Scientific Research Challenge

The scientific driver for this project is to understand the astonishing and, as yet, unexplained natural variability of past climate in terms of the dynamic behaviour of the Earth as a whole system. Such an understanding is an essential pre-requisite to increase confidence in predictions of long-term future climate change.

The figure above shows the changes in carbon dioxide, temperature and methane over the last four glacial cycles recorded in the Vostok ice core (Petit et al. 1999). The causes of these major glacial-interglacial cycles that have dominated the past few million years of Earth history remain highly uncertain. However, it is clear that changes in many components of the Earth system appear to have amplified rather weak orbital forcing. These include: land ice, sea ice and vegetation cover affecting Earth's albedo (reflectivity), CO2, CH4 and water vapour affecting the 'greenhouse effect', and ocean circulation affecting heat transport. Previous modelling and data studies (e.g. Imbrie et al. 1993, Shackleton, 2000, Berger et al. 1998) have revealed that non-linear feedbacks are important, and that these feedbacks extend beyond the physical subsystem to include biological and geochemical processes. For example, changes in the marine carbon cycle (Watson et al. 2000) and terrestrial vegetation cover (de Noblet et al. 1996, Claussen et al. 1999) are fundamental contributors to past climate change.

Hence our working hypothesis is that realistic simulations of long-term climate change require a complete Earth system model that includes, as a minimum, components representing the atmosphere, ocean, sea-ice, marine sediments, land surface, vegetation, soil, and ice sheets and the energy, hydrological and biogeochemical cycling within and between components. The model must be capable of integration over multi-millennial time-scales. The design of the system will allow other components, such as atmospheric chemistry, to be added at a later stage. At present, state-of-the-art models of the essential components of the Earth climate system exist mostly as separate entities. Where several components have been coupled, as in the more elaborate versions of the Hadley Centre model (Cox et al. 2000), they are computationally too demanding for long-term or ensemble simulations. Conversely, existing efficient models of the complete system, e.g. CLIMBER-2 (Petoukhov et al. 2000) employ highly idealised models of the individual components, with reduced dimensionality and low spatial resolution.

Our objectives are to build a model of the complete Earth system which is capable of numerous long-term (multi-millennial) simulations, using components which are traceable to state-of-the-art models, are scaleable (so that high resolution versions can be compared with the best available, and there is no barrier to progressive increases in spatial resolution as computer power permits), and modular (so that existing models can be replaced by alternatives in future). Data archiving, sharing and visualisation will be integral to the system. The model will be used to quantitatively test hypotheses for the causes of past climate change and to explore the future long-term response of the Earth system to human activities.

All of the necessary component models have already been developed within the NERC community (representing a considerable investment of resources) and by our collaborators at the Hadley Centre. Further work will be required to produce compatible, computationally efficient components for Grid coupling and to represent the hydrological and biogeochemical cycling within and between components. Our initial scientific focus will be on one fundamental transition of the Earth system: from the last glacial maximum to the present interglacial warm period (the Holocene).

The figure above is a high resolution snow accumulation and temperature record from the Greenland GISP2 ice core, showing the rich behaviour of the Earth system during the last deglaciation (Kapsner et al. 1995). This interval has been chosen because it encapsulates both gradual and rapid climate changes, and high-resolution data records exist against which to test the model.

Our specific scientific objectives are to use the Earth system model to investigate:

• The timing of the Bølling-Allerød warm phase: General Circulation Model (GCM) based simulations using a simple ocean model suggest that this warming occurs earlier than would be expected from orbital theory alone.
  
• The magnitude and extent of the Younger-Dryas cold phase, and the anti-phase climate variations recorded in Antarctic and Greenland ice cores (Blunier et al. 1998): The links between the hemispheres and the extent of the Younger-Dryas beyond the Atlantic remain uncertain.
  
• The changes in carbon cycling during the deglaciation and the changes in vegetation and carbon storage during the Holocene (Clausen et al. 1999).
  
• The minimum complexity (in terms of system components, processes within components, and resolution) required to simulate these changes in the system.
  
• The predictability (or otherwise, due to chaotic behaviour) of the fully coupled system: Will small changes in initial conditions result in major changes to the glacial-interglacial transition?
  
• The robustness of predictions of carbon cycle feedback on global warming (Cox et al. 2000; Lenton 2000), and long-term projections of climate change and carbon cycling (Archer et al. 1998).

Overview | Vision | Scientific Research Challenge | Earth Modelling | Grid Technologies