This is not that hard to figure out: 12.5B tons is about 12.5e9 m3 of water. Earth's diameter is 12700 km. The area of a sphere (Earth is a geoid, sure!) of tha

This is not that hard to figure out: 12.5B tons is about 12.5e9 m3 of water. Earth's diameter is 12700 km. The area of a sphere (Earth is a geoid, sure!) of that diameter is 5e8 km2. 70% of the Earth's surface is water, so only about 3.5e8 km2. If we add 12.5e9 m3, or 12.5 km3, of water to the oceans, that 12.5 km3 will cover a depth of 12.5 km3 / 3.5e8 km2, or roughly 3.8e-8 km, or 3.8e-5 m, or 38 microns, or basically nothing to worry about. It won't add up to one meter until it happens every day for about 2.5e4 days, or about 65 years. I expect glacial melting to mess things up through other means than brute oceanic depth increase. Things like albedo increase, current modification. This is a stream-of-consciousness Fermi estimate level of precision, so don't expect it to be correct beyond order of magnitude.