CLIMATE
VARIABILITY
RESEARCH
An Introduction to Earth Systems Modelling
What is Earth Systems Modelling?
Earth Systems Modelling (ESM) is a new name for an old trick, the trick in question being replacing specified boundary conditions in a climate model with interactive representations of the part of the system previously represented by the imposed boundary conditions.
The term is usually only applied to attempts to model the climate system with extra interactive components in addition to an atmosphere or coupled ocean-atmosphere model. Obvious examples of ESM would be to include interactive representations of the biosphere or the major ice sheets in climate simulations.
Why is ESM relevant to UGAMP?
ESM allows specified boundary conditions to be replaced by supposedly more physical representations of the state and interactive evolution of the systems in question. An ESM approach therefore has the potential to improve the understanding and predictability of climate, particularly on intermediate to long timescales.
- Boundary conditions are usually specified within a climate model for one of three reasons:
- The system represented by the boundary conditions is expected to remain in steady state or change only very slowly on timescales of interest.
- The influence of variations in the system represented by the specified boundary conditions are expected to be small.
- The system represented by the boundary conditions is poorly understood, and explicit representation is not felt to be justified.
In all of the above cases, it is likely that it will become advantageous to replace boundary conditions with interactive simulation at some point. As interest grows in longer term prediction of climate change, systems which could previously be considered to be in steady state, for instance, the ice sheets and underlying lithosphere, must have their evolution considered explicitly.
Additionally, cases will almost certainly exist in which variability considered to be of minor influence in the majority of situations may prove critical to understanding certain features of climate or climate change. The interaction of vegetation with climate in determining the state of the Sahel region of Africa appears to be such an case.
Finally, the ability to simulate systems which are currently poorly understood will almost certainly improve in the future, not least through research by groups such as UGAMP into ESM and related disciplines. Such advances may allow, for instance, a future Earth System Model to treat atmospheric carbon dioxide as a prognostic variable rather than as a parameter to be specified.
What ESM resources are available to UGAMP?
The UKMO Unified Model, in either atmosphere only or coupled ocean-atmosphere mode, is likely to form the basis of the majority of ESM experiments.
Vegetation-climate interactions can be simulated using the MOSES scheme which is part of the Unified Model, along with one of several vegetation models. The TRIFFID model is designed as a counterpart to MOSES, and ensures a compatible hydrological cycle is achieved. Other vegetation models available include DOLY, SDGVM, BIOME and MAPSS; the latter in particular has a highly realistic representation of the hydrological cycle, superior to that of most GCMs. The majority of these vegetation models have already been used in conjunction with the UGCM, and their use with the UM should prove possible.
A two dimensional high resolution ice flow model coupled to a simple model of lithospheric rebound under the weight of the ice sheets is available at Reading and can be forced either directly by GCM predictions of net snow accumulation or via a downscaling surface model which resolves many of the difficulties arising due to the differing horizontal resolutions of the two models.
Many processes of interest to Earth System Modellers take place on extremely long timescales, at least compared to those usually considered for climate modelling. It is therefore sometimes useful to have access to simple climate models which allow very long integrations without excessive computational requirements. To fulfil this need, a simple energy balance climate model coupled to a crude ice sheet/lithosphere model is under development at Reading and will allow simulations of climate over timescales up to a few million years in just a few days on a powerful workstation.
ESM and Palaeoclimate modelling
Earth Systems Modelling and Palaeoclimate modelling are very closely related. Many ESM components and concepts find ready application in palaeoclimate research, or, in fact, derive from such research in the first place. Conversely, palaeoclimate simulations can often provide the only independent test of Earth Systems Models against the behaviour of the real Earth System, through comparisons with proxy data from climate states significantly different to the present day.
What Next?
As I write this article, I have yet to begin officially as CGAM Earth Systems Modelling coordinator. I have outlined above some of my thoughts on ESM and why it is important. As you will surely have noticed, my own interests are in the study of vegetation-climate and particularly ice-climate interactions. Of course, the success of an ESM project within UGAMP is going to rely on other people becoming involved and performing science using Earth Systems Models. I would like to hear from people who have ideas about where UGAMP ESM should be heading, or who would like to use or develop ESM components.
Robin Glover
CGAM, University of Reading
robin@met.rdg.ac.uk
