Friday, November 16, 2018

Rising Sea Levels – by How Much, and Why? A Current Commentary.



The following was published in the journal Science Progress recently, of which I am an editor. Since this blog typically covers issues of environment and energy, I am including the present topic, which I hope will be of interest to its regular readers, and indeed to anyone else with concerns about the direction of "the changing climate".





1. Introduction.
The term “sea level rise” normally refers to an increase in the global mean sea level (GMSL), caused by an increase in the volume of water in the Earth’s oceans, primarily as a result of thermal expansion, the addition of further water  from the melting of land-based ice sheets and glaciers, and  to a  smaller degree from changes in land-water storage, including the transfer of groundwater that has been pumped from aquifers1. As measured on the local scale (Section 4), sea levels may be higher or lower than the global mean value, as a result of various factors, including land subsidence, glacial rebound, tectonic effects, and the influence of currents, local temperatures, winds, tides, storms, and variations in  local barometric pressure2 among  the particular locations where the measurements are  being made. There is strong evidence that the GMSL is increasing, and as a result of long response times from various components of the climate system, this  process may continue over the course of centuries3. It has been estimated that more than half of the observed sea level rise during the 20th century was due to global warming4. According to satellite altimetry measurements, the GMSL is currently5,6 increasing by 3.2 ± 0.4 mm yr-1, which is about double the rate determined to have prevailed throughout the 20th century6, and it has been predicted that, for each degree Celsius rise in the  mean global temperature (MGT),  over the next two millennia  an increase in the   GMSL  of  at least 2.3 m   can be expected7. Not surprisingly, the consequences of rising sea levels are expected to be most severe  for those living in coastal regions and on small islands, in terms of coastal flooding, and saltwater intrusion, with further impacts on marine ecosystems8. As can be seen from Fig. 1, the ocean heat content has increased markedly during the past few decades, and it has been inferred that significant and comparable contributions to the total  GMSL rise are made by thermal expansion of the oceans, and by the melting of land-based ice, although estimates of their relative amounts have varied over time9.

[Fig. 1]

The latter variation may be due partly to real changes in the amount of sea level rise caused by each of these phenomena, but also to the development of more accurate methods with which to determine them9. There is evidence than an exacerbation of both effects has occurred during the past few decades, particularly that from melting land ice, and currently, more than one third6,9 of the increase in the global mean sea level appears to be occurring through thermal expansion, with  much of the remainder arising from the melting of land-based glaciers and ice sheets 6,9. As is indicated in subsequent sections of this article, projections of sea level rise throughout the present century (and beyond) vary, but most fall within a range of a half to one metre increase by 2100; however, there are more dramatic predictions, based on a significant degree of melting of the Greenland and Antarctic ice sheets. Fig. 2 shows a projection of the effect of a uniform global increase in sea levels by 6 m.

[Fig. 2]

 

2. Methods for measuring Sea levels.

2.1. Tide gauge.

There exists a global network of tide gauges, information from which, along with that from satellite altimetry measurements, led to the conclusion that10 the GMSL had increased by 19.5 cm, between 1870 and 2004, at an average rate of close to 1.44 mm yr-1. During the 20th century, a slightly faster rate of 1.7 mm yr-1 was deduced, and it was apparent that a significant increase had occurred from around 1930, signifying an acceleration of 0.013 ± 0.006 mm yr-2, in accord with simulation models which predict that global warming will drive an acceleration of sea level rise10. Predictions of what the sea levels will be in the future are of vital importance to low lying regions, for example the Netherlands, where around two thirds of its land area is vulnerable to flooding11. Thus, sea level measurements were established at the Amsterdam Ordnance Datum as far back as 1675, where they have been made ever since, thus providing the world’s longest continuous sea level record12. From a comparison of measurements made in Tasmania (using a sea level benchmark on a small cliff on the Isle of the Dead, near the Port Arthur convict settlement), which began on 1 July 1841, with more recently determined tide gauge data, the conclusion is supported that the global mean sea level (GMSL) rose by 1.0 ± 0.3 mm yr-1 during the period 1841—2002 13. In the United States, sea level measurements are complicated by the fact that in some regions the land is rising, whereas in others it is sinking; hence, a variation in the apparent sea level rise has been noted, from around +9 mm yr-1 on parts of the Louisiana Coast (where the land is sinking), to an ostensible sea level fall of -17 mm yr-1 in parts of Alaska (where the land is still rising as a result of post-glacial rebound)14.

