I believe that the compass in which Richard give to Locke to give to him only to give back to Locke and so on and so on..... is infact the only variable that can change the future. Think about it, Richard has it for 50-60 years, gives to Locke-gives back to Richard in the 50's thus Richard has it again for another 50-60 years. Seriously, how old is this compass by now, and how is it still working? The compass is the only object that we know of that has been in a continuous time loop. My theory being that, the compass may have to either be destroyed or altered in some way for the losties to change the outcome of the future since the compass seems to be the only "constant" in Lost's time travel universe.
