I'd just add a few points to some of the other answers here ...
Humans have no magic crystal ball we can look into to see the future. What we can do in some circumstances is take the available theories and run computational simulations and models to try and see what the outcome might be. These aren't perfect but they're the best we can do. And hopefully, as we learn more, we get better at doing it.
Even if models give us a ballpark figure, it's better than nothing in terms of planning. If lots of different groups have slightly different data, analyse that data differently, make slightly different assumptions and all these groups come out with different predictions that still is useful - if all the models agree that the volume of ice being lost will be more than X amount, then at least we can plan for probably at least Y amount of sea level rise.
The problem I have is that some seem to be demanding absolute certainty before we take any action, whether it be on sea level rise predictions, glacier volume loss prediction, temperature predictions, etc. This absolutism is not applied to anything else. We don't apply it to earthquakes, tornadoes, viral outbreaks, and so on, yet no one argues that even though the models may not be as accurate as everyone wants, they aren't useful or helpful. Yet on the issue of global warming, suddenly it becomes a unique case where action is only warranted if and when 100% certainty is achieved.