Global warming, sea level rise and coastal inundation
Global warming, sea-level rise and coastal inundation<br />Professor Simon K. Haslett<br />Centre for Excellence in Learning and Teaching<br />Simon.firstname.lastname@example.org<br />8th July 2010<br />
Introduction<br />The level of the sea is not constant; it is always rising and falling, whether through the passing of waves, meteorological influences, or gravitational effects in the form of the earth’s geoid.<br />In the long term, sea-level has been controlled by changes in the volume of landlocked ice sheets.<br />Recently, however, anthropogenically enhanced warming of the earth has been melting this ice and causing sea-levels to rise at a faster rate than ever before.<br />The Earth’s average temperature has risen approximately 0.7˚C in the last 100 years (IPCC, 2007).<br />Sea-level change is one of the main factors in stimulating coastal change and if sea-level rises, then coasts must dynamically adapt, or risk drowning.<br />This presentation covers; the principal mechanisms for sea-level change, future predictions and impacts of sea-level rise, and ways of managing sea-level rise.<br />
Sea-level change over the last 100 years 1<br />Records of eustatic change have been compiled from tide gauge stations.<br />All major studies have concluded that sea-level is rising at a rate of between 0.5 mm to 3 mm/yr-1.<br />E.g. Gornitz and Lebedeff (1987) used tide gauge data from 130 stations with at least a 20 year record to estimate average rate of sea-level change over the period 1880-1982. Using two different averaging techniques on their data, they obtained estimates of 1.2 ± 0.3 mm/yr and 1.0 ± 0.1 mm/yr.<br />Barnett (1988) analysed 155 stations over the period1880-1986 and obtained a rate of 1.15 mm/yr.<br />Both of these estimates are in close agreement, however, when their eustatic curves are compared it is apparent that while Gornitz and Lebedeff’s curve appears linear over the entire period, Barnett’s curve suggests a steeper rate of rise through the period 1910-1980 – approximately 1.7 mm/yr.<br />
Sea-level change over the last 100 years 2<br />These differences are perhaps due to errors introduced by isostatic movement.<br />Gornitz and Lebedeff corrected their curve by taking isostatic movement into account by using extensive data from radiocarbon dated Holocene sea-level index points, whilst Barnett did not.<br />The International Panel on Climate Change (IPCC) (2007) concludes that sea-level has risen by between 12 and 22 cm in the twentieth century because of global warming.<br />A rate of about 1.2-2.2 mm/yr.<br />For the period 1993 to 2003, however, this rate has increased to around 2.8 ± 0.7mm yr–1.<br />
Possible contributors to sea-level change 1<br />The IPCC have recognised four major climate-related factors that could possibly explain and a rise in global mean sea-level over the last 100 years:<br />Thermal expansion of the oceans<br />The volume of the oceans varies with changes in the density of sea water, which is inversely related to temperature. Therefore, as the oceans warm due to global warming sea water density decreases, the oceans expand, and sea-level rises.<br />This is called a STERIC rise in sea-level.<br />Salinity changes can also affect sea water density and although this can be significant on a local scale, the effect is relatively minor at the global scale.<br />Estimating oceanic expansion can be achieved either empirically (observational) or by models.<br />Observational data are scant, but Thomson and Tabata (1987) examined a steric height record over 27 years from the northeast Pacific and found that steric heights are increasing at around 0.9 mm/yr. However, the accuracy of this and similar estimates is doubted because interannual variability creates too much ‘noise’ to be confident, and the regional nature of the study makes inference on a global scale problematic.<br />In order to fill data groups and overcome these problems the World Ocean Circulation Experiment (WOCE) was designed, and within a few decades should produce more accurate results.<br />
Possible contributors to sea-level change 2<br />Thermal expansion of the oceans continued:<br />Because of the lack of empirical data, numerical models have been designed.<br />Wigley and Raper (1993) used such a model and showed that for the period 1880-1990, the resultant range of sea-level rise due to thermal expansion is about 3.1-5.7 cm.<br />Since that time a number of studies have reported new results for steric height trends (Antonovet al., 2005).<br />Bindoffet al. (2007) estimate that the average contribution of thermal expansion to sea-level rise was 0.4 ± 0.1 mm yr–1 for the period 1961 to 2003. But between 1993 and 2003, that contribution increased four-fold (1.6 ± 0.5 mm yr–1).<br />Source: Figure TS.16. Solomon, S., Qin, D., Manning, M., Alley, R.B., Berntsen, T., Bindoff, N.L., Chen, Z., Chidthaisong, A., Gregory, J.M., Hegerl, G.C., Heimann, M., Hewitson, B., Hoskins, B.J., Joos, F., Jouzel, J., Kattsov, V., Lohmann, U., Matsuno, T., Molina, M., Nicholls, N., Overpeck, G., Raga, G., Ramaswamy, V., Ren, J., Rusticucci, M., Somerville, R., Stocker, T.F., Whetton, P., Wood, R.A. and Wratt, D. 2007.Technical Summary. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Avervt, K.B., Tignor, M. and Miller, H.L. (eds.)]. Cambridge University Press, Cambridge.<br />Right: Global sea level change due to thermal expansion for 1955 to 2003 based on Levituset al. (2005), Ishii et al. (2006), and Willis et al. (2004).<br />
Possible contributors to sea-level change 3<br />Glaciers and small ice caps<br /><ul><li>The majority of non-polar valley glaciers have been retreating over the last 100 years.
