The achievements of AR4 can not be properly understood without reference to the Coupled Model Intercomparison Project Phase 6. CMIP6, organized by the World Climate Research Programme (WCRP), is a global initiative, designed to standardize and enhance climate model comparisons. Compared to earlier CMIP project phases, CMIP6 offers several key enhancements (see Box 1):

Box 1. Improvements in climate modeling introduced by CMIP6

Improved model resolution and complexity
CMIP6 models feature higher spatial resolution than those in CMIP5, enabling more precise simulations of atmospheric and oceanic processes. The models integrate more complex interactions within the Earth system, including cloud-aerosol dynamics, biogeochemical cycles, and ice sheet behaviors.
Updated scenarios
CMIP6 introduces SSPs, combining projections of societal development with climate outcomes. This framework enhances the integration of climate change impacts with mitigation and adaptation strategies.
Advanced experimental design
CMIP6 expands the suite of Model Intercomparison Project (MIP) experiments,
(1) covering a broader range of Earth system components and human activities, such as those in the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) and the Land Use Model Intercomparison Project (LUMIP), and
(2) models simulating longer historical and future periods, providing insights into long-term climate patterns and variability.
Increased focus on regional climate
Enhanced downscaling techniques in CMIP6 improve regional climate projections, crucial for assessing local impacts of extreme weather events like heatwaves, floods, and droughts.
Improved data accessibility CMIP6 continues to make its data freely available, supporting widespread scientific analysis and policy use. The increased data volume reflects the higher resolution and complexity of the models, with robust quality control ensuring reliability.

Benefits and advances of CMIP6 over previous phases can thus be summarized as follows:
• Enhanced climate projections: CMIP6’s improvements in resolution and complexity yield more accurate climate projections, essential for understanding future climate risks.
• Better quantification of uncertainties: a broader range of experiments and scenarios allows for a more comprehensive assessment of uncertainties, aiding in more informed decision-making.
• Integration of human and natural systems: the inclusion of Shared Socio-economic Pathways (SSPs) fosters a more interdisciplinary approach, bridging climate science with social sciences to support holistic policy development.
• Focus on regional and extreme events: The increased attention to regional climate and extreme weather in CMIP6 meets the growing demand for localized climate information, especially for vulnerable regions.
• Support for IPCC assessments: CMIP6 data is integral to AR6, which relies heavily on these outputs for understanding climate change impacts.