"Water, water everywhere but not a drop to drink."
A lot can happen in 100 years so it's hard to predict the defining moments in the first 14. But I think one of the defining issues (after global warming) will be the struggle for clean water on a planet where three quarters of its surface is covered by water. Of course this struggle is already here for many regions of the Earth. For most of human existence access to water has not been a limiting factor. But global warming, privatization of sources and pollution from industrial uses all seem to be heading in a direction that impacts the developed world as much as the undeveloped parts.
100 years is a long time so perhaps this will all prove to be a blip because the immense crisis we are facing is solved by a technical innovation equally large. If the energy requirements are solved, desalination of sea water would turn the oceans from a desert of drinking water into a fountain of abundance. But until then, the irony will be that we have made ourselves desperately thirsty while surrounded by water.