Scientists have identified some geological feature in the Antarctic that, combined with already rising sea levels, will cause a sudden collapse sometime in the not-so-distant future. They've identified two specific glaciers in west antarctic that are way past their tipping point beyond which collapse is unavoidable. Their estimate is 50% collapse within the next 100 years.
Collapse of glaciers into the ocean causes sea level rise. The melting of sea ice does not cause sea level rise.