Freshwater ice (river and lake ice), a key component of the cryosphere, plays a dominant role in the hydrology of northern climates. Although freshwater ice has been modelled at small geographic scales, it remains the only major unquantified component of the cryosphere. Therefore, the goal of this thesis is to quantify peak freshwater ice across the Northern Hemisphere using a regionally defined degree-day ice-growth model. To address this the ecological and climatic importance of freshwater ice are reviewed, as well as the physical processes that govern freshwater-ice growth, the existing approaches to modelling freshwater ice, and the major climate classification methods. Using a degree-day ice-growth model, ice-growth coefficients are defined by hydro-climatic region, and validated using maximum observed seasonal ice thickness values from across the Northern Hemisphere. The maximum seasonal extent of freshwater ice is then estimated over a 44-year temporal period and the areal extent and volume of freshwater ice quantified. / Graduate