The most widely feared consequence of global warming appears to be sea level rise (SLR). Environmental advocacy groups are polluting the airwaves and internet with lurid images of flooding of Bangladesh and Pacific islands, and raising the specter of hundreds of millions of environmental refugees. Even sober scientists, while not endorsing such obvious scare stories, predict an acceleration of the ongoing global rise, which a system of tidal gauges places at about 18 cm (7 inches) for the past century, Other scientists stoutly maintain that there has been no accelerationeven during the strong global warming of 1920-1940.
Recall that Obama even predicted a deceleration of SLR when he accepted his partys nomination in 2008: This was the moment when the rise of the oceans began to slow, and our planet began to heal. So far, the existing data can be used to support all three sides. Some tidal-gauge data tend to show deceleration starting in 1960 (Holgate).
With estimates of past SLR all over the place, how does one proceed? The principal data have come from gauges, which measure not only tides but storms and everything else. And from these measurements one extracts a steady rise in local sea level. There are about two dozen stations in the world with long-enough records dating back to the early 1900s, which have been used by the international tidal gauge network, located in Liverpool, England.
Leading researcher Bruce Douglas terms SLR a puzzle (Physics Today March 2003), while famed Scripps Institution oceanographer Walter Munk calls it an enigma (ProcNatlAcadSci). Maybe we should use Churchills quote on Russia: A riddle wrapped in a mystery inside an enigma.
The difficulty with projections of sea level rise is nicely illustrated by the IPCC. The initial estimates of its firstassessment report (1990) showed a range of 10-367 cm for sea level rise in 2100. The second report published in 1996 narrowed the range to 3-124 cm. Its third report published in 2001 showed 11-77 cm. The fourth assessment report published in 2007 showed 14-43 cm in draft form but changed it to 18-59 cm in the final printed version. As can be seen, the maximum SLR decreased successively as estimates improved. All these IPCC projections are very much smaller than the extreme values of about 600 cm (20 feet!) by activist-scientist James Hansen (and by climate multi-millionaire Al Gore)which assume excessive melting of the Greenland icecaps.
This narrowing of estimates by the IPCC produced great concern among alarmists who feared that the IPCC was being too conservative. Probably as a result of this peer-pressure, estimates have now increasedas will be seen in the fifth assessment report, due in September 2013. As a reviewer of IPCC reports, I have been able to look at the second order draft, which was recently leaked to the press. It gives values of 45-110 cm (16-40 inches)about double what IPCC estimated just six years ago in their fourth report. But there is no guarantee that these values will survive in the final printed version. Still, they are very much smaller than some of the extreme estimates that have been written up in newspapers and magazines and blamed on Global Warming (GW) from carbon dioxide released in the burning of fossil fuels.
But recent observations and new analyses of existing data suggest an opposite result: A climate warming could slow down SLR not accelerate it. To understand this counterintuitive result, one must first get rid of false leadsjust as in a detective story. The misleading argument here is the oft-quoted statement that the climate warmed by 1 F (0.6 C) in the last 100 yearsand that sea level rose by 18 cm. Both parts of the statement are true; but the second part does not necessarily follow from the first.
The first clue that there might be something amiss with the IPCC logic comes from the IPCC report itself. According to this authoritative source, the contribution to SLR of the past century comes mainly from three sources: (i) thermal expansion of the warming ocean contributed about 4 cm; (ii) the melting of continental glaciers about 3.5 cm. [Note that the melting of floating sea ice does not raise SL] (iii) the polar regions, on the other hand, produced a slight net lowering of sea level, with most of it coming from the Antarctic [IPCC 1996; Table 7.3]. (The mechanism is intuitively easy to understand but difficult to calculate: A warming ocean evaporates more water, and some of it rains out in the Polar Regions, thus transferring water from the ocean into snow and ice at the polar ice caps.)
The major new result here is that when one simply adds up all these estimated contributions (neglecting the large uncertainties), they account for only about 20 percent of the observed rise of 18 cm. The temperature rise since 1900 cannot account for the observed SLR. Something is missing here.
