A water issue that we can easily point out today is the rise of the sea level. It is mainly cause by two phenomena: global warming and ozone depletion. As the ocean temperature gets warmer, the water expanded. Also, both phenomena have direct effects on the vast land of ice lying on the two poles. Antarctica and Greenland regroup the largest amount of ice on land. Consequently, when it melts, it falls in the ocean and increases the amount of sea level. We can observe this by piling up many cubes of ice in a glass of water. After the ice melt, the level of water in the glass is higher then before.
It works the same way on earth. The melting of glaciers over land raises the sea level by approximately one inch per decade and could eventually flood part of the land we have now. The problem goes bad to worse when we take into consideration that one third of the world population lives in a range of 60 km from the coast. Throughout history, population did settle themselves on coasts to benefit from the commerce related to it. In a case of flood, this massive population would have to move away from the coasts, unless they build a barrier like Netherlands. But realistically speaking, this is a very expensive and complex procedure that most countries of the third world wouldn’t be able to put in place.
The rise of the sea level due to the melting of glaciers would inevitably cause massive migration like never seen before. It would create all kind of conflicts with other residence, then unprepared for their accommodation. History showed that massive migration causes not only conflicts, but it also spread diseases. World War 1 is one example, where more death was cause after the war due to diseases then during 1914-1918. Now let’s imagine the chaos when one third of the population tries to fit in new areas.