Snow is a pillar of the Earth’s climate system, affecting all its components with critical impacts for Nature and human societies. Perennial snow evolves to firn and ice, providing unique records of the past climate. Yet today no snow model adequately simulates relevant snow variables worldwide, not to mention their inability to represent firn processes and snow/permafrost interactions. I argue that this is because current models focus on a limited number of physical processes and none suitably consider snow microstructure. IVORI’s goal is to build a microstructure-based model encompassing all the relevant snow and firn physical variables. Drawing on advanced observations of snow and firn, the proposal has three objectives: (1) Understand the role of water vapour transport in snow and its subsequent impacts on the ground thermal regime governing permafrost evolution; (2) Understand how initial changes in surface snow microstructure are transferred deeper into the firn and affect ice core records; (3) Determine the contributions of snow-climate feedbacks, triggered by changes in the albedo and insulating capacity of snow to the past and future of snow cover and ground temperature. To this aim, I will build a microstructure-based model, with a novel physics core, unifying the evolution of snow and firn. IVORI will also deliver unprecedented season-long observations of snow microstructure in the Arctic, Alps and Antarctica using X-ray tomography. These observations will significantly advance our understanding of the physical processes involved and be used for a thorough evaluation of the model. The model will provide a reliable assessment of snow-climate feedbacks in a changing climate and a rigorous appraisal of the modelling uncertainties. When completed, this work will pave the way for crucial advances in our understanding of glaciers, ice sheets and past climate through ice core records, with many fallouts for sea ice and permafrost evolution.