1. What is a climate model?

Global climate models or General Circulation models (GCMs) are the most complex and precise models for understanding climate systems and predicting climate change. These models aim to mathematically describe the Earth’s climate system based on the laws of physics (e.g. first law of thermodynamics, Stefan-Boltzmann law), fluid motion (e.g. Navier-Stokes equations) and chemistry. They use mathematical equations to quantify observable Earth system processes, i.e. characterize how energy and matter interact and get transported in different parts of the atmosphere, land, ocean and sea ice (Fig. 1).

The atmospheric component of the climate model simulates clouds, aerosols and the transport of heat and water around the globe. The land surface component simulates surface characteristics such as vegetation, snow cover, soil water, rivers, and carbon storing, whereas the ocean component simulates current movement, mixing and ocean biogeochemistry. The sea ice component modulates solar radiation absorption, air-sea heat and water exchanges.

Building a complex global climate model incorporating all these components requires the division of the Earth’s surface into three dimensional grid cells (Fig. 1). The size of these grid cells defines the spatial resolution of the model (typically about 100 km x 100 km x 30 vertical layers). Climate models also incorporate the dimension of time, measured in time steps. The temporal resolution refers to the size of these time steps (typically about 30 minutes) used in the model. Powerful supercomputers iteratively solve the mathematical equations for every single spatial grid cell and step the model forward in time to produce a precise climate model for a specific time interval. Models with smaller grid cells as well as smaller time steps lead to better resolution, but also need considerably more computing power.

Fig. 1: Schematic representation of GCMs (https://www.gfdl.noaa.gov/climate-modeling/)



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