Although there are regional differences due, in part, to local subsidence and emergence rates of coastal land, global sea level is rising. We see this rise both in the century-long record from tide gauges around the world, and in
Climate Change: Observed Impacts on Planet Earth
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FIGURE 1 Maps of the north and south polar regions showing ice sheets and place names used in the text. Modified from Ref. .
the shorter record from satellite monitoring of the ocean surface elevation. In recent decades, the rate of global sea-level rise has been more than 3 mm/year. This rise is made up from various contributions: thermal expansion of ocean waters; changes in the mass of water contained in mountain glaciers, reservoirs and ground-water acquifers and changes in the ice-sheets of Antarctica and Greenland.
This rate of sea-level rise may not sound serious but unlike some other climate variables, sea-level tends to change smoothly, and the current rate of rise is likely to continue and most probably grow in the future. The cumulative sea-level rise over coming decades will have surprising and profound and impacts on coastal ecosystems, human populations and the stability of some economies. Climate change is very likely to accelerate most of the individual contributions to sea-level rise, and thus accelerate the rise in global sea-level. It is still not entirely clear that accelerating rates of sea level rise in the late twentieth century indicate that this acceleration has already begun , but there is very little doubt that sea-level rise will accelerate substantially during the twenty-first century.
The Fourth Assessment Review of the Intergovernmental Panel on Climate Change1 (IPCC 4AR)  contains the most authoritative assessment and projection of sea-level rise so far undertaken. The review includes discussion of all the major contributors to sea-level rise, including, the contribution of the Antarctic and Greenland ice sheets. In summary, it predicts that by 2090 2099, sea-level will have risen 20 60 cm compared to 1980 1999. However, there are strong statements within the IPCC report that retain the possibility that the IPCC-4AR projections of sea-level rise maybe incomplete and potentially too conservative, and that the potential contribution from the ice sheets holds substantial uncertainty. These statements are most succinctly summarised in the Summary for Policymakers ,
Models used to date [within the IPCC 4AR review] do not include uncertainties in climate carbon cycle feedback nor do they include the full effects of changes in ice sheet flow, because a basis in published literature is lacking. The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993 to 2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM.3 would increase by 0.1 to 0.2 m. Larger values cannot be excluded, but understanding ofthese effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise.
The IPCC assessment has already been widely criticised by studies that range from the scientific  to those that are almost ideological in approach . To fully understand the difficulties that the IPCC have faced, and the potential for
1 The IPCC is a group of largely government nominated specialists who are tasked with produc ing the most complete assessment of the science, impacts and potential responses to anthropo genic climate change.
resolution of this important question, we must first discuss the workings of the ice sheets, and the recent observations that have led the IPCC to be so cautious.
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