By Luisa Cristini, PhD, University of Hawaii at Manoa.
[Note from the editor: This is the eighth in a series of blog entries that will focus on introductory topics in climate dynamics and modeling, and will be a great insight into the current understanding of the science.]
Simply stated, climate models are mathematical representations of the climate system, or of parts of it, based on our best knowledge of the natural processes. These representations are expressed by equations. Some climate models can be very complex and require the use of supercomputers to solve the equations.
Many climate models have been developed to perform future climate projections, i.e., to simulate and understand climate changes in response to the emission of greenhouse gases. Models can also be powerful tools to improve our knowledge of the most important characteristics of the climate system and of the causes of climate variations. Climatologists cannot perform experiments on the real climate system to identify the role of a particular process or to test a hypothesis. Therefore, climate models can be used to perform experiments in a virtual world.
For a climate model describing nearly all the components of the system, only a relatively small amount of data is required. This data can include solar irradiance (the amount of solar radiation arriving at a specific spot on the Earth at a specific time), the Earth’s radius and period of rotation, the land topography and bathymetry (the underwater “topography”) of the ocean, some properties of rocks and soils, etc. Data are also important during the development phase of the model, as they provide essential information on the properties of the system that is being modeled. In addition, large numbers of observations are needed to test the validity of the models in order to gain confidence in the conclusions derived from their results.
The models used for future global climate projections are called General Circulation Models (GCMs) and try to account for all of the important properties of the system at the highest affordable resolution. They are divided into Atmospheric General Circulation Models (AGCMs) and Ocean General Circulation Models (OGCMs). For climate studies using interactive atmospheric and oceanic components, the acronyms AOGCM (Atmosphere Ocean General Circulation Model) or CGCM (Coupled General Circulation Model) are generally chosen.
GCMs provide the most precise and complex description of the climate system. They compute the values of model variables at a given time on a horizontal grid across the surface of Earth. These values provide enough information to reconstruct an approximation of the corresponding field over the whole studied area. Currently, the horizontal resolution of GCMs is typically on the order of 100 to 200 km. Also, nowadays, GCMs take more and more components into account, and many new models include sophisticated components for sea ice, carbon cycle, ice sheet dynamics and atmospheric chemistry. Because of the large number of processes included and their relatively high resolution, GCM simulations require a large amount of computer time. For instance, an experiment covering one century typically takes several weeks to run on the fastest computers.
The interactions between the various components of the system (atmosphere, ocean, sea ice, land surface, marine biogeochemistry, and ice sheets) play a crucial role in the dynamics of climate. Some of the interactions are quite straightforward to compute from the model state variables, while more sophisticated parameterizations are required for others.
There is no perfect model suitable for all purposes. This is why a wide range of climate models exist and, depending on the objective or the question, a certain type of model could be selected. On the other hand, combining the results from various types of models is often the best way to gain a deeper understanding of the dominant processes in action.
Since their invention in the 1950s, GCMs have been further developed and there has been a lot of work and research behind them in each discipline connected to climate science (physics, chemistry, biology, mathematics, geology, and so on). Modern models are very accurate in the representation of the global climate system and are now able to give us insights into both past climate changes and future climate projections.
Goosse H., P.Y. Barriat, W. Lefebvre, M.F. Loutre and V. Zunz, (2012). Introduction to climate dynamics and climate modeling. Online textbook available at http://www.climate.be/textbook.