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Description: This chapter assesses the current state of knowledge of the rate of change of global-averaged and regional sea-level in relation to climate change. We focus on the 20th and 21st centuries.However, because of the slow response to past conditions of the oceans and ice sheets and the consequent land movements, we consider changes in sea level prior to the historical record, andwe also look over a thousand years into the future.Past changes in sea levelFrom recent analyses, our conclusions are as follows:since the Last Glacial Maximum about 20 000 years ago, sea level has risen by over 120 m at locations far from present and former ice sheets, as a result of loss of mass from these ice sheets. There was a rapid rise between 15 000 and 6000 years ago at an average rate of 10 mm/yr.based on geological data, global average sea level may have risen at an average rate of 0.5 mm/yr over the last 6000 years and at an average rate of 0.1 to 0.2 mm/yr over the last 3000 years.vertical land movements are still occurring today as a result of these large transfers of mass from the ice sheets to the ocean.during the last 6000 years, global average sea-level variations on the time scales of a few hundred years and longer are likely to have been less than 0.3 to 0.5 m.based on tide gauge data, the rate of global average sea-level rise during the 20th century is in the range 1.0 to 2.0 mm/yr, with a central value of 1.5 mm/yr (as with other ranges of uncertainty, it is not implied that the central value is the best estimate).based on the few very long tide-gauge records, the average rate of sea-level rise has been larger during the 20th century than the 19th century.no significant acceleration in the rate of sea-level rise during the 20th century has been detected.there is decadal variability in extreme sea levels but no evidence of widespread increases in extremes other than that associated with a change in the mean.Factors affecting present day sea level changeGlobal average sea level is affected by many factors. Our assessment of the most important is as follows.Ocean thermal expansion leads to an increase in ocean volume at constant mass. Observational estimates of about 1 mm/yr over recent decades are similar to values of 0.7 to 1.1 mm/yr obtained from Atmosphere-Ocean General Circulation Models (AOGCMs) over a comparable period. Averaged over the 20th century, AOGCM simulations result in rates of thermal expansion of 0.3 to 0.7 mm/yr.The mass of the ocean, and thus sea level, changes as water is exchanged with glaciers and ice caps. Observational and modelling studies of glaciers and ice-caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between Ð0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long term adjustment to past climate changes.Changes in terrestrial storage of water over the period 1910 to 1990 are estimated to have contributed from Ð1.1 to +0.4 mm/yr of sea-level rise.The sum of these components indicates a rate of eustatic sea-level rise (corresponding to a change in ocean volume) from 1910 to 1990 ranging from Ð0.8 mm/yr to 2.2 mm/yr, with a central value of 0.7 mm/yr. The upper bound is close to the observational upper bound (2.0 mm/yr), but the central value bound is less than the observational lower bound (1.0 mm/yr), i.e. the sum of components is biased low compared to the observational estimates. The sum of components indicates an acceleration of only 0.2 mm/yr/century, with a range from Ð1.1 to +0.7 mm/yr/century, consistent with observational finding of no acceleration in sea-level rise during the 20th century. The estimated rate of sea-level rise from anthropogenic climate change from 1910 to 1990 (from modelling studies of thermal expansion, glaciers and ice-sheets) ranges from 0.3 to 0.8 mm/yr. It is very likely that 20th century warming has contributed significantly to the observed sea level rise, through thermal expansion of sea water and widespread loss of land ice.Projected sea-level changes from 1990 to 2100Projections of components contributing to sea-level change from 1990 to 2100 (this period is chosen for consistency with the IPCC Second Assessment Report), using a range of AOGCMs following the IS92a scenario (including the direct effect of sulphate aerosol emissions) give:thermal expansion of 0.11 to 0.43 m, accelerating through the 21st century.a glacier contribution of 0.01 to 0.23 m.a Greenland contribution of -0.02 to 0.09 m.an Antarctic contribution of -0.17 to 0.02 m.Including thawing of permafrost, deposition of sediment, and the ongoing contributions from ice sheets as a result of climate change since the Last Glacial Maximum, we obtain a range of global-average sea-level rise from 0.11 to 0.77 m. This range reflects systematic uncertainties in modelling.For the 35 SRES scenarios, we project a sea-level rise of 0.09 to 0.88 m for 1990 to 2100, with a central value of 0.48 m. The central value gives an average rate of 2.2 to 4.4 times the rate over the 20th century. If terrestrial storage continued at its present rates, the projections could be changed by -0.21 to 0.11 m. For an average AOGCM, the SRES scenarios give results which differ by 0.02 m or less for the first half of the 21st century. By 2100, they vary over a range amounting to about 50% of the central value. Beyond the 21st century, sea level rise will depend strongly on the emission scenario.The West Antarctic Ice Sheet (WAIS) has attracted special attention because it contains enough ice to raise sea level by 6 m and because of suggestions that instabilities associated with its being grounded below sea level may result in rapid ice discharge when the surrounding ice shelves are weakened. The range of projections given above makes no allowance for ice-dynamic instability of the WAIS. It is now widely agreed that major loss of grounded ice and accelerated sea-level rise are very unlikely during the 21st century.Our confidence in the regional distribution of sea level change from AOGCMs is low because there is little similarity between models. However, models agree on the qualitative conclusion that the range of regional variation is substantial compared with the global average sea-level rise. Nearly all models project greater than average rise in the Arctic Ocean and less than average rise in the Southern Ocean.Land movements, both isostatic and tectonic, will continue through the 21st century at rates which are unaffected by climate change. It can be expected that by 2100 many regions currently experiencing relative sea-level fall will instead have a rising relative sea level.Extreme high water levels will occur with increasing frequency (i.e. with reducing return period) as a result of mean sea-level rise. Their frequency may be further increased if storms become more frequent or severe as a result of climate change.Longer term changesIf greenhouse gas concentrations were stabilised, sea level would nonetheless continue to rise for hundreds of years. After 500 years, sea-level rise from thermal expansion may have reached only half of its eventual level, which models suggest may lie within ranges of 0.5 to 2.0 m and 1 to 4 m for CO2 levels twice and four times pre-industrial, respectively.Glacier retreat will continue and the loss of a substantial fraction of the total glacier mass is likely. Areas that are currently marginally glaciated are most likely to become ice-free.Ice sheets will continue to react to climate change during the next several thousand years even if the climate is stabilised. Models project that a local annual-average warming of larger than 3°C sustained for millennia would lead to virtually a complete melting of the Greenland ice sheet. For a warming over Greenland of 5.5°C, consistent with mid-range stabilisation scenarios, theGreenland ice sheet contributes about 3 m in 1000 years. For a warming of 8°C, the contribution is about 6 m, the ice sheet being largely eliminated. For smaller warmings, the decay of the ice sheet would be substantially slower.Current ice dynamic models project that the WAIS will contribute no more than 3 mm/yr to sea-level rise over the next thousand years, even if significant changes were to occur in the ice shelves. However, we note that its dynamics are still inadequately understood to make firm projections, especially on the longer time scales.Apart from the possibility of an internal ice dynamic instability, surface melting will affect the long-term viability of the Antarctic ice sheet. For warmings of more than 10°C, simple runoff models predict that an ablation zone would develop on the ice sheet surface. Irreversible disintegration of the WAIS would result because the WAIS cannot retreat to higher ground once its margins are subjected to surface melting and begin to recede. Such a disintegration would take at least a few millennia. Thresholds for total disintegration of the East Antarctic ice sheet by surface melting involve warmings above 20*C, a situation that has not occurred for at least 15 million years and which is far more than predicted by any scenario of climate change currently under consideration.

