There is an error in our equations 1 and 2, defining the SSA stress balance equations and the effective viscosity computation. 

Equation 1 should be,

∇·(Hµ (∇u + ∇^Tu + 2∇·u I ) ) + τ_d - τ_b = 0,

where I is the identity matrix and the other symbols are as defined in the manuscript.  This version differs from the version in the manuscript by a removal of a factor of 1/2, by the inclusion of the divergence of horizontal velocity, and by moving the driving and basal stress terms to the left hand side.  Meanwhile, equation 2 should be,

µ = 1/2 B ε₀^(1-n)/n,

where all symbols are defined as in the manuscript.  This version differs from the version in the manuscript by a factor of 1/2.

We will correct both of these equations in the next revision. 

Much thanks to Christian Schoof for pointing out our mistake in an email.