D6 Climate Models and Projections

Topic

Climate models are mathematical representations of the climate system that allow for the simulation of its evolution over time. The most detailed models divide the atmosphere and ocean into a three-dimensional spatial grid of cells and solve the physical equations describing their behavior within them. Neighboring cells exchange mass, energy, and momentum, enabling the simultaneous representation of atmospheric and oceanic circulation evolution on a planetary scale.

General circulation models are among the primary tools used to conduct these simulations. In the atmosphere, they solve the primitive equations expressing the conservation of mass, momentum, and energy, while incorporating the effects of Earth's rotation, the relationship between air pressure, temperature, and density, and the conservation of water vapor. The atmospheric component is also coupled with models of the ocean, sea ice, and land surface, such that changes occurring in one component affect the others during the simulation.

The typical spatial resolution of these models is on the order of tens of kilometers—in both the atmosphere and the ocean—and numerous vertical levels are employed. However, many important processes—such as convection, cloud formation, surface turbulence, or certain atmospheric waves—occur at scales smaller than a single grid cell. Since they cannot be resolved explicitly, their effects are incorporated via parameterizations: simplified representations that estimate their average influence on the variables calculated by the model.

Earth system models expand upon this representation by incorporating biogeochemical processes and, specifically, an interactive carbon cycle. In these models, the atmospheric concentration of carbon dioxide can result from the balance between emissions and the uptake by terrestrial ecosystems and the ocean. This allows for the representation of interactions between the climate and the carbon cycle: for a given level of emissions, greater terrestrial or oceanic uptake results in less CO₂ accumulation in the atmosphere, while climate changes can, in turn, alter the capacity of these reservoirs to sequester it.

At the lower end of the complexity spectrum lie energy balance models. Instead of explicitly representing a three-dimensional grid of the planet, they describe the climate response using a limited number of equations. One example is the two-box model, which represents a surface component and a deep-ocean component that exchange heat. Its parameters make it possible to study how temperature responds to a specific forcing and to distinguish between transient and equilibrium responses. These models allow for the analysis of global climate system properties at a much lower computational cost than three-dimensional simulations.

Models of intermediate complexity lie between these two extremes. They retain some of the climate system's spatial structure—including the geographical distribution of continents and oceans—but simplify numerous processes that general circulation models represent in greater detail. They can also incorporate certain biogeochemical cycles, albeit less comprehensively than Earth system models.

Energy balance models, models of intermediate complexity, general circulation models, and Earth system models thus form a continuous spectrum of tools. At one end are models with few equations—fast and useful for understanding fundamental mechanisms and global climate responses; at the other are three-dimensional models that provide a detailed representation of the planet's physical and biogeochemical interactions. The choice depends on the problem being studied: a simple model may suffice for investigating mechanisms or running numerous simulations, whereas a detailed analysis of regional patterns, interactions between components, or carbon cycle feedbacks requires models with greater physical, spatial, and computational complexity.

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