 

2.2 Satellites.


[Fig. 3]

Figure 3 shows changes in the GMSL over the period 19932018, as determined using satellite altimetry. The satellite, TOPEX/Poseidon15 was launched on August 10th, 1992, as a device to map the surface topography of the oceans, and was a joint project between the U.S. space agency, NASA, and the French space agency, CNES. Unfortunately, despite its success, in January 2006 the satellite suffered a malfunction which ended its working life16. One of the roles of TOPEX/Poseidon was to make radar altimetry measurements from which to determine changes in sea levels15, an effort which has been continued using other appropriate satellites, and it has been determined17 that the rate of sea level rise during the period 1993-2017 was ~3 ± 0.4 mm yr-1. In 2015, it was identified that the TOPEX/Poseidon satellite had a minor error in its calibration18, which had indicated the 1992—2005 sea levels to be higher than they actually were, and masked the true acceleration in sea level rise, as evidenced from tide gauge data. Until this point, it was not clear why the two different methods were giving conflicting results18. The Jason-115  satellite was launched in 2001 to advance measurements of the topography of sea surfaces, and for three years it flew in tandem with TOPEX/Poseidon, thus providing double the coverage of the sea surface, and enabling the determination of smaller features than was possible using either satellite alone. Follow-on projects were inaugurated, in the form of the Ocean Surface Topography Mission, using the Jason-2 satellite, which was launched in June 200815, followed by the Jason-3 mission, which was launched on January 17th, 2016 15.

3. Changing sea levels: from past to present.
It has been determined that GMSL rose by over 125 m from the melting of vast ice sheets than existed at the end of the last ice age (Fig. 4). However, during the  late Holocene epoch, which extends from around 2,500 calendar years ago to the present time, the sea level remained relatively stable, prior to the recent rising trend which began approximately in1850 19. The current5,6 GMSL rise is 3.2 mm yr-1, which is about twice the rate that occurred throughout the 20th century10, i.e. 1.7 mm yr-1. However, from a recently reconstructed20 trend in GMSL rise, made using an area-weighting technique for averaging tide gauge records, which included contemporary data for vertical land motion (VLM) and allowed for local geoid changes that are caused by melting of ice, and changes in land storage of freshwater, it was concluded that the GMSL prior to 1990 was 1.1 ± 0.3 mm y−1, but this had risen to 3.1 ± 1.4 mm y−1 during the period 1993—2012. While the rate before 1990 is lower than had been previously estimated, the 1993—2012 value is in accord with satellite altimetric measurements, and hence the results suggest that the acceleration of GMSL is greater than hitherto had been thought20. As recorded in April 2018, the GMSL stood at 85 ± 4 mm above the level measured in 1993 when satellite altimetric monitoring was first introduced, and accords5 with a rate of 3.2 ± 0.4 mm yr-1.

[Fig. 4]

On the basis of a time series of precision satellite altimeter data, taken from the TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3 satellites, Nerem et al. have determined17 an acceleration of the GMSL rise over the last 25 years of 0.084 ± 0.025 mm y-2, which is in good accord with climate model projections. In combination with an average GMSL of 2.9 ± 0.4 mm y-1, determined over the same time period, simple extrapolation of the quadratic predicts that, by 2100, the GMSL will have risen by some 65 ± 12 cm above the level in 2005, which is in reasonable agreement with the projections published by the IPCC in their 5th Assessment Report (AR5)6.  As a result of the acceleration, which is driven by an enhanced melting of ice in Greenland and Antarctica, the increase in GMSL is more than twice that expected on the basis of a merely linear elevation of 3 mm yr-1. It is possible17 that by the end of the present century, the GMSL could be increasing by 10 mm yr-1. Nerem is quoted21 as saying:

And this is almost certainly a conservative estimate. Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that’s not likely.”

