  We present results for the nucleon axial charge g_A at a fixed lattice
spacing of 1/a=1.73(3) GeV using 2+1 flavors of domain wall fermions on size
16^3x32 and 24^3x64lattices (L=1.8 and 2.7 fm) with length 16 in the fifth
dimension. The length of the Monte Carlo trajectory at the lightest m_\pi is
7360 units, including 900 for thermalization. We find finite volume effects are
larger than the pion mass dependence at m_\pi= 330 MeV. We also find that g_A
exhibits a scaling with the single variable m_\pi L which can also be seen in
previous two-flavor domain wall and Wilson fermion calculati ons. Using this
scaling to eliminate the finite-volume effect, we obtain g_A = 1.20(6)(4) at
the physical pion mass, m_\pi = 135 MeV, where the first and second errors are
statistical and systematic. The observed finite-volume scaling also appears in
similar quenched simulations, but disappear when V\ge (2.4 fm)^3. We argue this
is a dynamical quark effect.
