Why is the sea level rising?

Waves crashing on black rocks at the sea with words Why is the sea level rising

Since the last report of the Intergovernmental Panel on Climate Change (IPCC) on sea level rise in 2013, global Sea Level Rise (SLR) has been attracting much attention. This can be can be attributed to two main reasons [1]:

  • one, the sea level has been rising menacingly from a stable equilibrium to 3.1 mm yr-1 during the past century as a result of human action and
  • second 21st century SLR may be 3-5 faster, thus putting the coastal zones all over the world in great danger.

As such, scientists and policymakers from the four corners of the Earth are getting alarmed as to what the future may hold for the coastal population, especially those on low lying deltas such as Bangladesh or on the small islands like the Maldives. To understand sea level rise though a certain amount of data on oceanic processes and observed sea level changes is required. Today, with advances on many fronts such as sea data from GRACE (Gravity Recovery and Climate Experiment) twin satellites, Argo floats, SMB (Surface Mass Balance) estimates for ice loss, satellite altimetry readings, better glacioisostatic adjustment and evidence for multi-decadal sea level variability from tide gauges, the factors causing sea level changes can be better understood.

Factors causing sea level rise

1.      Steric expansion

Steric expansion is the widening of the oceans as the water molecules absorb heat and thus expand in size. Global warming is believed to be the main reason behind the steric expansion of sea water thus leading to the present day accelerated sea level rise. Normally, when the volume of water increases, the density decreases; this phenomenon is observed in regions of high temperatures and/or pressure. Over time, this chemistry shift can disrupt the thermohaline circulation and affect oxygen and nutrient transport in the sea.

2.      Ice sheets

The second most important factor in today’s SLR is the melting of the ice sheets on Earth. An ice sheet is a huge mass of glacial land ice covering an area of over 50,000 km2. Though in the past glaciers used to cover a large portion of the northern hemisphere, today only the Greenland Ice Sheet and the Antarctic Ice Sheet remain. Melting of the ice sheets has been studied by many scientists and it is believed that they have been contributing to 0.35 mm per year SLR equivalent. A complete meltdown of the Greenland Ice Sheet alone would cause the sea to rise by 7.2 m [2]

3.      Glaciers and Ice caps

Glaciers and ice caps are ice masses less than 50, 000 km2 in size. As the temperature rises, it is causing the melting of small glaciers and ice caps. It has been noted that since the year 2000, glacier loss has increased by 0.77 mm per year to 1.4 mm per year based on various distinct parameters. Melting of these ice chunks both in the sea and on the land can have drastic consequences. On the one hand, as sea ice melts in the ocean, marine dependent creatures like polar bears and seals lose their hunting grounds. On the other hand, as mountain ice caps melt, such as those on the Andes or the Himalayas, the melt water runs into the ocean rising the sea level.

4.      Ice sheet outlet glaciers

An outlet glacier is an ice -filled valley that forms between mountains which drains meltwater from melting of ice caps, ice sheets or ice fields. Basically ice sheets and glaciers lose mass due to warm surface temperature, warm bottom water and side break off. In fact, many factors like temperature, local glacier-ice dynamics, bedrock configuration, depth of fjord and glacier thickness play a role in outlet glacier dynamics. Nonetheless, a small rise in temperature can cause glacier mass loss to speed up. Today, many outlet glaciers near the Greenland and Antarctic Ice Sheets are melting rapidly as a result of ocean warming as has been noticed at the Jakobshavn Isbrae Glacier and the Sermilik Fjord.

5.      Glacial Isostatic Adjustment (GIA)

Also called post glacial rebound, GIA refers to the re-distribution of mass on the surface of the Earth when ice sheets expand or contract thus causing vertical land movement. This land movement together with coastal sediment analysis, has been a key element in understanding past sea level changes and is now incorporated in all SLR studies. An excellent example of GIA is the uplift occurring in the Gulf of Bothnia, in the Fenno-Scandinavian region, where an ice sheet of 2/3 km thick had forced down the land during the last ice age. GPS studies have revealed that an uplift of 8 mm per year has been ongoing in the Gulf of Bothnia as the ice load is slowly retreating.

6.      Global groundwater withdrawal

Groundwater is extracted massively around the world, with estimates being in the range of 982 km3 per year thus making it the world’s most extracted resource [3]. Much of that water is used for irrigation and for drinking and after serving its purpose, the water finds its way back into the sea resulting in sea level rise. A study by Konikow (2011) [4] estimated sea level rise due to inflow of water into the oceans from 2000 to 2008 at 0.4 mm per year accounting for 13% of the 3.1 mm per year global sea level rise during that period.

