Melting of polar ice sheets has added 11mm to global sea levels over the past two decades, according to the most definitive assessment so far. 

More than 20 polar research teams have combined forces to produce estimates of the state of the ice in Greenland and Antarctica in a paper in Science. 

Until now different measurement means have produced a wide range of estimates with large uncertainties. 

But sea-level rise is now among the most pressing questions of our time. 

Polar ice has a tremendous capacity to cause massive rises - with huge potential impacts on coastal cities and communities around the world. The new estimate shows that polar melting contributed about one-fifth of the overall global sea level rise since 1992; other factors include warming that causes the seawater to expand. 

The study does not seek to forecast future change. 

Supported by US and European space agencies Nasa and Esa, the research brought together data from satellites measuring the surface altitude, the flow of the glaciers and the gravitational effect of the ice mass to produce the first joint assessment of how the ice sheets are changing.