Sealevel RiSE

An almost certain consequence of global warming on the ocean is that sea level will rise, if only because as water warms it expands. In the oceans, the only way that an increased volume of the ocean can be accommodated is by an increase in sea level, and sea level has indeed risen over the past several decades, as illustrated in figure 7.6. Sea level is estimated to have risen about 20 cm since records began in the late nineteenth century, and it rose at about 2 mm per year over the last half of the twentieth century, increasing to about 3 mm per year from 1993 to 2003. The fact that sea level has increased over the past century is, by the way, additional evidence that global warming is arising from an external cause, and not from internal variability involving the ocean giving up heat to the atmosphere. If the latter were the cause, then the ocean would be cooling and sea level would be falling (except possibly because of the effects of ice melt, but this is insufficient by itself to cause sea level to rise as observed).

Let us do a basic calculation to see how much sea level can be expected to rise from thermal expansion alone. From the equation of state for seawater, namely equation 2.1, the volume of the oceans will change with temperature according to

where DV is the change in volume, V is the current volume, T is the temperature, and bT is the coefficient of thermal expansion. This coefficient itself changes with temperature and pressure from an average value of about 2.4 X 104 K-1 at the surface in low latitudes to about 1.3 X 104 K-1 for the ocean below the thermocline (which of course is most of the ocean). Most of the increase in ocean volume will be accommodated by an increase in the depth of the ocean, and given that the ocean is on average 3.7 km deep and using bT = 1.4 X 104 K-1, we find that an increase in temperature of 1°C will result in an increase in sea level of, approximately, half a meter.

This is a significant overestimate of the rate of thermal expansion that has happened over the past century, or what is likely to happen in the near or medium-term future because the temperature of the deep ocean has changed only a little. Most of the temperature increase thus far has been concentrated in the upper ocean: in the mixed layer and in and above the main thermocline. If we suppose that the depth of the column of water experiencing this temperature increase is just 1 km, and using bT = 2.0 X 104 K-1 (appropriate for the warm upper ocean), we obtain a rough estimate of 20 cm of sea-level rise for each degree rise in temperature. As time progresses and the deep ocean begins to participate in the temperature rise, this rate can be expected to increase to the aforementioned 50 cm per degree, but that will only occur some time in the future. Still, in the long term, such an increase is unavoidable if global warming continues. If the equilibrium climate response to a doubling of co2 is 3°c, which is a typical estimate, and if temperatures reach and stay at that level for an extended period, sea level will eventually rise by somewhere in the vicinity of 1.5 m purely by thermal expansion, without any contribution from the melting of ice sheets.

Needless to say, there are other possible causes of sea-level rise as well as thermal expansion, the most important one being the decrease in ice over land (glaciers and ice sheets), but not, of course, the melt of ice floating on seawater. In the past few decades, thermal expansion of the oceans is estimated to have contributed a little more than half of the total sea-level rise, glaciers and ice caps a little less than a third; losses from the polar ice sheets contributed the remainder, with total sea level currently rising at about 2.4 mm per year (Domingues et al. 2008

and Bindoff et al. 2007). For the future, calculations using comprehensive climate models project that a global sea-level rise due mainly to thermal expansion (projected to be about two-thirds or more of the total) of a few tens of centimeters might be expected in the coming century if greenhouse gas emissions continue, but there is quite of lot of uncertainty and, of course, it depends on the rate of greenhouse warming itself. Such increases in sea level would mainly affect very low-lying communities such as the Maldives, in the Indian Ocean, and Bangladesh. It has been said that about half of Bangladesh would be flooded if the sea level were to rise by 1 m, creating millions of refugees. Even with a sea-level rise of 30 cm, the consequences would be severe.

Much higher rises in sea level would ensue if the great ice sheets covering Greenland and Antarctica were to melt. The ice on Greenland (about 2.8 million km3) would then contribute about 7 m to sea-l evel rise, the West Antarctic ice sheet (about 2.2 million km3), about 5 m. The East Antarctic ice sheet is the biggest of them all (about 28 million km3), and if it were to melt, sea level would increase by about 70 m, but such an increase is extremely unlikely, at least in the foreseeable future. It is also generally thought unlikely that there will be wholesale melting of the Greenland or West Antarctic ice sheets in the twenty-first century, even if global warming continues apace, if only because it takes a long time for the ice to melt and flow into the ocean. Nevertheless, our knowledge of the dynamics of land ice sheets remains rudimentary and there does remain the possibility that we have underestimated the risk of significant melting of land ice (glaciers as well as the big ice sheets), and hence underestimated the possibility of significant sea-level rise. Even if sea-level rise is modest in the coming decades, on timescales of more than 100 years the possibility of significant land-ice melt and of sea-level rise of much greater than 2 m should not be discounted.12 Rather interestingly, many climate models predict that the Antarctic ice sheet will experience increased snow fall in the coming decades (because climate is warmer so there is more moisture in the atmosphere) but will not experience additional melting (because it will still be too cold). Thus, the ice sheet may grow slightly, causing sea level to fall. In any case, the extent and pace of ice sheet melting is currently one of the more significant unknowns in the global warming business.

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