One of the biggest uncertainties that has caused scientists to tend towards conservative estimates of sea level changes in recent years (and result in the frequent announcements of increased projections in the past few years) is the rate at which the Earth’s polar ice sheets are melting. In the 2007 IPCC report, the sea level rise projection agreed upon was a conservative 60 cm (~ 2 ft.). This number did not account for the possibility of rapid ice flow from Greenland or the Antarctic into the sea. These two ice sheets alone hold enough water to raise sea levels by 65 meters compared to 0.4 meters from all the world’s mountain glaciers. But, at that time, researchers felt that there was insufficient understanding of the ice sheets to be certain, so the IPCC resisted putting a number on it.
The most recent IPCC report (2013) increased this estimate to 98 cm or almost 1 meter.
This number has been bumped up further since 2014 with the most recent projections ranging from 0.2 to 2.0 meters (See NOAA Sea Level Rise viewer information in Module 4 Lab for more on these ranges).
Eliminating the uncertainty around quantifying the contribution of ice sheet melt-water to sea level increases is attributed to observations from NASA/German Aerospace Center’s twin Gravity Recovery and Climate Experiment (GRACE) satellites. These data indicate that between 2002 and 2016, Antarctica shed approximately 125 gigatons of ice per year, causing global sea level to rise by 0.35 millimeters per year.
In 2002, NASA launched the GRACE satellites, which track both ocean and ice mass by measuring changes in the Earth's gravitational field. The paired satellites orbit the Earth together and are spaced roughly 200 kilometers apart. Ice and water moving around the Earth exert different gravitational forces on the GRACE satellites. The satellites can sense the minuscule changes in the distance between one another caused by the change in gravitation force, which they measure and use to track water and ice mass change. It's thanks to GRACE that we know where the water flowing into the ocean came from. According to GRACE, melting of ice in Greenland increased sea level by 0.74 mm/year and melting in Antarctica by 0.25 mm/year since 2002. (Source: Smithsonian)
The science involved in understanding the behavior of ice sheets is growing rapidly. More data will no doubt be revealed in the upcoming years which will help to increase the accuracy of sea level rise projections. Other work to measure the rate of change in the Antarctic is described in these NASA articles: