You can't see them on the horizon, and could easily cross one without noticing. Also there is no turning back - at least not on a human timescale. 

Ice sheets with enough frozen water to lift sea levels more than a dozen meters, powerful ocean currents that keep bone-chilling winters at bay on both sides of the Atlantic, monsoon rains upon which hundreds of lions in Asia depend for food- all are at a risk of irretrievable disruption.