The Earth System
The Earth is a very complex system where different realms and processes are intertwined along vast space and time scales. Earth's climate is one of the best examples in this sense, connecting atmospheric, land and oceanic domains through the carbon cycle. Just like any other planet of the Solar System, there is little room for experiments and hence, exploration of the present and future impacts on climate change are done through numerical modeling.
Earth System Models
These models try to represent the main processes that drive the climate system on structured grids. They are a combination of an atmospheric, a land surface, an ice and an ocean model. All of these models can be run separately to study inherent processes and variables making past, present and future projections.
Coupling on ESMs
Supercomputing facilities are now able to deal with the large amount of calculations that these models require and with the storage of the huge model output files containing all the spatial and temporal information. The ultimate goal is to fully couple all the individual models and analyze the joint evolution of climate variables.
Oceanic models include both ocean circulation and ocean biogeochemistry, with a wide range of complexity, from basic NPDZs to more complex biogeochemical modules with dynamical stoichiometry, various Plankton Functional Types (PFTs) or full Iron cycle representation. A detailed model performance and model intercomparisons can be found in analysis from Bopp et al, 2013, and Steinacher et al, 2010, as well as Cocco et al., 2013 for previous model generation.
Forced with variables such as temperature, wind or humidity, land models are focused on the fluxes of water, energy and carbon. Of particular interest are the CO2 fluxes and their relation to forest, grasslands, cropfields, hydrological cycles together with other energy transfers. A description of the carbon cycle capabilities in terrestrial models can be found in Anav et al., 2014, or an individual model description in Krinner et al., 2005.
Atmospheric models are based on physics and fluid dynamics to create the global atmospheric circulation. This includes convection, clouds, turbulent features and CO2 transport including sources and sinks through the interactions with the terrestrial and oceanic models. Hourdin et al., 2006 provides a detailed description of a paradigmatic example of atmospheric model.
The Coupled Model Intercomparison Project 5 (CMIP5, Taylor et al., 2012) defines a common set of variables and experiments to be run among several Earth System Models in a joint effort to study present and future climate scenarios. These variables diagnose different processes, being the carbon cycle one of the most important targets of this study. Ocean Acidification, as a by-product of increasing CO2 concentrations, can be characterized through these variables.