We evaluate the representation of the 20th century Northern Hemisphere, North American, and Eurasian snow cover extent, frequency, and mass by the Geophysical Fluid Dynamics Laboratory coupled climate model, version 2 (CM2) and then explore the 21st century trends and changes in these quantities. The CM2 simulations of 20th century climate capture the seasonal cycle in Northern Hemisphere snow cover extent and produce a mean annual snow area of 25 ¿ 106 km2 that equals the satellite-based observations for the period 1973--2000. The simulated snow cover frequency and snow mass generally decline from north to south, but longitudinal gradients in these variables are also found. Snow mass over North America, especially during spring, is underestimated by CM2. Simulations of 21st century climate using three Intergovernmental Panel on Climate Change Special Report on Emission Scenarios reveal strong trends in Northern Hemisphere snow cover extent, frequency, and mass. These simulations suggest that the annual Northern Hemisphere mean snow cover extent (total snow mass) will decrease by 12 to 26% (20 to 40%) by 2100 from their 21st century mean values. Large declines in 21st century snow cover frequency (up to 50%) and snow mass (up to 100 kg m-2) arise during fall, winter, and spring over southern Canada and the northern United States, the Western Cordillera of North America, and western Eurasia compared to the 20th century CM2 simulations.