Description: The HadCM3 AOGCM has been coupled with a dynamic 3D model of the Green-landice sheet that includes a visco-elastic solid Earth model. The AOGCM exchangesinformation with the sheet model once a year. Precipitation and temperature anomalies are passed to the ice sheet model, which calculates ablation (using a degree-day scheme), ice dynamics and basal rebound. The ice sheet model passes back to the GCM an updated orography and freshwater fluxes. Iceberg calving fluxes are applied evenly to the sea region adjacent to Greenland whilst runoff enters the ocean at coastal points. When a GCM grid cell changes from ice-covered to ice-free or vice-versa, the surface characteristics are modified appropriately. A multiple-century experiment is being undertaken, starting from the present-day ice sheet, with four times the pre-industrial atmospheric CO2 concentration, to determine the rate of ice ablation, the effect on oceanic circulation and local climate, and the feedback of orographic and climate change on the ice sheet mass balance. Over the first 200 years, the contribution to global average sea level rise as a result of loss of mass from the ice sheet is about 5 mm/yr.

Description: Accurate simulation of ice-sheet surface mass balance requires higher spatial resolution than is afforded by typical atmosphereocean general circulation models (AOGCMs),owing, in particular, to the need to resolve the narrowand steep margins where the majority of precipitation and ablation occurs. We have developed a method for calculating massbalance changes by combining ice-sheet average time-series from AOGCM projections for future centuries, both with information from high-resolution climate models run for short periods and with a 20 km ice-sheet mass-balance model. Antarctica contributes negativelyto sea level on account of increased accumulation, while Greenland contributes positively because ablation increases more rapidly. The uncertainty in the results is about 20% for Antarctica and 35% for Greenland. Changes in ice-sheet topography and dynamics are not included, but we discuss their possible effects. For an annual- and area-average warming exceeding 4:5G0:9 K in Greenland and 3:1G0:8 K in the global average, the net surface mass balance of the Greenland ice sheet becomes negative, in which case it is likely that the ice sheet would eventually be eliminated, raising global-average sea level by 7 m.