4. Local variations in sea level.
Local sea levels differ around the Earth as a result of effects such as spatially varying patterns of warming and sea water expansion around the globe, subsidence, local barometric pressure, glacial rebound, the planetary rotation, and gravitational effects of declining ice masses on sea levels, both locally and further afield, as ice sheets melt22. In 2017 it was reported23 that the surface ocean temperatures in the northern Indian Ocean had increased, as a result of weakening monsoon winds, which attenuated an ocean circulation (which typically brings cooler water up from the ocean depths), and resulted in sea level rise, especially in the Arabian sea, and significantly close to the Maldives23. Ports, urban conglomerations, and agricultural regions, which are built on river deltas, may experience land subsidence, and a substantial effective sea level rise. Land may also be caused to sink by the extraction of (previously supporting) groundwater, oil and gas, and from building levees and installing other flood control measures that restrain the accumulation of sediments, which otherwise would offset the natural settling of deltaic soils24. This rate of subsidence varies considerably, from perhaps several mm yr-1, up to maybe 25 cm yr-1 in parts of the Ciliwung delta (Jakarta)25. It has been estimated that the total subsidence, which may be attributed to human activities, is 3—4 m in the Rhine-Meuse-Scheldt delta (Netherlands), and in urban areas of the Mississippi River Delta (New Orleans) it is over 3 m, while the Sacramento-San Joaquin River Delta has subsided by more than 9 m 25.

Those regions of the Northern Hemisphere that were covered and compressed by massively weighing ice sheets, some 20,000 years ago, including the northeastern United States, have been slowly rebounding following the ice melt, and hence the sea levels appear to be rising more slowly, as measured against land that is simultaneously rising26. In a counterintuitive contrast, some land areas, which formerly occupied the edges of the ice sheets (for example the Chesapeake Bay region), are now subsiding. This is due to the fact that the pressure on the underlying rock in the Earth’s mantle drove the land to uplift; now, in the absence of the compressive weight of the ice, the land level is falling, thus enhancing the effective sea level there, and its consequences26.

The local barometric pressure varies around the globe, with an influence on the sea level at particular locations2. Thus, from an analysis27 of barometric pressure and sea level data, for twelve Tropical Pacific island countries, it was found that during mid-1997 and 1998, strongly positive pressure anomalies developed over the Pacific region, along with a marked fall in the sea level, and it is probable that these anomalies had a direct link to the delayed effect of the strong El Niño episode in 1997. In contrast, negative pressure anomalies and an increase in the sea level were observed for the La Niña episode, which followed in 1999. It was concluded that the influence of barometric pressure can cause abrupt sea level changes, and at least temporarily, can overshadow other dominant contributions to sea levels such as thermal expansion27.

Additional information was provided in a recent study28 regarding the influence of pressure on variations in sea level. Thus, from data taken at 27 tide gauge stations, distributed around the globe, it was shown that practically all the annual and semi-annual sea level variations are a consequence of atmospheric pressure changes, at regional and local scales (i.e. low pressure causes higher levels, and high pressure reduces them – “Inverted (Inverse) Barometer Effect”). However, non-linearities in the regional pressure data were noted that are statistically significant, and which impacted on other variations in sea level, so compounding the effects of lunar nodal forcing, and generating lunar sub-harmonics with multi-decadal periods28.

The upward gravitational attraction on neighbouring sea waters by massive glaciers is diminished as they melt, which causes a fall in the seawater level, both locally and more widely. As an extreme example of this effect, it was reported that the melting of ice in Western Antarctica increases sea levels more profoundly in coastal regions of New York than those of Sydney, which are far closer29.

Due to the Coriolis force, the Earth’s rotation causes fluids in the Northern Hemisphere to be deflected (to swirl) to the right, and to the left in the Southern Hemisphere. Accordingly, as water is driven around coastlines, the sea may be seen to bulge-out in certain locations, but to recede into troughs elsewhere. It has been shown that this effect can be exacerbated by the outflow from rivers30, waters from which, as they flow into the oceans, are levelled upwards by the swirling current, as compared with those waters which precede  them.


