There is almost twice as much carbon in permafrost as in the atmosphere. The thawing of permafrost, by enabling the microbial degradation of this carbon, could lead to the release of enormous amounts of the greenhouse gases CO2 and CH4 to the atmosphere, representing one of the most important climate feedbacks. However, the fate of permafrost carbon is still poorly understood, resulting in large uncertainties in climate projections. Since 2012, in the eastern Canadian Arctic, we have investigated processes involved in the evolution of the permafrost thermal regime and more recently on processes relevant to the permafrost carbon budget. Benefiting from the network of bases of the Centre d’Études Nordiques, we have deployed snow, soil and atmospheric measurement stations at 4 sites between 55°N and 83°N representative of polar environments, from the northern open boreal forest with sporadic permafrost to polar deserts with thick continuous permafrost. We propose to continue to observe and report on permafrost evolution and to expand our process understanding of its key controls. Our observations include unique time series of snow and soil thermal conductivity data at 12 locations with different vegetation covers, which need to be maintained to detect and quantify snow-permafrost-vegetation-climate feedbacks. Process studies will progressively shift their focus from the permafrost thermal regime to its carbon budget, thanks to the addition of a new co-PI focusing on dissolved organic matter transport and fate, and in particular the mechanistic representation of its biophotochemical transformations in lakes. Aqueous phase processes will thus be investigated at 3 locations and the focus will be on lake carbon budget and on the fraction degraded to greenhouse gases. Regarding land processes, the focus will be on carbon inputs due to vegetation growth, because this aspect is a significant source of uncertainty in land surface models. The data obtained will produce updated model structures and novel parameterizations for snow physics, land surface energy and carbon budgets, as well as lake coupled biogeochemical cycling.