Description: We demonstrate that a hemispherically averaged upwelling-diffusion energy-balance climate model (UD/EBM) can emulate the surface air temperature change and sea-level rise due to thermal expansion, predicted by the HadCM2 coupled atmosphere-ocean general circulation model, for various scenarios of anthropogenic radiative forcing over 1860-2100. A climate sensitivity of 2.6 oC is assumed, and a representation of the effect of sea-ice retreat on surface air temperature is required. In an extended experiment, with CO2 concentration held constant at twice the control run value, the HadCM2 effective climate sensitivity is found to increase from about 2.0 oC at the beginning of the integration to 3.85 oC after 900 years. The sea-level rise by this time is almost 1.0 m and the rate of rise fairly steady, implying that the final equilibrium value (the commitment) is large. The base UD/EBM can fit the 900-year simulation of surface temperature change and thermal expansion provided that the time-dependent climate sensitivity is specified, but the vertical profile of warming in the ocean is not well reproduced. The main discrepancy is the relatively large mid-depth warming in the HadCM2 ocean, that can be emulated by (1) diagnosing depth-dependent diffusivities that increase through time; (2) diagnosing depth-dependent diffusivities for a pure-diffusion (zero upwelling) model; or (3) diagnosing higher depth-dependent diffusivities that are applied to temperature perturbations only. The latter two models can be run to equilibrium, and with a climate sensitivity of 3.85 oC, they give sea-level rise commitments of 1.7 m and 1.3 m, respectively.

Description: A probability distribution for values of the effective climate sensitivity, with a lower bound of 1.6 K (5-percentile), is obtained on the basis of the increase in ocean heat content in recent decades from analysesof observed interior ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthopogenic and natural radiative forcing of the climate system. Radiative forcing is the greatest source of uncertainty in the calculation; the result also depends somewhat on the rate of ocean heat uptake in the late 19th century, for which an assumption is needed as there is no observational estimate. Because the method does not use the climate sensitivity simulated by a general circulationmodel, it provides an independent observationally based constraint on this important parameter of the climate system.

Description: Warmer climate conditions persisting for a period of many centuries could lead tothe disappearance of the Greenland ice-sheet, with a related 7 m rise in sea-level. We address the question of whether the ice-sheet could be regenerated if pre-industrial climate conditions were re-established after its melting. We use the HadCM3 coupled atmosphere-ocean GCM to simulate the global and regional climate with preindustrial atmospheric greenhouse-gas composition and the Greenland ice-sheet removed. Two separate cases are considered. In one, the surface topography of Greenland is given by that of the bedrock currently buried under the ice-sheet. In the other, a readjustmentto isostatic equilibrium of the unloaded orography is taken into account, giving higher elevations. In both cases, there is greater summer melting than in the current climate, leading to partially snow-free summers with much higher temperatures. On the long-term average, there is no accumulation of snow. The implication of this result is that the removal of the Greenland ice-sheet due to a prolonged climatic warming would not be reversible.

Description: Sea level rise is an important aspect of future climate change because, without upgraded coastal defences, it is likely to lead to significant impacts. Here we report on two aspects of sea-level rise that have implications for the avoidance of dangerous climate change and stabilisation of climate. If the Greenland ice sheet were to melt it would raise global sea levels by around 7m. We discuss the likelihood of such an event occurring in the coming centuries. We also examine the time scales associated with sea-level rise and demonstrate that long after atmospheric greenhouse gas concentrations or global temperature have been stabilised coastal impacts may still be increasing.

Description: The HadCM3 AOGCM has been coupled to a 3D dynamic model of the Greenland ice sheet, which includes a visco-elastic solid Earth model. Once every year the AOGCM provides the ice sheet model with precipitation and temperature anomalies which it uses in order to calculate ablation, ice dynamics and basal rebound. A new orography and fresh water fluxes are passed back to the OAGCM to be utilised over the subsequent year. The water from the melting of calved Icebergs is applied evenly to the sea region adjacent to Greenland whilst runoff enters the ocean through 'river' outlets. A multiple century experiment starting from the present day ice sheet with an atmospheric CO2 concentration of four times pre-industrial levels is being undertaken to determine the rate of ice ablation and the impact of ice sheet changes on simulated sea level, and oceanic and atmospheric circulation. The effect of orographic changes in the ice sheet on its own mass balance is also of interest. The results from the first 180 years of the simulation indicate that the modelled surface air temperature over Greenland in the 4xCO2 climate is around 8 degrees warmer than in the pre-industrial control, compared with a global mean difference of 5 degrees. Precipitation is increased by 33% in the 4xCO2 experiment but the rate of ablation rises by 640%, causing a direct sea-level rise of 5mm per year. To understand the mechanisms of change we will examine the spatial patterns of temperature and precipitation anomalies for the model control and 4xCO2 experiments and compare them with data from anomalously warm years determined from in situ (ice core) data.