5. Predictions of future sea level rise.

Sea level rise is modelled using two general approaches, one of which is process-based31, where those physical processes that are deemed to be important are incorporated as parameters within a simulation model; however, this approach is limited by the degree to which all relevant processes are sufficiently well understood. Hence, a semi-empirical approach may be useful, which uses inputs of historical geological data, to reflect how the sea level has actually responded to increases in global temperatures, within a basic physical modelling strategy, which employs sophisticated methods of statistics31. Due to the complexity of the Earth systems and the limitations of climate models, the reliability of current predictions of how far the sea levels will rise over time is a matter for posterity. Figure 5 indicates a prediction of what the sea level rise would be, if its only driver were thermal expansion of the oceans, caused by either a doubling or quadrupling of the atmospheric CO2 concentration. Clearly, the total rise would be enlarged by the addition of water to the oceans from melting bodies of ice, albeit to an uncertain degree, though the effect might be substantial, as has been emphasised by Hansen et al., who predict that the growing sea level rise is non-linear, possibly “reaching several meters over a timescale of 50–150 years32.

[Fig. 5]

In its Fifth Assessment Report6, the IPCC determined that the present global average sea level rise can be dissected into major contributions from: thermal expansion of the oceans (1.1 mm yr-1), melt-water from glaciers (0.76 mm yr-1), glaciers in Greenland (0.1 mm yr-1), the Greenland ice sheet (0.33 mm yr-1), the Antarctic ice sheet (0.27 mm yr-1), and the transfer of water stored on land (0.38 mm yr-1). The IPCC further concluded6 that a sea level rise of 0.52—0.98 m (relative to a 19862005 baseline level) could occur, were greenhouse gas emissions to maintain according to their worst emissions scenario; even on the basis of the lowest case projection, a rise of 0.28−0.61 m is expected6 by 2100. In a 2017 paper33, which assumed that a high consumption of fossil fuels will prevail during the remainder of the present century, to underpin the demands of a strongly growing global economy, it was predicted that an average sea level rise of up to 1.32 m would occur, with an extremum of perhaps 1.89 m, by 2100. The study also emphasised the mitigating effect of meeting the emissions targets set out in the Paris climate agreement, which it predicted would limit the rise to a median of 0.52 m by 2100 33.  On the basis of a semiempirical “constrained extrapolation” model, anthropogenically driven GMSL rises of 0.28–0.56 m, 0.37–0.77 m, and 0.57–1.31 m (in 2100) were predicted31 for three alternative greenhouse gas concentration scenarios, which may be seen to overlap with the process-based projections published by the IPCC6.

The final determinants of GMSL rise may prove to be the Greenland and Antarctic ice sheets34, since they contain 99.4% of global land ice. Thus, the volume of ice contained within them translates to a potential sea level rise of 7.4 m from Greenland, 58.3 m from Antarctica, to be compared with a far more modest 0.4 m from global glaciers34.

