Changes in hydrologic fluxes and in sea level will cause enormous changes in the global extent and nature of wetland and estuarine systems. Coastal systems will experience the dual pressures of development and increased sea level. Much of the world's wetland area lies in permafrost regions between 50 and 70 degrees north latitude, and will obviously be subject to dramatic changes associated with global warming. Two major aspects of global climate effects on wetlands must be considered. (1) Wetlands are major sources of methane, one of the more potent greenhouse gases. Changes in the extent and function of wetland ecosystems could have significant feedback on climate. (2) Coastal wetlands are the interface between terrestrial and aquatic systems. Their role as sinks or sources of nutrients and contaminants will be of substantial local and regional concern. While many of the concerns about changes in coastal and wetland systems can be anticipated by specific experiments (e.g. thermal effects, increased flooding) and more accurate measurements (e.g. gas flux), certain issues require the scale of whole system experiments. Among those are: