Sea levels are rising

Sea levels are rising

Sea level change can be difficult to measure. Sea level changes over the last century have been derived mainly from tide-gauge data, where the sea level is measured relative to a land-based tide-gauge benchmark. Local and regional sea level is subject to natural variation due to tides, waves, storm surges, and seasonal temperature effects. Such influences can generally be readily characterised and accounted for to reveal over-riding trends in long-term records. Observed trends, however, can be complicated by the fact that the land can experience vertical movements (e.g. from ISOSTATIC effects related to post-glacial rebound, NEOTECTONISM causing subsidence or uplift, and sedimentation or erosion). Recent improved methods of filtering out these effects, as well as a greater reliance on the longest tide-gauge records for estimating trends, have provided greater confidence that the volume of ocean water has indeed been increasing, causing the sea level to rise.

The two main components contributing to sea level rise are:

Reliable tide-gauge records indicate that sea level rose at a rate of about 1.7 millimetres per year during the 20th century. Recent satellite altimetry data, in agreement with recent tide gauge measurements, show that this rate is increasing, and sea level has risen at about 3 millimetres per year since 1993.

Reconstructed, observed and projected sea level rise

Reconstructed, observed and projected sea level rise

Time series of global mean sea level (deviation from the 1980–1999 mean) in the past and as projected for the future. For the period before 1870, global measurements of sea level are not available. The grey shading shows the uncertainty in the estimated long-term rate of sea level change. The red line is a reconstruction of global mean sea level from tide-gauges, and the red shading denotes the range of variations from a smooth curve. The green line shows global mean sea level observed from satellite altimetry. The blue shading represents the range of model projections for the SRES A1B scenario for the 21st century, relative to the 1980 to 1999 mean, and has been calculated independently from the observations. Beyond 2100, the projections are increasingly dependent on the emissions scenario. Over many centuries or millenia, sea level could rise by several metres.

Source: Intergovenmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 5 observations—oceanic climate change and sea level, FAQ 5.1, Figure 1, p. 409.

Much of the observed rise in sea level is directly related to the concurrent rise in global temperature over the last 100 years. Water expands as it gets warmer. As shown in the table below, ocean warming and the consequent thermal expansion of the oceans has been estimated to account for about 25 per cent of the observed sea level rise from 1961 to 2003, with the contribution increasing to more than 50 per cent from 1993 to 2003. Melt water related to the retreat of glaciers and ice sheet melting is the other major contributor. The ice sheets remain a major source of uncertainty in accounting for past changes in sea level because of insufficient data over the last 100 years. There is also significant uncertainty about the possible contribution of ANTHROPOGENIC changes in land water storage to changes in sea level, including groundwater extraction (and eventual discharge to the ocean), destruction of wetlands and other land-use changes (again adding to ocean storage), increased evaporation from surface water diversions for irrigation and industry, and storage in dams (reducing the amount of water flowing to the ocean). The IPCC Fourth Assessment Report suggests that these contributions are either small (contributing less than 0.5 millimetres per year to sea level rise) or are compensated by other factors not sufficiently understood. However, these uncertainties may explain some of the discrepancy between the sum of the estimated contributions in the table below, and the observed contributions. This difference is likely to be significant for the period 1961–2003, with better closure of the budget for the period 1993–2003.

Estimates of contributions to sea level rise in recent decades

Source
Sea level rise (mm per year)
1961–2003
1993–2003
Thermal expansion
0.42
1.6
Glaciers and ice caps (including polar)
0.50
0.77
Greenland ice sheet
0.05
0.21
Antarctic ice sheet
0.14
0.21
Sum
1.1
2.8
Observed
1.8
3.1

Source: Intergovenmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 5 Observations—oceanic climate change and sea level, Table 5.3, p. 419.

It is interesting to note that the observed sea level rise since 1990 has been tracking the upper limit of model projections, as shown in the figure below from CSIRO. Recent global ANTHROPOGENIC emissions have also been maintained at the higher end of the emissions scenarios, but there is a lag between ocean thermal expansion, surface warming and atmospheric greenhouse gas concentrations that precludes a concurrent correspondence between rates of sea level rise and emissions.

Sea level observations versus projections

Sea level observations versus projections

Recent observations show the observed sea levels from tide gauges (blue) and satellites (red) are tracking near the upper bound (black line) of the IPCC 2001 projections (grey shading and black lines) since the start of the projections in 1990. This upper limit leads to a global-averaged sea level rise by 2100 of 88 centimetres compared to 1990 values. These observations do not necessarily indicate that sea level will continue to track this upper limit—it may diverge above or below this upper limit. However, the ice sheet uncertainties are essentially one-sided—i.e. they could lead to a significantly larger sea level rise than current projections but are unlikely to lead to a significantly smaller rise. Note also that greenhouse gas emissions are now tracking just above the highest of the SRES emission scenarios used in calculating these projections.

Source: CSIRO, Sea level rise—Understanding the past, improving projections for the future, Observations vs projections.

Projections for the 21st century

At this stage the pattern for the longer-term regional distribution of projected sea level rise is uncertain, but most models indicate maximum sea level rise in the Arctic Ocean and a minimum sea level rise in the Southern Ocean south of the Antarctic Circumpolar Current. In addition, transfer of mass from the ice sheets to the oceans results in changes in the gravitational field and vertical land movements and thus affect the height of the ocean relative to the land. These large-scale changes, plus local TECTONIC movements, affect the regional impact of sea level rise. Withdrawal of groundwater and drainage of susceptible soils can cause significant subsidence. Subsidence of several metres during the 20th century has been observed for a number of coastal megacities. Reduced sediment inputs to deltas are an additional factor which causes lowering of land elevation relative to sea level.

