Storage of carbon dioxide beneath the seafloor forms an important component of climate change mitigation. A crucial element of such storage projects is the monitoring of carbon dioxide accumulation in the storage reservoir - we need to be sure that it is permanently stored and not leaking back into the ocean/atmosphere system. Storage monitoring is normally done using seismic techniques, but the seismic velocity of rocks partially saturated with gas is relatively insensitive to gas content except at very small contents. The electrical resistivity has much greater sensitivity to gas content at higher contents. In this project we explore how repeated controlled source electromagnetic surveys might be used to monitor carbon dioxide accumulation in a reservoir, particularly in the context of active and proposed offshore carbon dioxide storage projects in Norway.