Recent development of global sea level rise
Country leaders from all around the world came to Madrid in December 2019 for the United Nations Climate Change Conference to agree on a blueprint for limiting climate change. Contingent on the development of global warming global sea level will rise either less than one meter in a realistic best-case scenario or up to two meters in a high greenhouse gas future scenario by the end of the century. In the Paris Agreement the United Nations target a limitation of global surface temperature of under 2 degree Celsius (°C) or preferable an optimum of 1.5 °C relative to pre-industrial temperature among other things because sea level rise should be limited. In the following, an overview over recent sea level rise research is given as well as projections for future sea level rise in different global warming scenarios and probable consequences for regions affected.
The latest Intergovernmental Panel on Climate Change’s (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate states that different observation methods show a rise and acceleration of the global mean sea level from 1.4 mm per year in the period 1901-1990 to 3.6 mm per year in the period 2006-2015 (2019: 323). The IPCC defines global mean sea level rise as “an increase in the volume of ocean water caused by warmer water having a lower density, and by the increase in mass caused by loss of land ice or a net loss in terrestrial water reservoirs” (IPCC, 2019: 330).
There are five contributors which have the dominant influence on the global mean sea level (Golledge et al., 2019: 3). The first one is the Greenland Ice Sheet which lost ice mass in the period 2006-2015 resulting in an approximate global sea level rise of 0.77 mm per year. The second is the Antarctic Ice Sheet which lost ice mass equivalent to approximately 0.43 mm per year of global sea level rise between 2006 and 2015. The third one would be mountain glaciers which contributed to global sea level rise by approximately 0.61 mm per year between 2006 and 2015 (Golledge et al., 2019: 3; IPCC, 2019: 6). In addition, storage of land water and the thermal expansion of the ocean, with warmer water having a lower density than colder water and therefore a higher volume, contribute the most to the rise of the global sea level (Golledge et al., 2019: 3).
Antarctica alone lost 40 Gt per year in the period of 1979-1990 and 252 Gt per year in the period 2009-2017 showing an acceleration in total ice mass loss of 280 % (Rignot et al., 2019: 1099). One main factor of mass loss in Antarctica is the increased pace of glacier movement resulting in a higher loss of ice mass at the periphery whereas the rate of snow fall did not increase so that more ice melts in contact with sea water than ice can accumulate in the interior (Rignot et al., 2019: 1095). Many more factors influence the melting of ice sheets in the Antarctica with some regions being better explored and understood than others resulting in an occurrence of uncertainties within predictions for future sea level rise scenarios (Rignot et al., 2019).
By looking at the reconstructed global mean sea level during the two past warm periods Pliocene being approximately 3 million years ago and the last interglacial being approximately 130,000 to 115,000 years ago, the probability arises that ice sheets in Antarctica are considerably sensitive to a warming climate (DeConto and Pollard, 2016: 1). The carbon dioxide concentration in the atmosphere during the Pliocene was almost as high as today with approximately 400 parts per million by volume (ppmv) and sea level is reconstructed 10 to 30 m higher than today (DeConto and Pollard, 2016: 1). Global mean sea level during the last interglacial was 6 to 9 m higher than the sea level today, while carbon dioxide concentration in the atmosphere were less than 280 ppmv and the global temperature was not warmer than 2 °C (DeConto and Pollard, 2016: 1). As a consequence the contribution of Antarctica to that sea level rise is required and estimated to be 3.6 to 7.4 m (DeConto and Pollard, 2016: 1). In comparison, global atmospheric carbon dioxide concentrations in November 2019 were 407.65 ppmv with a visible linear inclining trend since 1980 where carbon dioxide concentrations averaged approximately 340 ppmv (Tans and Keeling, 2019). In an unabated emissions scenario it is possible that Antarctica contributes about one meter of global mean sea level rise by 2100 and about 15 meters by 2500 (DeConto and Pollard, 2016: 1). The main driver of ice mass loss in that scenario would be atmospheric warming with interconnected ocean warming delaying the recovery of ice masses for a few thousands of years (DeConto and Pollard, 2016: 1).
The global mean rise of the sea level since 1970 due to global warming is mainly caused by human activities (IPCC, 2019: 323). Severe changes in land use, aerosol and especially greenhouse gas emissions, such as carbon dioxide and methane, leading to an increased surface temperature on earth by approximately 1 °C in 2019 relative to pre-industrial temperatures (Otto and Climate Science Communications Group, 2019: 1).