Ice melt and the destabilisation of large polar ice sheets have the potential to result in a large rise in sea level. It has been suggested that there is a temperature threshold beyond which the Greenland ice sheet will become unviable. There is disagreement over where this threshold lies, with estimates ranging from as low as 1°C to 1.9–4.6°C rise in global average surface temperature. Model projections indicate that this threshold is likely to be reached under mid-range emissions scenarios. Loss of the Greenland ice sheet would contribute to a sea level rise of about 7 metres, which would occur over a long period of more than 1000 years if ice melting were the primary cause. However, if the dynamical loss of ice due to increased flow rates of outlet glaciers becomes a significant contributing factor, overall loss of Greenland ice could occur at a faster rate.

A rapid collapse of the West Antarctic Ice Sheet would lead to an additional sea level rise of several metres, but this is considered very unlikely during the 21st century. In fact, accumulation of snow due to enhanced precipitation on the Antarctic ice sheets is projected to exceed losses due to warming, thereby making a negative contribution to sea level over the next century (i.e. causing a net transfer of water from the oceans to the Antarctic continent in the form of snow and ice). See Glaciers, sea ice and ice sheets for more information.

Sea level rise projections for the 21st century and beyond are developed by simulating how the climate responds to increasing atmospheric concentrations of greenhouse gases, and the influences and feedbacks of the changes in climate with ocean temperature and land-based ice stores. Melting of sea ice and ice shelves over the oceans does not contribute to sea level rise because the ice is already in the ocean; however, reduction in ice shelf mass can affect the flow of adjacent ice over land. The rate and magnitude of sea level change is likely to vary from region to region around the globe. However to date, there is little agreement as to the pattern of sea level rise. The IPCC Fourth Assessment Report projects a sea level rise of between about 20 and 60 centimetres between 1990 and 2100 concurrent with a global average surface temperature rise of between 1.1 and 6.4°C (representing the extremes of the 'likely range' under the various emissions scenarios). This projection excludes any contributions from dynamical processes associated with the large polar ice sheets. The possibility of larger sea level rises cannot be excluded, as it is unclear whether dynamical contributions from the Greenland and Antarctic ice sheets may be enhanced at higher temperatures. The figure below shows the relative contributions to sea level rise from each of the major inputs under one medium-high emissions modelling scenario.

Components of sea level rise, 1860–2100

Components of sea level rise, 1860–2100

Sea level will change due to expansion of oceans as they warm, and due to the influx of water from melting of glaciers and other snow and ice, and changes in the two large ice sheets in Antarctica and Greenland. Above is a plot of changes in sea level predicted by the Hadley Centre model from the years 1860 to 2100, due to each of these contributors, under a Medium–High Emissions scenario. The dotted blue line shows the expected change of sea level due to changes in the Greenland ice sheet. The green line shows predicted changes due to melting of glaciers and snow on land. The red line shows the major component of sea level rise which is thermal expansion of ocean waters. Adding these components together gives a predicted sea level rise from the middle of the last century to 2100 of about 0.4 metres, about 0.1 metres of which should have already occurred (rather less than has actually been observed), leaving a rise of about 0.3 metres over the
next 100 years.

Source: Met Office Hadley Centre, Climate change and the greenhouse effect—a briefing from the Hadley Centre, December 2005, p. 48. Image © British Crown Copyright 2005, the Met Office.

 

Thermal expansion is a slow process, so that sea level rise from ocean thermal expansion may only be half of its eventual level after 500 years. Even if greenhouse gas concentrations in the atmosphere were stabilised, sea level rise would continue for hundreds of years, because of the time lags involved, as illustrated in the following graph from the Hadley Centre, UK Met Office. This means that under all emissions scenarios and regardless of mitigation strategies, global sea level will continue to rise and thus require adaptation over the next several centuries to manage its effects.

Long-term sea level rise after rapid doubling then stabilisation of CO2 concentrations

Long-term sea level rise after rapid doubling then stabilisation of CO2 concentrations

Greenhouse effect heating in the atmosphere is rapidly transferred into surface ocean waters. It then slowly penetrates deeper and causes more and more of the ocean depth to expand and, hence, leads to further sea level rise. This figure shows the sea level rise due to ocean thermal expansion, estimated from a climate model experiment where CO2 concentration in the atmosphere was hypothetically increased by 1 per cent per year from time zero to 70 years (that is, until it had doubled) and was then stabilised at that concentration, that is, no further increase occurred. The initial blue line shows thermal expansion while the CO2 concentration was rising, the continuing red line shows sea level rise after CO2 concentration had been stabilised. Despite the fact that CO2 in the atmosphere did not change after year 70, the sea level carries on rising for many hundreds of years, with only a slow decrease in the rate of rise.

Source: Met Office Hadley Centre, Climate change and the greenhouse effect—a briefing from the Hadley Centre, December 2005, p. 51. Image © British Crown Copyright 2005, the Met Office.

Further reading:

CSIRO, Sea level rise—understanding the past, improving projections for the future.

J.A. Church, N.J. White, J.R. Hunter and K. Lambeck, Post-IPCC AR4 update on sea level rise, Antarctic Climate and Ecosystems Cooperative Research Centre, 2008.




 

1 September, 2009

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