7.      Artificial terrestrial water storage

Conversely to groundwater withdrawal, impoundment of water in reservoirs and artificial lakes has reduced the flow of water into the sea. Some 8000 km3 of water is stored in reservoirs and more than 90% of this capacity was created after the 1950s thus leading to a decrease in sea level of 30 mm [5]. Today, however, excessive water mining associated with other factors like deforestation and loss of wetlands, have led to an increased flow of water into the oceans. In regions such as Lake Urmia and the Aral Sea, heavy use of water for agricultural practices has led to a decrease in land water storage and a rise in SLR.

8.      Meridional Overturning Circulation and Sea Surface Height

The Meridional Overturning Circulation (MOC) is a system of circulation between the surface and deep ocean water, allowing the exchange of materials like nutrients, oxygen between the surface and deep water layers. It is a very important component of the climate system and can influence sea surface height (SSH) based on temperature and salinity content in various regions around the globe. For example, sea dynamics is lower in the North Atlantic region because of deep water circulation as compared to the North Pacific region; studies have shown that there have been decadal changes in sea level in the North Atlantic region since the mid-19th century due to changes in sea surface temperature and the strength of the MOC [6].

9.      Long term uplift

Throughout the history of the Earth, the movement of tectonic plates has shaped the ocean basins profoundly resulting in dramatic changes in the sea level. Though this process has been very slow and over large time scales, plate tectonics has caused continental land to be pushed towards or away from other land masses thus either creating large expanses or small water bodies between or around them. Land uplift is the elevation of the Earth’s crust due to the movement of tectonic plates; coupled to GIA, land uplifting has been seen to be contributing to sea level rise near continental margins [1]. For instance, Adams et al., (2010) [7] modeling land uplift near the Florida shorelines over the last 1.6 Ma found it to be at a rate of 0.02 to 0.05 mm per year based on global sea level, rainfall and platform exposure.

10. Earth Elastic Crustal Deformation (ECD), gravity and rotational effects

The Earth’s elastic crustal deformation capacity refers to the capacity of the crust of the Earth to stretch and fold under pressure and return back to its original state because of its elasticity. If not for that particular effect, factors like GIA or hydrological processes would have broken the crust of the Earth. As such, when glaciers melt, they leave behind a particular fingerprint that can be traced to the source of melting. As it is, each glacier and ice sheet has a particular location and size, so, each one creates a unique pattern in the sea when it melts just like a fingerprint. Basically, the sea level is higher near ice sheets and glaciers as they exert a gravitational pull on the sea around them. When such glaciers melt, the gravitational pull decreases, causing the sea level to drop in that specific area, but at the same time, the land rises due to the crustal deformation of the Earth’s crust inevitably leading to a further drop in the sea level there.

11. Subsidence due to other processes

Besides plate tectonic related movement, regional sea level can change due to many other factors such as sediment dewatering and compaction, lithospheric down-warping, sediment starvation and biophysical tidal marsh accretion processes.

Present day sea level rise

Right now, the causal factors behind global sea level rise is believed to be due to thermal expansion of the ocean and the melting of glaciers and ice caps as a consequence of anthropogenic global warming. The IPCC reports that over the next few decades, it is very likely that 95% of the world’s ocean will rise [8] and this will be greatly determined by other factors such as regional dynamic changes, GIA, land uplift and subsidence.

References:

  1. Cronin, T.M. (2012). Rapid sea level rise. Quaternary Science Reviews, 56, pp. 11-30.
  2. Sciencedaily. (n.d). Ice Sheet. [Online] Available at https://www.sciencedaily.com/terms/ice_sheet.htm [Accessed 13/12/19]
  3. NGWA. (n.d). Facts about global groundwater usage. [Online] Available at https://www.ngwa.org/what-is-groundwater/About-groundwater/facts-about-global-groundwater-usage [Accessed 13/12/19]
  4. Konikow, L. F. (2011). Contribution of global groundwater depletion since 1900 to sea-level rise. Geophys. Res. Lett., 38, L17401.
  5. Wada, Y., Reager, J.T., Chao, B.F. et al. (2017). Surv Geophys, 38: 131. https://doi.org/10.1007/s10712-016-9399-6
  6. Bingham, R.J., Hughes, C.W. (2009). Signature of the Atlantic meridional overturning circulation in sea level along the east coast of North America. Geophysical Research Letters, 36.
  7. Adams, P.N., Opdyke, N.D., Jaeger, J.M. (2010). Isostatic uplift driven by karstificatio and sea-level oscillation: modeling landscape evolution in North Florida. Geology, 38, pp. 531-534.
  8. Church, J.A.,Clark, P.U.,Cazenave, A.,Gregory, J.M., Jevrejeva, S., Levermann, A.,Merrifield, M.A., Milne, M.A., Nerem, R.S.,Nunn, P.D., Payne, A.J., Pfeffer, W.T., Stammer D. and Unnikrishnan,A.S. (2013). Sea Level Change. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

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