Measurement of Long Term Sea Level Change - Sea level has varied throughout geologic time, yet is difficult to quantify. It is crucial to distinguish between relative sea level, the height of the ocean surface relative to a specific point on land, along a coastline, and eustatic sea level, which reflects the relation between the volume of the world’s ocean basins and the volume of the world’s ocean water. Relative sea level is easily measured by tide gauges, deployed throughout the harbors of the world, and which have produced a very useful record for over a century. If one could account for all the tectonic vertical motions of those coastlines (epeirogeny), it would be possible relate these records to eustatic variations. This has been attempted by many researchers over the decades, with some success.  The problem with measuring eustasy is that we measure land elevation with respect to sea level, while at the same time, we measure sea level with respect to the land surface along the coasts. This is untenably circular reasoning. In order to properly measure eustasy directly, it is necessary to define an appropriate reference frame. The lack of simple frames of reference has caused some researchers to give up on the concept of eustasy altogether. Reference frames are difficult to come by, as you must essentially measure the average height of the ocean surface relative to the center of the Earth (which we can do now with satellite altimeters, but we didn’t have those in the Cretaceous). However, one proxy for this for the Mesozoic and Cenozoic time frame may be found on the Russian Platform, where flat-lying Mesozoic sediments are found at the surface, having never been buried, and thus remain unconsolidated (Jurassic beach!). On the basis of the stratigraphic sequences found on the Russian Platform and surrounding basins, it has been possible to construct a eustatic sea level curve for the Jurassic and Cretaceous, when the region was generally flooded by high seas levels. 

 

Drivers of Long Term Sea Level Change - At multi-million year timescales, (eustatic) sea level is determined by the relation between the volume of the Earth’s ocean water and the volume of the Earth’s ocean basins. Basin volume is controlled by the average age of ocean floor, which determines depth, and continental stretching or collision, which determines area. An additional influence of variations in global sedimentation rate may play a role as well. The volume of the ocean water is determined by the mass of liquid water NOT in ice sheets, and by the average ocean water temperature. (Water, like anything else, expands when heated.) As such, water volume is strongly controlled by climate, and shorter-term sea level variability is almost completely climate-controlled. There is an additional interaction between climate and tectonics through the ocean, in that the temperature of the ocean bottom water sets the upper thermal boundary condition on the ocean lithosphere. On time scales of millions of years, this can expand or contract the entire ocean lithosphere (thermal expansion again), thus making the ocean basins deeper or shallower, thus controlling sea level.

 

Global Change and Short-term Sea Level - Climatically-driven variations in monsoon intensity may cause changes in internally-draining lake levels. These variations have been found to affect local aquifers as well (since lakes are just the surface expression of the water table). In addition, the hydrogeology of frozen aquifers at high latitudes may play an important role in global change as post-glacial rebound and melting lead to aquifer drainage. Variations in continental water storage on time scales of 103-105 years may account for at least some of the stratigraphic and isotopic inferences of "rapid" sea level fluctuations that have been often attributed to glacial variations despite evidence for high temperatures at high latitudes during the Cretaceous and other periods. On a shorter time scale, a first-order calculation shows that we have directly raised 20th century sea level by mining aquifers, in addition to other anthropogenic factors (irrigation, deforestation, etc.). 

 

Continental Hydrology and Water Impoundment - The interaction between continental surface and ground water reservoirs, as well as their net flux in and out of the ocean bears on problems of water resource management and ultimately affects sea level. There are many human activities that have transferred water from the continents to the ocean, thus raising sea level. A few of these include ground water mining of fossil aquifers such as the Ogallala, draining of wetlands, and deforestation. On the other hand, a globally significant volume of water has been impounded on the continents by dam construction in the last 100 years, and the total rate of new dam construction appears to outweigh the rate of anthropogenic activities that add water to the oceans. When a new dam is constructed, while the reservoir fills for the first time, it holds back water that would have gone to the ocean. Once filled, the reservoir plays no further role in reducing the rate of sea level rise, but another dam is always being built somewhere (until recently). So the rate of water impoundment can be tallied and the extent to which people have reduced the rate of sea level rise can be figured. It turns out that the rate of reservoir filling closely balanced the rate of positive contributions to sea level rise, so the net anthropogenic impact would be zero. However, while there are tallies of the volume impounded by large dams, there are no estimates of the amount stored in small reservoirs down to the size of the ubiquitous farm ponds and rice paddies, which may cumulatively account for as much water as that stored in large reservoirs. Because a surface lake is merely a place where the ground surface dips below the water table, it is also necessary to take into account the effect of ground water impoundment in response to dam construction. This has never been done in any water resource compilations. As such, people may have been masking the true rate of sea level rise by as much as 2 mm/yr. If new dam construction is not maintained at the same pace as it has for the last 50 years, the observed rate of sea level rise will increase in the 21st century, even if all other (e.g. climatic) factors remain equal. Indeed, we have essentially stopped building dams (except Three Gorges), and it may be no coincidence that we have seen a recent increase in the rate of sea level rise from the 1.5-1.75 mm/yr of the 20th century, to over 3 mm/yr in the last decade.