6. Contributions to sea level rise from melting land-ice.

6.1 Antarctica.
[Fig. 6]
 
From a recent systematic review35, it was reckoned that between 1992 and 2017, Antarctica
(Fig. 6) lost 2,720 ± 1,390 billion tonnes (Gt) of ice: during the period 1992—2002, the average
 rate of ice-loss was 49 ± 67 Gt per year; however, a substantial escalation of the process
occurred during the period 2012—2017, to an average of 219 ± 43 Gt per year. It is the melting
of the West Antarctic Ice Sheet that is the main culprit, with some positive contribution from the
Antarctic Peninsula. It is likely that the East Antarctic ice sheet was formerly (1992—2012)
increasing its ice mass, but the indication is that, during the period 2012—2017, a loss of ice
occurred; however, the values determined for this particular ice-mass are the least certain of
all35. The data indicate that the ice-loss from the West Antarctic Ice Sheet has increased from
an average of 53 ± 29 Gt per year, during the period 1992—2017, to an average of 159 ± 26 Gt 
between 2012 and 2017, as a result of ocean-driven melting35. It is thought36 that part of the
reason for this enhancement in ice-loss is an appreciable acceleration of outflow glaciers in the
Amundsen Sea Embayment, and that some 250 km3 of ice per year is being discharged by the
Amundsen Sea sector of the West Antarctic Ice Sheet, which amounts to a 60% greater volume
than is  being accrued from precipitation in the catchment areas. A sea level rise of 0.24 mm yr-1 
can be expected from this source alone36. It is reckoned that the rate of ice-loss from the
Antarctic Peninsula has increased from 7 ± 13 Gt per year, during the 1992—1997 period, to an
annual average of 33 ± 16 Gt, between 2012 and 2017, due to ice-shelf collapse35. A
comparison can be made between the above estimates35 and those determined from a
synthesis of recent satellite measurements and statistical modelling37, which provides an
overview of the global land mass trends during the period 1992—2016: specifically for the West
Antarctic Ice Sheet, the East Antarctic Ice Sheet, the Greenland Ice Sheet, and Arctic glaciers
and ice caps37. The results indicate37 that the loss of ice from the West Antarctic Ice Sheet
increased from −55 ± 30 Gt yr-1, during the 1992—1996 period, up to −172 ± 27 Gt yr-1,
between 2012 and 2016, which is in fair agreement with the study by Shepherd et al.35
t was found37 that Antarctica as a whole contributed some 29% of the global land ice contribution
 to sea level rise, of which the vast majority was from the West Antarctic Ice Sheet, (giving an increase
in sea level of 0.48 mm yr−1), with the East Antarctic Ice Sheet having undergone a shift toward
greater ice loss during the 20022016 period37; again, broadly in accord with the results of Shepherd
et al.35.

6.2 Greenland.

The Jakobshavn Isbræ is Greenland’s largest outlet glacier, and drains around 6.5% of the ice sheet area. On the basis of remote sensing measurements, its velocity was found to be highly variable over time, since it slowed down in its movement towards the sea from 6.7 km yr-1, in 1985, to 5.7 km yr-1, in 1992, but later accelerated, reaching 9.4 km yr-1 in 2000, and 12.6 km yr-1 in 2003. An explanation for this may be found in terms of a thickening of the glacier in the early 1990s followed by its rapid and progressive thinning38. It has been proposed that a tipping point was reached in 1997, and that Greenland’s glaciers and ice caps will continue to melt, rendering the main ice sheet increasingly unstable39. In 2012, pronounced surface melting of the ice sheet was observed37, with a mass-loss of certainly 400 Gt, which was exceptional since at least 195837; however, this declined in subsequent years (e.g. less than 100 Gt in 2013)37. Thus, it has been concluded that to make a “simple extrapolation of the trends over the last 20 years forward in time is, clearly, unwise and unjustified”40. It has recently been deduced37 that the Greenland ice sheet is making the greatest (37%) land-ice contribution to the measured sea level rise, which, during the 2012—2017 period, amounted to 0.69 mm yr-1. The results show a monotonic increase in mass transfer from the Greenland Ice Sheet to the oceans, which may be deduced at a sea level equivalent (SLE) of 0.13 ± 0.22 mm yr-1 during 1997–2001, but which had risen to 0.69 ± 0.04 mm yr-1 for the 2012–2016 period37.

6.3 Glaciers and sea ice.


From a recent study37, it was concluded that the contribution from global glaciers and sea ice had risen from −117 ± 44 Gt yr-1, during the 1992—1996 period, to −227 ± 31 Gt yr-1, from 2012 to 2016. This corresponds to an accelerating rate of sea level increase, from 0.32 ± 0.12 mm yr-1 to 0.63 ± 0.08 mm yr-1, during these same periods; currently, 34% of the total contribution from land ice to the GMSL is from this source37. Not being land-based, the melting of sea ice makes a very minor contribution to global sea level rise41, since the liquid water that it produces has close to the same volume as the proportion of the floating ice mass (e.g. icebergs) that lies below   sea level.  However, because the salinity of the sea water is greater than that of the sea ice, and freshwater has a slightly greater volume than salt water per unit mass (and the effect is one of “mass” displacement, according to “Archimedes’ Principle”), melting sea ice can confer a small elevation to the sea level. Thus, it has been estimated41 that if that all floating ice shelves and icebergs were to melt, the sea level would rise by just 4 cm.


