Humans are very clever; they have figured out that object have mass. Mass is often thought of as weight but scientifically it is more than weight. It can describe the properties of a body when it moves, or the properties of a body when it active or passive gravity is applied to it. Einstein told us that energy is simply mass multiplied by the square of the speed of light. In simple concepts mass is part of energy and energy is part of mass. Humans are also clever enough to know that it follows that we can measure mass of an object from space using satellites which can measure the gravitational mass of an object.
So, when satellites measure the gravitational mass of ice sheets at the poles than can know whether the ice is shrinking or growing or remaining stable. They have to make adjustments for known physical reactions when ice is resting on land (isostatic rebound), but having made these adjustments they conclude that both the East and West Ice Sheets are losing mass. These measurements are to scientists simple physics and unless Einstein and a whole bunch of scientists before him are wrong, we can now know exactly whether the ice at the poles is getting thinner or not.
Traditional surface ice measurements at the Antarctic have indicated in the past twenty years that the West Ice Sheet has been melting but the East Ice Sheet seems to have remained constant or may have marginally increased. It has been one of those anomalies beloved of climate change deniers.
Now the gravity mass measurements from satellites proves that the East Ice Sheet has been losing mass (in other words melting) over the past three years. So far climatologists have calculated rising sea levels based on the Greenland Ice Sheet and the West Antarctic Ice Sheet melting; those two events would raise sea level by about six and a half metres; if the West Ice Sheet is also melting then sea levels would rise by sixty metres.
There is no fear that both ice sheets will melt rapidly; the Antarctic is very cold and it is possible that the isostatic rebound adjustments are not completely accurate, but the evidence is accumulating that the ice at the poles is melting. We do not need a clearly warning.