Fortnightly variations in the flow velocity of Rutford Ice Stream, West Antarctica

Ice sheets like that of Antarctica lose much of their ice via fast-flowing ice streams. Hilmar Gudmundsson has monitored the flow in one such stream, the Rutford Ice Stream in West Antarctica, over a period of two months. The findings are a surprise: surface flow velocities here fluctuate by as much as 20% over a period of two weeks, perhaps under the influence of the spring–neap fortnightly tidal cycle. Although the mechanism that couples these tides to ice flow has yet to be elucidated, these observations caution against the use of velocity measurements over limited periods to infer long-term changes in flow rate.