Two men argue over sea level rise, one from Alaska, one from Alabama. The man from Alaska claims the sea level is falling while the man from Alabama claims the sea is rising. Both are correct. Many physical influences determine local sea level fluctuation. It’s a surprise to most that the surface of the ocean is not level around the globe. The average global sea level does not respond immediately to dynamic changes like a bathtub. Instead, the surface reacts at different rates around the globe in response to a number of local factors. This article will uncover truths about sea level rise that are hidden from plain view and will provide a background on sea level fluctuation that has occurred historically as a result of geophysics.
A report published in the Bulletin of the Atomic Scientists proposes that a global sea level average is practically meaningless as there are several factors that contribute to local sea level measurements. Before we get started, I’d like to set in stone the fact that sea level rise is happening now, is happening on a human-timescale, and needs to be addressed starting on a local, state and Federal level - but I will save that for further discussion. To the facts.
Fact #1: Large ice masses pull water towards them by their own force of gravity. This effect of gravity results in a higher recorded sea level in regions up to 2,000 km near solid ice masses. This also means that regions far away from ice masses historically observe lower sea level measurements. Snapshot measurements of sea level height are only useful when they are compared to other measurements. In other words, the rate over time of sea level rise and fall is still the most important measurement. Polar regions may record a higher sea level, but the polar sea level’s overall rate of change demonstrates that the sea level is dropping. Data represents that if all ice melting were to immediately cease, sea level recordings globally would continue to exhibit the types of changes we are experiencing now. Why? Geomorphic features. Geomorphic features are large, observable masses of earth, such as canyons, mountains, rifts, valleys, etc., that are formed by natural processes which continue to shape the earth’s surface through physical, chemical and biological interactions.
Fact #2: The earth’s solid crust is, in fact, not so solid. The earth’s surface is viscous, allowing it to experience plastic deformation. Plastic deformation is the ability of a material to undergo tensive, compressive and torsional stresses and return to its original state. The earth’s surface responds to immense weight, such as the weight of enormous ice bodies, by compressing and ultimately lowering the land mass the ice sits atop. This ongoing dynamic response of the earth is called ‘glacial isostatic adjustment’ or GIA. When the earth is relieved of this massive weight by the melting of sea ice or glaciers, it decompresses and returns to equilibrium with the surrounding crust. During this period, as the land rises the sea level will effectively fall. Regions that historically contained large ice masses may exhibit an overall sea level fall. Although, not all land is rising, instead, there are certain regions that are also sinking. New Jersey is sinking.
Figure 1: Arrows depict relative change in sea level. Image: NOAA https://oceanservice.noaa.gov/facts/sealevel-global-local.html
Fact # 3: Subsidence. Subsidence is the gradual sinking of an area of land. In New Jersey, sea level rise is occurring at an accelerated rate due to subsidence, which is enhanced by the loss of subsurface support. New Jersey sits on top of a karst terrain, which is described as a “topography that indicates dissolution of underlying carbonate rocks,” such as limestone and dolomite, by surface water or groundwater over time. This process leaves pockets of low-density sediment that will vertically compress, eventually becoming solid rock. In addition, sedimentary compaction occurs as groundwater evacuates the water table. Groundwater evacuation can occur naturally, but it is also accelerated by groundwater pumping for drinking water.
It is sometimes difficult to imagine such large, seemingly-static, masses of land and ice affecting each other over great lengths of time, but that is exactly what the data expresses. Always remember Newton’s third law: For every action, there is an equal and opposite reaction. In New Jersey, accelerated sea level rise due to glacial melting, combined with geomorphic processes, results in a significant overall increase in sea level compared to other regions.
All these factors together: ice masses attracting water, subsidence, sediment compaction, groundwater evacuation, and the ever-growing problem of erosion, contribute significantly toward accelerated local sea level rise. There is no doubt global sea level averages are rising, but it is important to recognize sea level rise and rates are substantially different in different regions, for different reasons. Humans have certainly contributed to the exponential melting of glaciers and sea ice. However, geological processes have been occurring long before anthropogenic effects on climate began. These truths about sea level rise are global and will have global consequences if humans do not act now to offset our effect on climate change. How will sea level rise affect the ever-growing coastal population evident across all coastlines in the US? How does sea level rise affect you in your community? Please let us know.