7. Effect of groundwater extraction on global mean sea levels.

According to a number of recent studies, it appears that, over the next half century, groundwater depletion (GWD) may become the most important positive terrestrial contribution to sea level rise, and equal those contributions from melting glaciers and ice caps42. Nonetheless, the models used to evaluate the GWD contribution, typically assume that practically all of the extracted (“mined”) groundwater eventually ends up in the oceans42. In its Fifth Assessment Report (AR5), the IPCC concluded that the contribution to GMSL rise from “land water storage” had increased6 from –0.11 [–0.16 to –0.06] mm yr-1 (during 1901–1990), to 0.38 [0.26 to 0.49] mm yr-1 (during 1993–2010). However, by means of a coupled, climate–hydrological model, Wada et al. deduced42 that only 80% of the GWD ends up in the oceans, and that its contribution to MGSL rise was 0.02 (± 0.004) mm yr1 in 1900, but this had increased to 0.27 (± 0.04) mm yr1 in 2000. The new model allows the oceanic net terrestrial water transfer (during 1993–2010) to be reckoned at +0.12 (± 0.04) mm yr1, which suggests that the figure given in the IPCC report6 is a factor of three too high42. The complex and interconnected issues of water impoundment in reservoirs and artificial lakes (which has reduced the outflow of water to the sea), and the increase in river runoff (due to groundwater mining, wetland and endorheic lake storage losses, and deforestation), are addressed in a follow-up paper, where the various components of positive and negative land-water contribution are considered separately in order to comprehend and appreciate the details of their recent changes toward sea level variations43. The broader aspects of land-sea water exchange were considered in the context of climate responses and sea level changes that were a feature of previous geological epochs44.



8. Potential consequences of sea level rise.

 

Some of the potential consequences of rising sea levels were considered in the section of the Contribution of Working Group II to the Fifth Assessment Report of the IPCC on “Climate Change and the Ocean”, most of which are expected to be detrimental8. Not surprisingly, it is coastal regions and low lying areas that are deemed to be most vulnerable, in terms of increased  erosion, more extensive flooding and submergence, greater flooding from storm-surges with potential loss of life, negative impacts on the quality of surface water and groundwater, adverse effects on agriculture and aquaculture caused by impairments in the quality of water and soil, the risks that property and coastal habitats may be lost through erosion and/or inundation, and negative impacts on tourism, recreation, transportation, and cultural resources. It is thought that increased ocean temperatures, and acidification, may cause further harm to these locations, and to marine ecosystems such as corals8.

It is generally agreed that the benefits to be had in providing protection in advance of increased coastal flooding, and loss of land from submergence and erosion, outweigh the costs of inaction, both in social and economic terms, although the actual evidence for this is limited8. However, the evidence is compelling that if we continue with business as usual (i.e. do nothing), by 2100, millions of people will suffer displacement due to land loss, mainly in regions of the East, Southeast, and South Asia. For the majority of developed countries, it is thought to make economic sense to protect against flooding and erosion8. Almost certainly, there will be considerable variation between different regions and nations, in terms of the relative costs of adaptation, and some low-lying developing countries (e.g., Bangladesh, Vietnam) and small islands (e.g., Maldives, Tuvalu) are expected to be particularly harshly impacted upon by sea level rise (and climate change more generally – storms, etc), protection of which may cost the equivalent of several percentage points of their gross domestic products (GDPs). The IPCC report8 stresses that those developing countries and small islands in the tropics, with a significant contribution to their GDP from coastal tourism, will be adversely affected by ocean acidification and coral bleaching, in addition to the impacts of future sea level rise and associated extreme weather phenomena8.