Considering global warming since the times of the industrial revolution and the already rise of the sea level it becomes certain to have visible impacts on coastal regions already today. Simon Albert and a small group of scientists could already identify five reef islands with vegetation in the Solomon Islands vanishing between 1947 and 2014 (Albert et al., 2016: 1). Other Solomon Islands experience severe recession at their shoreline leading to destroyed villages and the relocation of the communities living on those islands (Albert et al., 2016: 1). Shoreline recession on those islands is supported by a high wave energy linking to the interconnection of waves and sea level rise (Albert et al., 2016: 1). If the 2019 sea level were to rise by 0,1-0,2 m, more low latitude regions will likely be impacted by flooding events at their coast in a doubled frequency by 2050 (Golledge et al., 2019: 2). Therefore, to find good adaptation measures for coastal erosion and flooding of the islands it is important for local governments to know and understand the local factors leading to an increased vulnerability of the Solomon Island communities (Albert et al., 2016: 1).
Considering the impact of a relatively little rise in the global mean sea level between 1902 and 2015 of a total 0,16 m, in combination with already observed consequences predictions for future sea levels become really important (IPCC, 2019: 10). Especially because of the acceleration of the sea level rise i.e. the ice sheet from the Antarctic alone lost a mass between 2007 and 2016 three times that of the mass lost between 1997 and 2006 and Greenland lost ice mass between 2007 and 2016 two times the mass it lost during 1997-2006 (IPCC, 2019: 10).
For future projections, the 2019 IPCC report makes use of two different climate change scenarios called Representative Concentration Pathway 2.6 (RCP2.6) and RCP8.5. These scenarios are mainly modeled with CMIP5 simulations and include different concentrations of greenhouse gases and various land use options (IPCC, 2019: 8). The RCP2.6 scenario illustrates a future with little greenhouse gas emissions in the atmosphere and a likely range of temperature change on the earth’s surface of 0.9 to 2.4 °C until 2100 relative to 1850-1900 (IPCC, 2019: 8). RCP8.5 represents a high greenhouse gas emission future with a likely range of global mean temperature change on the earth’s surface of 3.2 to 5.4 °C or a global mean surface temperature change of 4.3 °C until 2100 relative to 1850-1900 (IPCC, 2019: 8).
In the RCP2.6 scenario, glaciers will probably lose 18 % of their mass between 2015 and 2100 contributing approximately 0.94 m to sea level rise (IPCC, 2019: 17). In addition Greenland’s ice sheet would contribute to global mean sea level rise of 0.07 m while the Antarctic ice sheet contributing 0.04 m by 2100 (IPCC, 2019: 17). In the unabated emission scenario RCP8.5, glacier’s projected mass reduction is 36% and its contribution to sea level rise is 2 m between 2015 and 2100 (IPCC, 2019: 17). Plus the Greenland ice sheet contribution to sea level rise by 0.15 m and Antarctic ice sheet contribution of 0.12 m by 2100 (IPCC, 2019: 17). The change in the cryosphere is projected to increase in its rate and magnitude after 2050 in RCP8.5 and the projected ice mass loss rate of Antarctic and Greenland ice sheets is increasing in the 21st century and the following centuries in both future scenarios (IPCC, 2019: 17).
More than 100 million people all over the world are projected to be displaced due to sea level rising and coastal inundation during the next century (Barnard et al., 2019: 1). As a result of the already risen global mean sea level, cyclone winds in tropical regions and rainfall increase as well as extreme waves, coastal hazards and flooding (IPCC, 2019: 10). In combination with other non-static environmental drivers such as the response to erosion and tidal non-linearity storms, and climate have an increasing impact on flood-hazard exposure (Barnard et al., 2019: 1). Sea level rise alone with a static tide will probably lead to humanitarian challenges, economic losses and national-security issues (Barnard et al., 2019: 1). Barnard et. al could show in a case study on California, USA, that the number of exposed population to dynamic flooding will be three times as high by 2100 as it would be if only a non-dynamic coastline and sea level rise are considered (Barnard et al., 2019: 1). For smaller sea level rise scenarios societal exposure to flooding at the coast is rather underestimated (Barnard et al., 2019: 1). By including storm conditions in projections for consequences of sea level rise the number of exposed populations and economic significance is up to seven times higher than for sea level rise alone (Barnard et al., 2019: 1). For a 0.50 m sea level rise scenario an additional 32% of land is estimated to be flooded in case of a 20-year storm and 41% during a 100-year storm respective to inundation due to sea level rise alone (Barnard et al., 2019: 3). For the sea level rise scenario of just 0.25 m, 48% more land would be estimated to flood during a 20-year storm and 77% additional land during a 100-year storm in comparison to sea level rise alone (Barnard et al., 2019: 3). Sea level rising 0.25 m could become a reality for California by the 2040s and 0.50 m by the 2060s (Barnard et al., 2019: 3).
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- Christina Wulf (Author), 2020, Recent development of global sea level rise. A short overview, Munich, GRIN Verlag, https://www.grin.com/document/957999