IPCC Second Scientific Assessment: Chapter 7 Summary

Changes in Sea-Level

Summary

The purpose of this chapter is to assess the current state of knowledge regarding climate and sea level change, with special emphasis on scientific developments since IPCC (1990). The main focus is on changes that occur on the time-scale of a century. We thus look for evidence of sea level change during the last 100 years, examine the factors that could be responsible for such changes, and consider the possible changes in sea level during the next 100 years as a result of global warming.

With respect to the past, recent analyses suggest that:

• global mean sea level has risen 10-25 cm over the last 100 years. This range is slightly higher than that reported in IPCC (1990) (i.e., 10-20 cm). The higher estimate results largely from the use of geodynamic models for filtering out long-term vertical land movements, as well as from the greater reliance on the longest tide gauge records for estimating trends.

• there has been no detectable acceleration of sea level rise during this century. However, the average rise during the present century is significantly higher than the rate averaged over the last several thousand years, although century-time-scale variations of several decimetres almost certainly occurred within that longer period. The exact timing of the onset of the present, higher rate of sea level rise remains uncertain.

It is likely that the rise in sea level has been due largely to the concurrent increase in global temperature over the last 100 years. The possible climate-related factors contributing to this rise include thermal expansion of the ocean and melting of glaciers, ice caps and ice sheets. Changes in surface water and ground water storage may also have affected sea level. The assessment of the scientific evidence suggests that:

• global warming should, on average, cause the oceans to warm and expand, thus increasing sea level. The various models, from simple upwelling diffusion models to complex coupled atmosphere-ocean GCMs, all agree that oceanic thermal expansion is one consequence of global warming. The thermal expansion over the last 100 years is estimated to be 2-7 cm. Large-scale observations of changes in sub-surface ocean temperatures are beginning to support these estimates.

• global warming should, on average, increase the melt rates of glaciers and ice caps, causing sea level to rise. Observational data indicates that, globally, there has been a general retreat of glaciers during this century. Based on both observations and models, recent analyses suggest that this enhanced melting may have increased sea level by about 2-5 cm over the last 100 years.

• with respect to the Greenland ice sheet, a warmer climate should increase the melt rates at the margins. The increase in melting should dominate over any increase in accumulation rates in the interior, causing sea level to rise. However, observational evidence is insufficient to say with any certainty whether the ice sheet is currently in balance or has increased or decreased in volume over the last 100 years.

• with respect to the Antarctic ice sheet, a warmer climate should increase the accumulation rates, causing sea level to fall. Here, too, the observational evidence is insufficient to say with any certainty whether the ice sheet is currently in balance or has increased or decreased in volume over the last 100 years.

• it is unclear how changes in surface water or ground water storage have affected sea level. Estimates vary widely of the net effects of activities (largely anthropogenic) such as dam construction and reservoir filling, which lower sea level, and ground water pumping, deforestation and wetland loss, which tend to raise sea level. However, the potential future effect on sea level from such sources is probably relatively small, of the order of a few centimetres during the next century.

An exact accounting of the past sea level rise is difficult, particularly in the light of the large uncertainties associated with the mass balances of the ice sheets. However, the observed rise lies well within the combined ranges of uncertainty of the above factors.

Projections of future changes in sea level as a consequence of greenhouse-gas-induced warming were made for each of the six IPCC IS92 emission scenarios, with and without the effect of aerosol changes after 1990, for the period 1990 to 2100. In addition, high, middle and low estimates, using a range of parameter values based on key model uncertainties, were made for IS92a (the emission scenario most comparable to the IPCC (1990) Scenario A, the so-called "Business-as- usual" scenario). The results showed that:

• for Scenario IS92a, sea level is projected to be about 50 cm higher than today by the year 2100, with a range of uncertainty of 20-86 cm;

• for the range of emission scenarios IS92a-f using "best-estimate" model parameters, sea level is projected to be 38-55 cm higher than today by the year 2100;

• the extreme range of projections, taking into account both emission scenarios and model uncertainties, is 13-94 cm;

• most of the projected rise in sea level is due to thermal expansion, followed by increased melting of glaciers and ice caps. On this time-scale, the contributions made by the major ice sheets are relatively minor, but are a major source of uncertainty.

It is evident that the choice of emission scenario makes relatively little difference to the projected rise in sea level, especially for the first half of the next century. This is because much of the rise has already been determined by past changes in radiative forcing, due to lags in the response of the oceans and ice masses. For this same reason, in model simulations sea level continues to rise over many centuries even after concentrations of greenhouse gases are stabilised. In contrast, the scientific uncertainties - as reflected partly in intra-model uncertainties in the choice of individual model parameter values, and partly in inter-model uncertainties in the choice of methods for climate, glacier and ice sheet modelling - make a very large difference in the estimate of future sea level rise.

A major source of uncertainty concerns the polar ice sheets. Not only is there a lack of understanding of the current mass balance, but there is also considerable uncertainty regarding the possible dynamic responses on time-scales of centuries. Concern has been expressed that the West Antarctic Ice Sheet might "surge", causing a rapid rise in sea level. The current lack of knowledge regarding the specific circumstances under which this might occur, either in total or in part, limits the ability to quantify the risk. Nonetheless, the likelihood of a major sea level rise by the year 2100 due to the collapse of the West Antarctic Ice Sheet is considered low.

The changes in future sea level will not occur uniformly around the globe. Recent coupled atmosphere-ocean model experiments suggest that the regional responses could differ significantly, due to regional differences in heating and circulation changes. In addition, geological and geophysical processes cause vertical land movements and thus affect relative sea levels on local and regional scales. Finally, extreme sea level events - tides, waves and storm surges - could be affected by regional climate changes but are, at present, difficult to predict.

Overall, the basic understanding of climate-sea level relationships has not changed fundamentally since IPCC (1990). The estimates of global sea level rise presented here are lower than those presented in IPCC (1990), due primarily to significantly lower estimates of global temperature change which drive the projections of sea level rise. Thus, if global warming were to occur more rapidly than expected, the rate of sea level rise would consequently be higher.