9. Adaptation.

Even if the Paris Climate Change targets45 are met, the delayed response of the Earth systems (“climate inertia”46) means that the MGT will continue to rise for centuries to come3, and probably millennia; hence, rising sea levels can be expected to continue over similar timescales. That noted, at the time of writing (August 21st 2018), it can be estimated47 that we have 17 years remaining within which to make the necessary reductions in global carbon emissions that will prevent the  MGT from rising further than “2.0 oC above pre-industrial levels”, but a mere 3 weeks, to keep it below the 1.5 oC limit. Hence, whatever course of action humankind may take, it will be necessary at adapt to the effects of sea level rise3, along with other probable consequences of climate change48.
As the IPCC has noted8:
“The analysis and implementation of coastal adaptation toward climate-resilient and sustainable coasts has progressed more significantly in developed countries than in developing countries (high confidence). Given ample adaptation options, more proactive responses can be made and based on technological, policy related, financial, and institutional support. Observed successful adaptation includes major projects (e.g., Thames Estuary, Venice Lagoon, Delta Works) and specific practices in both developed countries (e.g., Netherlands, Australia) and developing countries (e.g., Bangladesh). More countries and communities carry out coastal adaptation measures including those based on integrated coastal zone management, local communities, ecosystems, and disaster reduction, and these measures are mainstreamed into relevant strategies and management plans (high confidence)”.
Various means to manage sea level rise have been proposed49 including tidal barriers, coastal armouring, elevated development, floating development, floodable development, living shorelines, and managed retreat, each of which has its particular advantages and drawbacks, as is discussed49. The highly ambitious strategy of geoengineering the Greenland and Antarctic glaciers directly, to hold back water, and slow sea level rise, has recently been mooted50. It has been proposed51 that “Roman concrete” might prove a robust and enduring material with which to construct sea walls to protect coastal cities against the effects of sea level rise, and to fabricate tidal lagoons for electricity generation51.  Since about a quarter of the Netherlands lies below sea level, and around two thirds of its land area is vulnerable to flooding11, a comprehensive water management strategy has been advanced to deal with the effects of rising sea levels, which involves accommodating some of the additional water on land, rather than trying to simply repel  all of it52.

10. Note, added in proof.
Shortly after this article had been accepted for publication, a report was published53, which is a synthesis of results from around 50 separate research teams. On the basis of altimetric measurements, it was deduced that the GMSL was increasing at an average rate of 3.1 ± 0.3 mm yr−1, and that an acceleration of 0.1 mm yr−2 had occurred from1993 to the present time. The contribution to the total sea level rise from various components during that same time period was determined to be: ocean thermal expansion (42%), meting of temperate glaciers (21%), Greenland (15%), and Antarctica (8%). It was concluded that the GMSL balance can be closed to within 0.3 mm yr−1 (1σ), although there is considerable uncertainty over the contribution from land water storage53.

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Captions to Figures.
Figure 1. Ocean heat content. https://upload.wikimedia.org/wikipedia/commons/5/5c/Ocean_Heat_Content_%282012%29.png Credit: US National Oceanographic and Atmospheric Administration (NOAA). Credit: NOAA.

Figure 2. Map of the Earth with a sea level rise of 6 metres, represented in red (uniform distribution; actual sea level rise will vary regionally). Hotspots of sea level rise can be 3–4 times the global average, as is projected for parts of the U.S. East Coast. https://upload.wikimedia.org/wikipedia/commons/5/5c/6m_Sea_Level_Rise.jpg Credit: NASA.

Figure 3. Satellite measurements of sea level, in millimeters, 1993-2018 (April). Data source: Satellite sea level observations. https://upload.wikimedia.org/wikipedia/commons/c/cc/NASA-Satellite-sea-level-rise-observations-1993-April-2018.jpg Credit: NASA.

Figure 4. Changes in sea level since the last ice age. https://upload.wikimedia.org/wikipedia/commons/1/1d/Post-Glacial_Sea_Level.png Credit: Robert A. Rohde
Figure 5. Graph to illustrate the minimum projected change in global sea level rise if atmospheric carbon dioxide (CO2) concentrations were to either quadruple or double, based on the expected changes due to thermal expansion of sea water alone. The effect of melted continental ice sheets would further increase the total rise, albeit by an uncertain but possibly substantial factor.

https://upload.wikimedia.org/wikipedia/commons/a/a4/Projected_change_in_global_sea_level_rise_if_atmospheric_carbon_dioxide_concentrations_were_to_either_quadruple_or_double_%28NOAA_GFDL%29.png Credit: NOAA GFDL/Enescot

 

Figure 6. Map of Antarctica, showing West Antarctica, East Antarctica, the Antarctic Peninsula and the Transantarctic mountains. https://upload.wikimedia.org/wikipedia/commons/c/c0/Antarctica.svg Credit: NASA.