Riverine transport of organic carbon (OC) to the ocean is a significant component in the global carbon (C) cycle and the concentration of total organic carbon (TOC) in rivers and lakes is vital for ecosystem properties and water quality for human use. By use of a large dataset comprising chemical variables and detailed catchment information in ∼1000 Norwegian pristine lakes covering a wide climatic range, we were able to predict TOC concentrations with high accuracy. We further predict, using a ‘space-for-time’ approach and a downscaled, moderate, climate change scenario, that northern, boreal regions likely will experience strong increases in OC export from catchments to surface waters. Median concentrations of OC in these lakes will increase by 65%, from the current median of 2.0–3.3 mg C L−1. This is a long-term effect, primarily mediated by increased terrestrial vegetation cover in response to climate change. This increase OC will have severe impacts on food-webs, productivity and human use. Given the robustness of the estimates and the general applicability of the parameters, we suggest that these findings would be relevant to boreal areas in general.