The thinning and retreat of Arctic sea ice is one of the most dramatic manifestations of recent climate warming. Though ice extent can be routinely monitored by satellite, ice thickness is far more difficult to measure operationally. We show that small amplitude, long period waves - termed infragravity waves - can be used to measure ice thickness at basin scales by determining their travel time between measurement sites. The waves travel at a different speed in ice than in open water, the difference being a sensitive function of ice thickness. We present measurements from near the North Pole where the travel time of 15 s waves is reduced by around 7 h for a typical 2 m ice thickness. Our results demonstrate that a basin-scale observation network which can track the effect of global change on Arctic sea ice thickness is practical and feasible using current technology. (C) 2009 Elsevier B.V. All rights reserved.