Global climate models (GCMs)

Global Climate Models (GCMs)

Derek Rosendahl explains that a global climate model is a numerical simulation run on a supercomputer. Its computer code represents the physical processes that move heat, moisture, momentum, and chemicals through the atmosphere, oceans, land, and biosphere.

How GCMs Work

• A model divides Earth’s atmosphere, oceans, land, and sea ice into three-dimensional grid cells.
• Modern models may use horizontal cells about 50 miles across, with many vertical levels in the atmosphere and ocean.
• Simulations commonly begin with conditions resembling those of the early 20th century. They then calculate how the climate changes in response to shifts in Earth’s energy balance, including changes in solar radiation and greenhouse-gas or aerosol concentrations.
• Using equations that describe physics, fluid motion, and chemistry, the model calculates variables such as temperature, precipitation, wind, moisture, and pressure in each grid cell.
• These variables interact across locations and over time. For example, modeled winds can carry colder air into a region, causing its temperature to fall over successive time steps.

Processes Represented

GCMs account for many major climate processes, including:

• Radiation being transmitted, reflected, and absorbed by the atmosphere and Earth’s surface.
• Clouds, rain, snow, evaporation, plant transpiration, and water moving into soil.
• Ocean circulation, storms, fronts, jet streams, and trade winds.
• Exchanges of carbon dioxide with oceans and forests, as well as volcanic releases of sulfur dioxide.

Parameterizations and Limitations

Some processes are too small or short-lived to simulate directly at a model’s scale. Modelers use parameterizations—simplified representations of these processes’ effects on larger-scale conditions. Examples include:

• Convection and other small-scale vertical motions.
• Turbulence near the surface.
• The formation of cloud droplets, raindrops, and snowflakes.
• Molecular interactions with solar and terrestrial radiation.

GCMs have improved significantly, but they cannot reproduce the climate system perfectly. Some climate processes remain incompletely understood, and models are updated as scientific knowledge advances.

Why Climate Models Are Used

• To study climate processes and feedbacks, including in regions with relatively few measurements.
• To test how changes within a model affect climate—an alternative to experimenting on the single planet we have.
• To assess human influence by comparing simulations with and without human-caused greenhouse-gas emissions, known as fingerprint studies.
• To project possible future climates and help decision-makers understand how today’s choices may affect future climate conditions.

The complexity of the calculations and the vast number of grid cells require immense computing power, so GCMs run on supercomputers.

Speaker Featured

• Derek Rosendahl, postdoctoral research scientist with the South Central Climate Science Center.