ESMs simulate physical processes much like other climate models and weather prediction models. ESMs are composed of model components that simulate individual parts of the climate system (such as the atmosphere, ocean, land, and sea ice) and the exchange of energy and mass between these parts.

The atmosphere is where most weather occurs and therefore where humans mostly experience the climate. The atmospheric model component of an ESM simulates the movement of mass and energy over large distances, as well as the nanometer-scale interactions between cloud droplets and water vapor. In order to capture this broad range of scales, full-complexity atmospheric models divide the atmosphere into thousands of grid-boxes and solve the fundamental equations of motion and energy conservation within each grid-box, which might be 50-200 km in size. There are many important processes (such as clouds, precipitation, and radiation) that are smaller than a grid-box and are simulated using "parameterizations." Parameterizations, which treat small-scale processes as a function of average large-scale properties, are the source of many of the uncertainties in climate projections.

The Earth's surface plays a crucial role in earth system modeling, not only because humans live at the land surface, but also because 2/3 of the sunlight absorbed by the Earth is absorbed at the land and sea surface. Almost all of this energy is eventually transferred to the atmosphere. How this transfer occurs is critical for climate. For instance, if solar energy is absorbed by dry soil, it simply heats the soil. If absorbed by wet soil, some energy may evaporate the water, depriving the land and living things of moisture, providing moisture to the atmosphere, and limiting heating of the soil. Therefore, the land model component simulates how water moves from its sources (where it rains or snows) to sinks such as the ocean or aquifers, a process simultaneously involving physical, biological, and anthropogenic aspects. The model can consider the effect of topography on drainage or how water use by plants affects soil moisture. Since land is fixed in space, most of the processes simulated on land occur within just one grid-box: only water and any energy or nutrients that water carries moves between grid-boxes.

Like the land, the ocean exchanges sensible and latent heat (the energy associated with the evaporation of water) with the atmosphere. However, the ocean's great depth and water's high heat capacity give the ocean an energy storage capacity about a thousand times greater than the atmosphere. In contrast to the land, the ocean can transport energy from the warm tropics to colder high latitudes. Thus, as with the atmosphere, ocean models simulate large-scale movement of mass and energy. In addition, small-scale processes, such as the sinking of cold, salty water near the poles, are parameterized within the ocean model.

The cryosphere, the part of the earth system that is ice, covers a small proportion of the Earth's surface but plays a large role in the climate system. Sea ice has an albedo about ten times that of ocean water. In addition, sea ice insulates the atmosphere from the ocean and affects the exchange of energy and mass between them. ESMs therefore simulate sea ice formation and loss. Snow on land is also simulated within the land model, including modifications to energy and mass fluxes, as well as albedo. Most ESMs do not directly simulate the growth and decay of ice sheets on land, but ice sheet model components are being developed to address the potential for ice sheet collapse in the future.

ESMs are chiefly distinguished from climate models by their ability to simulate the carbon cycle. If the sum of all CO 2 emitted into the atmosphere between 1966 and 2008 is compared with the observed level of atmospheric CO 2 , approximately one of out of every two CO 2 molecules appears to be missing (Figure 2). This extra CO 2 has not vanished entirely. It has been incorporated into land and ocean reservoirs, often in carbon fixed by organisms during photosynthesis. Whether all of it will stay there and what proportion of future emissions will remain in the atmosphere are open questions, which have motivated the development of land model components that can predict the spatial distribution of vegetation, how its growth varies through the year, and the exchange of carbon between it and the soil. Similar model components exist to simulate the marine biosphere and chemistry.



Figure 2: The Global Carbon Sink. Integrated CO 2 emissions (Boden et al., 2010) outpace CO 2 concentrations at Mauna Loa, Hawaii (Keeling et al., 2009), suggesting that there are sinks for CO 2 other than the atmosphere. © 2013 All rights reserved.

ESMs are also distinguished by the sophistication of their atmospheric and oceanic chemistry. In the ocean, biological productivity is limited by the availability of nutrients, ranging from fundamental cellular model components such as N to trace nutrients such as Fe, which is a key ingredient in many enzymes. The distribution of these nutrients is controlled by transport and biology but also by reactions with dissolved organic matter and inorganic constituents. Nutrient cycling involving atmospheric transport is also important on land and can be affected by atmospheric chemical reactions. In addition, climate, as well as water and soil chemistry, can determine whether microbes decompose organic matter into methane or carbon dioxide. And once in the atmosphere, methane, a more efficient greenhouse gas, oxidizes to carbon dioxide, a less efficient greenhouse gas.