A slow-moving emergency is lapping at California’s shores — climate-driven sea-level rise that experts now predict could elevate the water in coastal areas up to 10 feet in just 70 years, gobbling up beachfront and overwhelming low-lying cities. The speed with which polar ice is melting and glacier shelves are cracking off indicates to some scientists that once-unthinkable outer-range projections of sea rise may turn out to be too conservative. A knee-buckling new state-commissioned report warns that if nothing changes, California’s coastal waters will rise at a rate 30 to 40 times faster than in the last century.