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: Global change is defined here for this review as changes caused by increasing greenhouse gas emissions resulting in a high CO2 world and the direct and indirect changes that ensue. Increasing green house gas emissions are causing three major impacts on the ocean: warming sea surface temperature (SST), ocean acidification and deoxygenation (Turley et al 2011). The latter two are poorly understood at present, particularly in Eastern Africa. Secondary impacts include sea level rise due to melting polar ice caps. Coral bleaching and death caused by SST rise has been extensively studied and measured (HoeghGuldberg 1999; Hughes et al. 2003), including in the WIO (Obura 2005, McClanahan 2009), with likely ecosystem phase shifts as coral reefs become dominated by macroalgae (brown algae such as Turbinaria, Sargassum spp.) (Bellwood et al. 2004; Hughes et al. 2005). Ocean acidification is likely to have enormous impacts on marine resources and hence fisheries (Turley et al. 2011), as ocean chemistry is changed and thus any marine organisms that rely on Ph sensitive chemical reactions will be affected. This field is still relatively new and early reports predict trophic level shifts as organisms with calcium carbonate skeletons, e.g molluscs, corals, are compromised. Deoxygenation is caused by ocean warming (oxygen becomes less soluble) which will result in less growth of most marine organisms and a shift to low oxygen tolerant organisms, often microorganisms. Coastal environments and people are also undergoing changes that are directly related to human pressures caused by development and other activities. These include increasing population, mechanisation (eg in fisheries), industrialisation (eg ports and coastal cities), pollution, and extraction of oil and gas.

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Description: The project focuses on safeguarding the biodiversity, natural resources and ecological processes in Kiunga Marine and Dodori national Reserves through the development and implementation of a consensus-based management plan, involving full participation of local communities.

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Description: Coral reefs in the Kiunga Marine National Reserve (KMNR) (40o 07’ E, 2o 00’ S) are located in a transition ecotone between the warmer East African coral reef bioregion to the south, and colder waters of the Somali Current to the north. The reefs have been monitored annually from 1998 to the present, documenting a range of ecosystem changes from large and small scale threats. Reefs in the area suffered ˜60% loss of coral cover due to mass bleaching in the 1998 El Niño event, and 25-40% loss of coral species at individual site levels. Recovery of coral community structure has been variable, with some reefs showing strong recovery, while others have declined further. A harmful algal bloom and coral disease in early 2002 further impacted these reefs, causing mass mortalities of fish and coral, and failure of coral recruitment in that year. Fishing impacts to the reserve are high, with a strong south-north decline in fish density due to easier access to the migrant and large fishing communities to the south of the reserve. Responsibility for management of the KMNR falls under multiple institutions, including the Kenya Wildlife Service, Fisheries and Forestry Departments, and the local council. Overlapping mandates, unclear relationships, limited information and understanding, and lack of resources have hampered effective management. The monitoring programme reported here is one aspect of new collaborative appro aches to coral reef and fisheries management, and has focused on improving the information and understanding of the biological and resource systems of the area. The ecosystem trends induced by larger scale threats and the south-north fish resource gradient caused by local use patterns will be analyzed in an attempt to develop sustainable management practices for the reserve.

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Description: Observations of sea level from coastal tide gauges indicates that global sea level has been rising at a rate of between 1 and 2 mm/yr during the 20th century. This is substantially faster than estimates of global sea-level rise of 0.5 mm/yr over the last 6000 years and 0.1-0.2 mm/yr over the last 3000 years. There is no discernible acceleration in the rate of sea-level rise during the 20th century.For the 21st century, models indicate that ocean thermal expansion will be the largest contributor to sea-level rise. The melting of glaciers and ice -caps also contributes to sea-level rise. For Greenland, increased ablation is estimated to be somewhat larger than increased precipitation. For Antarctica, increased accumulation is estimated topartially offset a rise in sea level from other sources. There will be a continuingcontribution to sea-level rise as a result of the response of the ice sheets to changes in climate since the Last Glacial Maximum. Changes in terrestrial storage is estimated to have partially offset sea-level rise from other sources. Combining estimates of each of these components results in a calculated rate of sea-level rise during the 20 thcentury which ranges from about zero to about 2 mm/yr, and with little computed acceleration over the period.Sea-level rise projections for 2100 are several tens of centimetres above 1990 levels. For the early decades of the 21st century, the range of projections is dominated by modelling uncertainties. For longer lead times, uncertainties in greenhouse gas emissions also contribute significantly to the range in sea -level projections. Beyond 2100, sea level will continue to rise for centuries after greenhouse gas concentrations have stabilised. After 500 years, sea-level rise from thermal expansion may onlyhave reached half of its eventual level. Ice sheets will continue to react to climate change during the next thousand years even if the warming is stabilised.

Description: Several existing statistical and dynamical reconstructions of past regional sea level variability and trends are compared with each other and with tide gauges over the 48-yr period 1960–2007, partially pre-dating the satellite altimetry era. Evaluated statistical reconstructions were built from tide gauge data (TGR), and dynamical reconstructions from ocean data assimilation (ODA) approaches. Although most of the TGRs yield global-mean time series of sea level with trends deviating within only ±0.1 mm yr −1 , the spatial anomalies of the trends deviate substantially between the reconstructions over the period predating altimetry. In contrast, TGRs match observed regional trend patterns fairly well during the satellite altimetry era. TGRs match tide gauge data better than ODA results; however, they exhibit less variability in the open ocean compared to altimetric data. Over the pre-altimetry period, all reconstructed regional sea level trend patterns deviate substantially from each other. In terms of detrended correlations in this earlier period, the reconstructions match tide gauges, and each other, much better in the Pacific than in the Atlantic. An ensemble of all TGR and ODA estimates provides some improvements in correlations and trends to both tide gauges and altimetry. Nevertheless, a lack of independent open-ocean sea surface height data predating altimetry makes impossible the validation of the ensemble for pre-altimetry open ocean sea level trends and variability. Estimating regional sea level changes prior to altimetry therefore remains an unsolved challenge.

Description: We note the presence in the literature of two different concepts of the term ‘active layer’ in relation to fluvial sediment transport. It has been used to represent the current dynamically active streambed surface, or to represent the depth of event-scale scour and fill. These concepts involve distinct length and time scales. We propose that, when the distinction is important, the concepts be distinguished as either a ‘dynamical active layer' or an ‘event active layer'. This article is protected by copyright. All rights reserved.