Manning and Petit (2003) also emphasize the importance of explaining the uncertainties to policy makers and non-experts. In our case, it makes sense to distinguish between those uncertainties associated with the understanding of past LSL changes and those related to predictions of future LSL. The understanding of past LSL variations is hampered by a lack of observations that would allow us to compute the thermal expansion more accurately. Vertical land motion has been monitored only at same locations and only since a few years or slightly more than a decade in most cases. The mass relocation in the global water cycle, including ice sheets, glaciers, and land water storage is also uncertain particularly for the earlier part of the previous century.