One thing I find handy for visualizing large quantities is to imagine it as a giant cube. 28 trillion tonnes of ice is a cube measuring 31 km on each side.

³√(28ᴇ15 kg / 916.8 kg/m³) ≈ 31 km

Though even this technique falls down in the face of such a massive number. It works better with lengths < 750m, which are just small enough to imagine inside one's regular environment. Things like yearly production of steel (a cube 616 m on each side) or copper (cube 140 m on each side). It's particularly effective if you like golfing and can recall the distances on a course you've played.