as PAGES (Past Global Changes, a core project of the International Geosphere-Biosphere Programme) and CLIVAR (Climate Variability and Predictability, a programme of the World Climate Research Programme) aim at providing a quantitative understanding of the natural variability of the Earth's past climate. High resolution records of long-term climatic changes are becoming available and more global palaeoclimate reconstruction is underway. Consequently comparison between simulation and reconstruction is providing a more global overview of the climate variability as well as a deeper understanding of the processes playing a role in this variability.
The ability of different three-dimensional global model simulate a wide range of different climate of the past is studied through the PMIP (Paleoclimate Modelling Intercomparison Project) research programme (for 6 and 21 kyr BP). The experiments performed within PMIP can be used to test MoBidiC against more complex models. The new version of the model developed within this research project is also used for IPCC (Intergovernmental Panel on Climate Change) simulations. The purpose is here to determine the climate response to different scenarios of atmospheric greenhouse gases concentration changes.
