What was done
The authors used satellite radar altimeter measurements from 1992 to 1996 to estimate the rate of change of the thickness of nearly two thirds of the grounded portion of the Antarctic Ice Sheet.  They also used snowfall variability data obtained from ice cores to ultimately calculate the mass balance of the interior of the continental ice sheet over the past century.

What was learned
The results showed that, at most, the interior of the Antarctic Ice Sheet has been "only a modest source or sink of sea-level mass this century."  The authors thus concluded that "a large century-scale imbalance for the Antarctic interior is unlikely," noting that this conclusion is in harmony with a body of relative sea-level and geodetic evidence "supporting the notion that the grounded ice has been in balance at the millennial scale."

What it means
The great grounded, or non-floating, portion of the Antarctic Ice Sheet has apparently neither gained nor lost significant mass over the past century, or maybe even the past millennium.  It thus appears to be impervious to climate changes of the magnitude of the Medieval Warm Period and the Little Ice Age, which is the type of change most likely to occur -- if there is any change at all -- in response to the ongoing rise in the air's CO2 content (see Journal Review A Natural Thermostat to Keep a Lid on Global Warming?).