Permafrost (thin lenses of ice 1-2 m deep beneath 0.5 m of peat) underlies much of the vast boreal wetlands of the northern hemisphere. Global warming will result in major changes in the hydrology of these systems and their coupling to the atmosphere. Increased erosion and subsidence of frozen peat can cause major problems for extant and future transportation systems. We do not know if melting of permafrost will cause flooding or drainage of boreal wetlands, or how this will alter wetland growth and areal extent. We do not know if methane fluxes will increase or decrease with a change in water level. These changes cannot be forecast from small-scale studies; ecosystem experimentation is essential.