Meier (1984) estimated that during the period 1900-1961 glacier retreat contributed 2.8 cm or 0.46 ± 0.26 mm/yr-1 to global sea-level rise.
Over the same period, global mean temperature rose by approximately 0.35°C.
This means that for every 1°C increase in temp., the sea-level rises by 1.3 mm/yr-1 due to the melting of glaciers.
Warrick et al. (1996) have now suggested 2-5 cm between 1890 and 1990.
Bindoffet al. (2007) consider that glaciers and ice caps have contributed 1.34-2.86 cm to sea-level rise between 1961 and 2003 and 0.55-0.99 cm between 1993 and 2003.</li></ul>The Greenland ice sheet<br /><ul><li>Estimating the contribution of the Greenland ice sheet to sea-level rise over the past century is hampered by a lack of data for the ice-sheet dynamics.
It has been suggested that as global temperature increases, precipitation would increase over Greenland leading to an increased in ice accumulation.
Indeed, for every 1°C rise in global temp., precipitation would increase by 4%, at around 0.3 ± 0.2 mm/yr-1 per °C.
However, this accumulation is thought to affect Greenland’s interior only, as most outlet glaciers around the coast of Greenland have retreated strongly over the last century, thus contributing to sea-level rise.</li></li></ul><li>Possible contributors to sea-level change 4<br />The Greenland ice sheet continued:<br /><ul><li>Over the last 100 years, melting of the Greenland ice sheet through outlet glacier retreat was, in 1990, thought to have contributed 23 ± 16 mm to sea-level rise at a rate of approx. 0.23 ± 0.16 mm/yr-1.
But, in 1996, the IPCC suggested the contribution from Greenland to sea-level rise appears to be less than straightforward and a range of -4 - 4 is proposed.</li></ul>More recently, the IPCC (2007) estimated that Greenland’s contribution was 0.05 ± 0.12 mm yr–1 during 1961 to 2003 and 0.21 ± 0.07 mm yr–1 during 1993 to 2003, meaning the Greenland ice sheet has contributed ~ 4.2 mm to sea-level rise in the last 50 years.<br />If the Greenland ice sheet was to completely melt over millennia, then sea-levels may rise by up to 7 m (IPCC, 2007).<br />The Antarctic ice sheet<br /><ul><li>In a manner similar to the Greenland ice sheet, might accumulate ice through an increase in precipitation brought about by global warming.
However, unlike Greenland, the Antarctic ice sheet has a positive balance, meaning it accumulates more ice than it ablates.
Budd and Smith (1985) estimate Antarctica is accumulating 209x1012 kg/yr of ice.
This positive balance corresponds to a rate of eustatic change of about -0.6 mm/yr-1.</li></li></ul><li>Possible contributors to sea-level change 5<br />The Antarctic ice sheet continued:<br /><ul><li>Therefore, the main Antarctic ice sheet is thought to be contributing negatively to sea-level change, in the order of -14 cm between 1890 and 1990 (IPCC, 1996).
THE WEST ANTARCTIC ICE SHEET deserves special attention, because the majority of it is grounded below sea-level, so it may be very sensitive to sea-level change.
The main worry is that sea-level rise may lift the ice sheet by floatation off the sea floor.
This would reduce floor stress and lead to a thinning of land-based ice.
Iceberg calving would increase leading to a positive contribution to sea-level rise of up to 14 cm between 1890-1990 (IPCC, 1996).
