Climate models are computer programs that simulate processes in the Earth’s climate system using physical laws and equations that have been mathematically programmed. They allow scientists to test hypotheses, explore future climate scenarios, and understand how different factors (like greenhouse gases) influence the planet. Models can range widely in scale and complexity and large models (often referred to as Earth System Models) simulate planetary patterns and long-term changes. Their results support everything from global climate assessments to local planning decisions.
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CMIP
The Coupled Model Intercomparison Project (CMIP) is a collaborative project from the World Climate Research Programme (WCRP) providing climate projections to understand past, present and future climate changes. Its main goal is to improve our understanding of past, present, and future climate by comparing how different models simulate key processes and respond to various forcing scenarios (like greenhouse gas increases). CMIP and its associated data infrastructure have become essential to the Intergovernmental Panel on Climate Change (IPCC) and other international and national climate assessments. Each phase of CMIP (e.g., CMIP5, CMIP6) introduces updated experiments and protocols to reflect advances in science and policy needs. By using common inputs and diagnostics, CMIP makes it easier to evaluate model performance and understand the sources of uncertainty in climate projections.
World map showing the increased diversity of modelling centres contributing to CMIP and CORDEX.
Figure 1.20 in IPCC, 2021: Chapter 1. https://www.ipcc.ch/report/ar6/wg1/figures/chapter-1/figure-1-20
doi: 10.1017/9781009157896.003 .
Important Data links and catalogues
- Earth System Grid Federation (ESGF) - The primary data portal for accessing CMIP (Coupled Model Intercomparison Project) simulations and other model output data from various modeling centers.
- World Data Center for Climate - Repository of several climate model datasets, hosted by DKRZ; part of the WMO/ICSU data system.
- IPCC Data Distribution Centre (DDC) - Hosts a subset of model output used in IPCC assessments, along with observational and socioeconomic data.
- Copernicus Climate Data Store (CDS) - Offers free access to a wide range of European climate data, including forecasts.
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Other resources
Fresh eyes on CMIP guide for Dummies
The fresh eyes on CMIP team have put together some valuable resources to understand and utilise CMIP data.. (link not out yet)
Important Modeling related terms
These are some concepts that are specifically important in relation to climate models:
- Grid Resolution - The size of each grid cell in a climate model, typically in kilometers. Higher resolution means finer spatial detail but also higher computational cost.
- Boundary Conditions - Fixed values or inputs (like greenhouse gas concentrations or land surface properties) provided to models to define the simulation environment.
- Spin up - An initial run period used to bring the model’s internal variables into a stable state before analyzing results. Especially important for long-term or Earth system simulations.
- Parameterization - A method used to represent small-scale processes (like cloud formation or turbulence) that can't be directly resolved in climate models. These are estimated using simplified equations.
- Tuning - The process of adjusting model parameters to better match observed climate without compromising physical consistency.
- Coupled Model - A model that links different parts of the climate system—such as atmosphere, ocean, land, and sea ice—so they interact dynamically during the simulation.
- Ensemble Simulation - A set of model runs used to explore uncertainty. Can vary in model physics, starting conditions, or forcing scenarios.
- Climate Sensitivity - A measure of how much the Earth’s temperature would eventually rise if CO₂ concentrations doubled. Often used to compare how responsive different models are to forcing.
- Internal Variability - Natural fluctuations in the climate system that occur without changes in external forcing—like El Niño or the Pacific Decadal Oscillation. Distinguished from forced changes (e.g., due to CO₂).