An additional fact is discussed in detail in my 1997 book Hot Talk, Cold Science (Independent Institute, 1997). During the strong warming of 1920-1940 there was no SLRindicating a rough balance between the opposing effects. In fact, scrutinizing the record, I can even discern a slight lowering of sea level, an over-compensation. Unfortunately, back then in 1997 we had no data on Antarctic ice accumulation; so the hypothesis was not publishable. However, now we do have sufficient data in support of such a scenario.
But if, as surmised, ice accumulation roughly balances ocean thermal expansion and contributions from melting mountain glaciers, why then is sea level rising? Another riddle requiring a solution.
The relevant clue comes from corals and from geological observations: It seems that sea level has been rising for the past centuries at about the same rate as seen by tidal gauges in the last 100 years. In other words, sea level was rising even during the colder Little Ice age, from about 1400 to 1850 AD. This provides further support for the hypothesis that the observed global SLR since 1900 is reasonably independent of the observed temperature rise. [It is also a killing argument against a widely quoted (semi-empirical) theory that assumes rate of SLR is proportional to global surface temperature.]
The full explanation to this riddle had been suspected for some time, based on historic data on SLR from corals (Fairbanks) and ice volume (Shackleton). But the picture was filled in only recently (Bindschadler 1998) through measurements of the rate of melting of the West Antarctic Ice Sheet (WAIS), by tracing the millennial-scale shrinking of the WAIS [Conway et al. 1999]. Note that the WAIS is not floating sea ice; like a mountain glacier, its melting contributes more water to the ocean, thus raising the global sea level.
We can therefore describe the broad scenario as follows: The strong temperature increase that followed the Last Glacial Maximum (LGM) about 18,000 years ago has melted enough ice to raise sea level by 120 meters (400 feet). The rate of rise was quite rapid at first and controlled by the melting of the huge ice sheets covering North America and the Eurasian land mass. These disappeared about 8000-5000 years ago; but then, as sea level rose, the WAIS continued to melt, albeit at a much lower rateand it is still melting at about this rate today.
The principal conclusion is that this melting (and corresponding SLR of about 18 cm [7 inches] per century) will continue for another several millennia, until the WAIS is all goneunless another ice age takes over. And there is nothing that we can do to stop this future sea level rise! It is as inevitable as the ocean tides. Fortunately, coral reefs will continue to grow, as they have in the past, to keep up with SLR. The rest of us will just have to adapt to future SLR, as our ancestors did some10,000 years ago. At least we are better equipped with technology to deal with such environmental changes.
Anthropogenic Global Warming (AGW) may not affect SLR
A final note: What about the effects of human-induced global warming on SLR? Will it really increase the rate above its natural value, as predicted by the IPCC? We do have a handle on this question by observing what happened when the climate warmed suddenly between 1920 and 1940, before cooling between 1940 and 1975. The answer is quite surprising and could not have been derived from theory or from mathematical models. The data show that SLR slowed down slightly when the climate warmed and accelerated when the climate cooled. Evidently, ocean-water thermal expansion and mountain-glacier melting were less important than ice accumulation on the Antarctic continent (which would of course act to lower sea level).
By analogy, a warming produced by an increase in greenhouse gases should give the same result: i.e., reduce the rate of rise of sea level. This is not a policy recommendation that burning more coal might save Venice from drowning. It is a modest appeal to politicians to take note of scientific developments and to make the necessary corrections to the course of negotiations now underway.
We can now try to answer our original question: Can Global Warming (GW) really lower sea level rise? It all depends on the time-scale: Yes, if GW lasts only for some decades or less. No, if warmer temperatures persist for millennia.
NB: This essay ignores many less important features of global SLR, such as mining of ground water and construction of dams. It also ignores important regional and local effects that depend on isostatic adjustments, ocean currents and wind patterns, land subsidence, etc. Efforts are underway to harmonize conflicting data from tidal gauges and from direct measurements of the ocean surface by satellites.