In general, Antarctica is thought to have contributed 0.14 ± 0.41 mm yr–1 and 0.21 ± 0.35 mm yr–1 to sea-level rise for 1961-2003 and 1993-2003 respectively (IPCC, 2007).</li></ul>Estimated contributions to sea-level rise. (Bindoffet al., 2007).<br />
Other factors<br />Sea-level could also be affected by increases or decreases in surface and groundwater storage.<br />Groundwater depletion, through pumping, has amounted to the extraction of over 2000 km3 or water during this century, which is equivalent to a 0.55 cm rise in sea-level (Warrick and Oerlemans, 1990).<br />However, substantial increases in surface storage have occurred since the 1930s.<br />Deforestation, however, leads to water loss from soils through increased surface run-off and evaporation, which constitutes a positive contribution to sea-level rise.<br />Warricket al. (1996) estimated a contribution to sea level of between -5 and 7 cm during 1890-1990, and Gornitzet al. (1997) supported a negative contribution to sea-level rise.<br />The estimates are too imprecise yet to make any conclusion of their contribution to sea-level change.<br />~ <0.5 mm yr–1 (IPCC, 2007).<br />
Synthesis<br />The combined contribution from thermal expansion, mountain glaciers and the Greenland ice sheet over 1890-1990 totalled 10.5 cm (IPCC, 1990).<br />Sea-level rose 2.6 cm between 1993 and 2003 as a result of these factors (IPCC, 2007).<br />So far, within the range of observed sea-level rise (12-22 cm) (IPCC, 2007).<br />
Future impact of sea-level rise 1<br />The IPCC presents its projections of future sea-level rise for six different greenhouse gas forming scenarios.<br />They each vary in their assumptions regarding future economic and population growth, technological change and resource use.<br />In scenario A1B (which assumes moderate resource use), global sea level reaches 0.22 to 0.44 m above 1990 levels by the mid 2090s (IPCC, 2007).<br />Source: FAQ 5.1, Figure 1. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Avervt, K.B., Tignor, M. and Miller, H.L. (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp.<br />Time series of global mean sea level. The blue area represents the range of model projections for the SRES A1B scenario. Beyond 2100, the projections are increasingly dependent on the emissions scenario used.<br />
Future impact of sea-level rise 2<br />The various contributions to sea-level change are assessed individually.<br />Projected global sea-level rise (SLR) between 1890-99 and 2090-99 and estimates of the rate of SLR during 2090-99. The ranges comprise output from six different climate change scenarios (Meehlet al., 2007).<br />
Future impact of sea-level rise 3<br />The projections show that even under scenarios of lower greenhouse gas emissions, sea-level will continue to rise throughout the 21st century.<br />This suggests that even if greenhouse gas emissions were cut completely now, sea-level would continue to rise.<br />This is mainly because of a lag effect caused by the ability of the oceans to store heat, in what is called THERMAL INERTIA.<br />Society is therefore ‘committed’ to a rise in sea-level. <br />However, global sea-level rise will have different impacts on different coastlines, mainly due to isostatic movements.<br />The main cause for concern for the majority of coasts is not the day-to-day increase in tidal range, which can be contained, but the increased ability of extreme events such as storm surges and very high tides to flood coastal lowlands and cause extensive coastal erosion.<br />
Managing global sea-level rise 1<br />With global sea-level rise there is going to be an increasing demand for management solutions to protect thousands of kilometres of coast.<br />Predicted sea-level rise will lead to coastal erosion, redistribution of sediments, wetland submergence, floodplain water-logging, and salt contamination of coastal aquifers.<br />Disruption will be caused to residential, industrial and commercial activities, transport routes will be severed, and agricultural land ruined.<br />Addressing these consequences will require a major redistribution of money at both the national and international levels.<br />Good coastal protection schemes, such as the Thames Barrier, are not viable worldwide, and governments need to accept the loss of coastal land.<br />The early anticipation of sea-level rise is of great value because it allows planning to be introduced gradually.<br />
Managing global sea-level rise 2<br />At present, management suggestions fall into three categories:<br />Reduce the level of greenhouse gas emissions, however, it is too late to prevent sea-level rise into the next century due to the lag effect from the oceans thermal inertia.<br />Counteract sea-level rise by technological and managerial tactics – coastal defenceprogrammes (often expensive!).<br />Redress the water balance of the earth. This management plan proposes to transfer large amounts of water from sea to land and was first suggested by Newman and Fairbridge (1986).<br /><ul><li>In order for such a scheme to succeed and offset any rise in sea level, international cooperation on a massive scale would be required.
A survey of possible ‘on-land’ storage sites, under the heading PROJECT NOAH was made by Newman and Fairbridge (1987).</li></li></ul><li>Managing global sea-level rise 3<br />Continued:<br /><ul><li>They have calculated that reservoir storage may have suppressed sea-level rise by 0.25 mm yr–1 since the 1950s, and that flooding natural basins could offset at least 50% of future sea-level rise before 2050, by transferring 6000 km3 of water from sea to land.
Long-term effects on climate and coastal waters are as yet unknown, and any consequences should be carefully considered beforehand.
One site singled out is the Dead Sea, between Israel and Jordan. A proposal has been made to raise the level of the Dead Sea to -300 m from the current level of -398 m, through the addition of 4x106 m3/yr-1 of water from the Mediterranean.
The transfer of water would generate over 300 GW of electricity per year.
However, while feasible, schemes like this are not likely to reach fruition due to local socio-economic and political conditions.</li></li></ul><li>Summary<br />Sea-level is not constant in either space or time.<br />It appears to have risen throughout the twentieth and early twenty-first centuries and is expected to continue to 2100 and beyond as a consequence of human activity and the enhancement of the Greenhouse Effect.<br />Global sea level is predicted to rise during the 21st century at a greater rate than between 1961 to 2003.<br />Contributors to sea-level rise include; thermal expansion of the oceans, melting of glaciers, ice caps and ice sheets, and loss of terrestrial storage surfaces.<br />The IPCC have made a wide range of predictions based on several different scenarios that assume different levels of future CO2 emissions. Each scenario then has a range of possible outcomes associated with it.<br />Management strategies designed to deal with sea-level rise fall into three categories: (a) reducing global warming, (b) integrated coastal management, or (c) through transferring sea water onto land.<br />
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