Learn more about global climate models
After reading this article, you will learn:
What are global climate models?
Global climate models are used to understand how the climate may change in the future. They embody state-of-the-art understanding of the physics and chemistry of the atmosphere, land surface, ocean, and cryosphere, and how these different climate spheres interact.
Global climate models represent key physical and chemical processes with mathematical equations that have been discovered through making observations of the natural world, and studying how different climate phenomena relate to and interact with one another.
There are many global models in existence, each designed by modeling centers from various parts of the world. They combine the different climate spheres in different ways, and describe the underpinning science using different mathematical relationships, each of which is grounded in robust scientific research.
Each model is a possible realization of the climate system and, together, the models provide an unprecedented perspective on how the climate could change in the future - there is no other source of information like it.
What is CMIP?
The World Climate Research Programme (WRCP) designed the Coupled Model Intercomparison Project (CMIP) to explore how climate could change in the future under a range of different socioeconomic scenarios.
CMIP takes advantage of this worldwide collection of climate models by including multiple realizations of each one - ensembles of model possibilities containing numerous individual simulations.
With multiple model simulations, CMIP can represent:
- the natural variability in the climate system,
- the uncertainty associated with the mathematical relationships underpinning the science, and
- the possible trajectories the climate may follow depending on future developments in policy.
CMIP5 versus CMIP6
CMIP5 was the 5th phase of CMIP, released in 2012. The model data it created has been used worldwide to advance our understanding of how the climate could change in the future. CMIP5 model data underpinned the climate science described in the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report.
CMIP5 used Representative Concentration Pathways (RCPs) to represent possible future physical climate scenarios with differing greenhouse gas emissions. RCP8.5 is the high emissions scenario, which warms the entire planet by over 3°C on average by the end of the century. RCP2.6 is the low emissions scenario which projects an average global temperature rise of under 2°C by 2100.
CMIP6 is the latest phase of CMIP, and includes over 50 modelling centres from around the world. CMIP6 model data combines physical climate change described by the RCPs with possible socioeconomic changes in the 21st century by employing Shared Socioeconomic Pathways (SSPs). Each SSP is designed to represent possible pathways forward in time, ranging from a green, sustainability-driven society (or, Paris-aligned), to a fossil fuel-driven society (or, Business as usual). These scenarios were established based on the outcomes of the Paris Agreement of 2015, and refer to policy developments relative to that agreement.
Which CMIP data do we use for each signal?
For all atmospheric signals (heat stress, precipitation, drought, and extreme wind) in EarthScan, the future climate scenarios are based on CMIP6 SSP scenarios; specifically, SSP1-2.6 (Paris-aligned), SSP2-4.5 (Emissions peak by 2040), and SSP5-8.5 (Business as usual).
Future projections of river discharge are taken from the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) hydrological models. These models are driven using four independent CMIP5 models. ISIMIP is currently developing an update to their discharge models that utilizes CMIP6 models. Once this is published, we will update the riverine flooding datasets accordingly.
Coastal flooding uses IPCC AR6 sea-level rise data, generated by NASA, for future projections. These are independent of CMIP and include state-of-the-art understanding of processes contributing to global sea level rise.