Very small changes in the Earth's gravity field—which might result from shifts in deep ocean currents, melting of ice sheets in Antarctica, or accumulation of seasonal snow pack in the mountains—could be detected and mapped using newly developed instruments on future satellite missions. A new report from the National Research Council recommends the satellite missions, which would detect gravity changes that are 100 to 100,000 times smaller than those measured previously, to map the Earth's gravity field and provide new data for a variety of scientific fields.