By sea level, we refer roughly to the height of the sea or ocean surface. Sea level increases when the volume of water in the ocean increases and vice versa. One of the chief environmental processes that control sea level is climatic change. For example, the rise in temperature across the globe can cause polar icebergs and glaciers to melt, and this can increase the sea level greatly. In fact, one of major threats of global warming of the earth we are facing nowadays is the rise in sea level, leading to submerging of coastal areas (which means loss of many things, including human life). Besides global rise in temperature, tectonic changes or earth crust mechanisms like movement of lithospheric plates, addition of crust layers, volcanic activity near oceanic ridges, opening, closing and widening of sea basins, etc. can also cause changes in the sea level. Such changes in the sea level are mostly expressed on a wider scale.
A relatively predictable phenomenon is “tidal currents”, which is nothing but periodic rise and fall of sea level. The processes that control tidal waves are gravitational forces manifested by the moon and the sun with respect to the rotation of the Earth. The moon is the key controller as it is much nearer to earth as compared to the sun. Other forces that bring a change in the sea level are storms, floods, cyclones, tsunamis, etc. Such sea level changes are, however, not necessarily permanent and happen locally.