CMIP6 is the latest phase of collaboration under the Coupled Model Intercomparison Project (CMIP). CMIP6 data are the most current global climate model data available. The data are scientifically robust and provide the foundation for the Intergovernmental Panel on Climate Change’s Sixth Assessment Reports.
CMIP is an international scientific collaboration under the United Nations World Climate Research Programme. This initiative involves climate modelling teams from around the world. CMIP sets the experimental design protocol and recommends core sets of experiments for the climate modelling community to undertake. The goal of CMIP is to better understand past, present and future climate changes through assessments of model performance during the historical period and quantification of the causes of the range in future projections. The CMIP collaboration designs a set of standard simulations to allow for cross-comparison of results to detect where models agree and disagree. The CMIP process furthers our understanding of climate modelling, enables modelling improvements, and ensures scientifically robust future climate projections.
Forty-nine climate modelling groups, running 100 climate models, are taking part in this latest phase of CMIP. In comparison, CMIP5 included 40 global climate models created by 20 climate-modelling groups.
CMIP6 models generally have increased complexity (more components) and spatial resolution, representing the atmosphere, oceans and small-scale processes (such as clouds, water vapour, and aerosols) in more detail. This increase in spatial resolution means, among other things, that the representation of temperature and precipitation in mountainous areas is improved compared to CMIP5 simulations.
Another notable difference between CMIP5 and CMIP6 is the emissions scenarios used to project future levels of global climate change. Representative Concentration Pathways (RCPs) were used to drive the CMIP5 generation of models. CMIP6 utilizes an enhanced set of emissions scenarios, which are based on Shared Socio-Economic Pathways (SSPs). SSPs complement RCPs by exploring the socio-economic conditions behind various emissions levels in a standardized manner. While some RCPs and SSPs exhibit approximately the same increase in radiative forcing by the end of the century (e.g., RCP8.5 and SSP5-8.5), there are differences in the GHG emissions associated with each of these emissions pathways.
Finally, some models included in CMIP6 have a higher equilibrium climate sensitivity than those in CMIP5. See below for a description of climate sensitivity.
Different climate models have different spatial resolutions. Over time, resolution has generally increased as climate modelling techniques have improved and computing technology has advanced. For example, for the scenario experiments exploring the evolution of future climate in response to changing greenhouse gas (GHG) emissions, the French model IPSL-CM5-LR (used in CMIP5) had a resolution of 1.9° latitude x 3.75° longitude. The latest version of this model (IPSL-CM6A-LR) used in CMIP6 has a resolution of 1.25° latitude x 2.5° longitude. However, some climate modelling centres chose to keep the same resolution, e.g., CanESM2 (CMIP5) and CanESM5 (CMIP6) from the Canadian Centre for Climate Modelling and Analysis both have a spatial resolution of 2.8° latitude x 2.8° longitude.
Both the CMIP5 and CMIP6 datasets on ClimateData.ca have been downscaled to the same spatial resolution, approximately 6 x 10 km (0.0833° latitude x 0.0833° longitude).
The future modelling in CMIP6 is based on an updated set of future emissions scenarios. CMIP5 model runs were based on four Representative Concentration Pathways (RCPs) and their associated greenhouse gas (GHG) concentrations starting in 2006. CMIP6 model runs use GHG concentrations stemming from Shared Socioeconomic Pathways (SSPs) which relate emissions levels with socio-economic conditions starting in 2015.
CMIP5 models served as the basis for IPCC’s Fifth Assessment Report (AR5) published in 2013; however, these model experiments were started a number of years before being reported on in AR5. Modelling efforts for CMIP6 began shortly after AR5 was published, and were reported on in the Sixth Assessment Report (AR6; 2021). The earlier CMIP5 experiments, therefore, followed observed GHG emissions until 2005, while the more recent CMIP6 experiments followed observed GHG emissions until 2014.