Climate model

Climate models are based on physical laws such as conservation of energy, mass, and momentum, and they simulate the interactions of the atmosphere, oceans, land surface, and ice. They range from simple energy balance models to complex Earth system models that include biogeochemical cycles. The most common types are general circulation models (GCMs), which solve equations on a three-dimensional grid, and Earth system models (ESMs) that add interactive carbon and aerosol cycles.¹

The first numerical weather prediction model was developed by Lewis Fry Richardson in 1922, but it was not until the 1950s that digital computers enabled practical simulations. In 1956, Norman Phillips produced a two-layer model that successfully simulated the general circulation of the atmosphere. By the 1970s, models began to include ocean components, and in the 1990s, coupled atmosphere-ocean models became standard. The Intergovernmental Panel on Climate Change (IPCC) has coordinated model intercomparisons since 1990, leading to the Coupled Model Intercomparison Project (CMIP).

Climate models are used for seasonal forecasting, attribution of extreme events, and long-term climate projections. They are also used to study past climates, such as the Last Glacial Maximum, and to assess the impacts of geoengineering proposals. However, models have limitations due to computational constraints, leading to parameterizations of processes like cloud formation. Uncertainties arise from incomplete understanding of feedbacks, such as cloud and carbon cycle feedbacks, and from the need to downscale results for regional impact studies.²

Beyond the mainstream, climate models have niche applications: they are used to simulate the climates of exoplanets, to study the potential for abrupt climate change, and to test the effects of solar radiation management. A lesser-known fact is that early climate models were developed by military and nuclear research institutions, such as the Lawrence Livermore National Laboratory, which contributed to the understanding of nuclear winter. Additionally, the concept of 'tipping points' was popularized through model studies of the Atlantic Meridional Overturning Circulation. Some models now include 'human dimension' components, such as land use and economic activity, to better represent anthropogenic influences.³

General Circulation Model (GCM)

A model that simulates atmospheric and oceanic circulation using mathematical equations.

Earth System Model (ESM)

A model that includes interactive biogeochemical cycles, such as the carbon cycle.

Parameterization

A method of representing sub-grid scale processes, such as clouds, using simplified statistical relationships.

CMIP

Coupled Model Intercomparison Project, a framework for coordinating climate model experiments.