What was done
The authors determined changes in ice mass "from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2."

What was learned
Zwally et al. report that "the Greenland ice sheet is thinning at the margins (-42 ± 2 Gt a^-1 below the equilibrium-line altitude (ELA)) and growing inland (+53 ± 2 Gt a^-1 above the ELA) with a small overall mass gain (+11 ± 3 Gt a^-1; -0.03 mm a^-1 SLE (sea-level equivalent))." Likewise, they say that "the ice sheet in West Antarctica (WA) is losing mass (-47 ± 4 Gt a^-1) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 ± 11 Gt a^-1) for a combined net change of -31 ± 12 Gt a^-1 (+0.08 mm a^-1 SLE)." Hence, they report that "the contribution of the three ice sheets to sea level is +0.05 ± 0.03 mm a^-1." Furthermore, although not impacting sea level, they note that "the Antarctic ice shelves show corresponding mass changes of -95 ± 11 GT a^-1 in WA and +142 ± 10 Gt a^-1 in EA."

What it means
We often hear horror stories about the potential for Greenland and Antarctica to add many meters to the level of the seas in response to global warming. However, Zwally et al. put things in proper perspective by noting that the real-world data they processed indicate that the ongoing contribution of the Greenland and Antarctic ice sheets to sea-level "is small relative to the potential contribution from ice sheets." How small? At the current sea-level-equivalent ice-loss rate of 0.05 millimeters per year, it would take a full millennium to raise global sea level by just 5 cm, and it would take fully 20,000 years to raise it a single meter. In addition, Zwally et al. report that "the contribution of the ice sheets is also small compared to the most recent estimate of current sea-level rise of 2.8 ± 0.4 mm a^-1 from satellite altimetry (Leuliette et al., 2004)," which in their words, "further confounds possible explanations of the causes of contemporary sea-level rise."

In conclusion, the real-world findings of Zwally et al. suggest that the climate-alarmist hype about global warming causing sea levels to rapidly rise to dangerous heights due to the mass wasting of earth's great ice sheets is simply false. This outrageous claim is nothing more than a scare tactic designed to persuade the public to accept the bitter pill they prescribe for the solving of a patently obvious non-problem.

Reference
Leuliette, E.W., Nerem, R.S. and Mitchum, G.T. 2004. Calibration of TOPEX/Poseidon and Jason altimeter data to construct a continuous record of mean sea level change. Marine Geodesy 27: 79-94