|Working Group I: The Scientific Basis|
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Sea level change is an important consequence of climate change, both for societies and for the environment. In this chapter, we deal with the measurement and physical causes of sea level change, and with predictions for global-average and regional changes over the next century and further into the future. We reach qualitatively similar conclusions to those of Warrick et al. (1996) in the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter SAR). However, improved measurements and advances in modelling have given more detailed information and greater confidence in several areas. The impacts of sea level change on the populations and eco-systems of coastal zones are discussed in the IPCC WGII TAR (IPCC, 2001).
The level of the sea varies as a result of processes operating on a great range of time-scales, from seconds to millions of years. Our concern in this report is with climate-related processes that have an effect on the time-scale of decades to centuries. In order to establish whether there is a significant anthropogenic influence on sea level, the longer-term and non-climate-related processes have to be evaluated as well.
“Mean sea level” at the coast is defined as the height of the sea with respect to a local land benchmark, averaged over a period of time, such as a month or a year, long enough that fluctuations caused by waves and tides are largely removed. Changes in mean sea level as measured by coastal tide gauges are called “relative sea level changes”, because they can come about either by movement of the land on which the tide gauge is situated or by changes in the height of the adjacent sea surface (both considered with respect to the centre of the Earth as a fixed reference). These two terms can have similar rates (several mm/yr) on time-scales greater than decades. To infer sea level changes arising from changes in the ocean, the movement of the land needs to be subtracted from the records of tide gauges and geological indicators of past sea level. Widespread land movements are caused by the isostatic adjustment resulting from the slow viscous response of the mantle to the melting of large ice sheets and the addition of their mass to the oceans since the end of the most recent glacial period (“Ice Age”) (Section 22.214.171.124). Tectonic land movements, both rapid displacements (earthquakes) and slow movements (associated with mantle convection and sediment transport), can also have an important effect on local sea level (Section 11.2.6).
We estimate that global average eustatic sea level change over the last hundred years is within the range 0.10 to 0.20 m (Section 11.3.2). (“Eustatic” change is that which is caused by an alteration to the volume of water in the world ocean.) These values are somewhat higher than the sum of the predictions of the contributions to sea level rise (Section 11.4). The discrepancy reflects the imperfect state of current scientific knowledge. In an attempt to quantify the processes and their associated rates of sea level change, we have critically evaluated the error estimates (Box 11.1). However, the uncertainties remain substantial, although some have narrowed since the SAR on account of improved observations and modelling.
Eustatic sea level change results from changes to the density or to the total mass of water. Both of these relate to climate. Density is reduced by thermal expansion occurring as the ocean warms. Observational estimates of interior temperature changes in the ocean reported by Warrick et al. (1996) were limited, and estimates of thermal expansion were made from simple ocean models. Since the SAR, more observational analyses have been made and estimates from several Atmosphere-Ocean General Circulation Models (AOGCMs) have become available (Section 11.2.1). Thermal expansion is expected to contribute the largest component to sea level rise over the next hundred years (Section 126.96.36.199). Because of the large heat capacity of the ocean, thermal expansion would continue for many centuries after climate had been stabilised (Section 188.8.131.52).
Exchanges with water stored on land will alter the mass of the ocean. (Note that sea level would be unaffected by the melting of sea ice, whose weight is already supported by the ocean.) Groundwater extraction and impounding water in reservoirs result in a direct influence on sea level (Section 11.2.5). Climate change is projected to reduce the amount of water frozen in glaciers and ice caps (Sections 11.2.2, 184.108.40.206) because of increased melting and evaporation. Greater melting and evaporation on the Greenland and Antarctic ice sheets (Sections 11.2.3, 220.127.116.11) is also projected, but might be outweighed by increased precipitation. Increased discharge of ice from the ice sheets into the ocean is also possible. The ice sheets react to climate change by adjusting their shape and size on time-scales of up to millennia, so they could still be gaining or losing mass as a result of climate variations over a history extending as far back as the last glacial period, and they would continue to change for thousands of years after climate had been stabilised (Section 18.104.22.168).
Sea level change is not expected to be geographically uniform (Section 11.5.2), so information about its distribution is needed to inform assessments of the impacts on coastal regions. Since the SAR, such information has been calculated from several AOGCMs. The pattern depends on ocean surface fluxes, interior conditions and circulation. The most serious impacts are caused not only by changes in mean sea level but by changes to extreme sea levels (Section 22.214.171.124), especially storm surges and exceptionally high waves, which are forced by meteorological conditions. Climate-related changes in these therefore also have to be considered.
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