Why Are Sea Levels So Low?
The last interglacial period (LIG)—the Eemian—is commonly believed by scientists to have been warmer than the current Holocene interglacial. Along with that balmier climate there is evidence that sea levels were significantly higher than today. Previous studies have pegged Eemian sea levels at 4 to 6m higher than today. Recently, a new investigation raises that estimate, reporting that ancient sea levels peaked between 6.6 and 9.4 m (~20 to 30 feet). Modern day accounts of flooding in low lying coastal areas and tropical islands abound, with ominous suggestions of links to global warming. How high the oceans will rise is a topic of debate for IPCC members, the news media and assorted climate alarmists, but they are asking the wrong question. Instead, they should ask why are sea levels so low?
Perhaps no speculative outcome of anthropogenic global warming (AGW) is more threatening than a significant rise in sea level. Given that most of Earth's human population lives in littoral regions, the prospect of flooded cities and millions of refugees fleeing inundated coastal plains is a threat that strikes fear in the hearts of people everywhere. Predicting future sea-level rise requires an understanding of various climate conditions and potential ice-sheet instability. Unfortunately, efforts to accurately forecast the rising and falling of sea levels have proven inadequate. To try and gain some insight into the mechanisms at work, scientists have been studying the warm period prior to our own—the Eemian interglacial that occurred ~125,000 years ago.
In a report in the July 13, 2012, edition of Science, A. Dutton and K Lambeck present a new assessment of Eemian sea levels based on a a new global database of U-Th ages and elevations of fossil corals. In “Ice Volume and Sea Level During the Last Interglacial,” the authors argue that during the previous warm period significantly more Antarctic ice must have melted to generate the calculated ancient sea levels. Here is the article’s abstract, which sums up their findings nicely:
During the last interglacial period, ~125,000 years ago, sea level was at least several meters higher than at present, with substantial variability observed for peak sea level at geographically diverse sites. Speculation that the West Antarctic ice sheet collapsed during the last interglacial period has drawn particular interest to understanding climate and ice-sheet dynamics during this time interval. We provide an internally consistent database of coral U-Th ages to assess last interglacial sea-level observations in the context of isostatic modeling and stratigraphic evidence. These data indicate that global (eustatic) sea level peaked 5.5 to 9 meters above present sea level, requiring smaller ice sheets in both Greenland and Antarctica relative to today and indicating strong sea-level sensitivity to small changes in radiative forcing.
Working with coral samples from around the world, the authors were careful to correct for possible problems in the proxy data, based on uranium and thorium ratios, and the dating of the coral samples. They also considered glacio-isostatic processes, the deformation of Earth's crust due to the weight of glacial ice and its rebound after weight removal due to melting. These and other factors were used to estimate local, or relative sea level (RSL), and the global or eustatic sea level (ESL). “Therefore, if RSL data from multiple sites have not been corrected for isostatic contributions, their superposition can result in synthetic sea-level oscillations that misrepresent the ESL history,” the paper states. The resulting local sea levels for various locations are shown in the figure below.
This is not the first study to suggest peak Eemian sea levels on the order of 9 m more than modern levels. In “Probabilistic assessment of sea level during the last interglacial stage,” R. E. Kopp et al. also concluded that sea levels 125,000 years ago were much higher than today. “We find a 95% probability that global sea level peaked at least 6.6 m higher than today during the last interglacial; it is likely (67% probability) to have exceeded 8.0 m but is unlikely (33% probability) to have exceeded 9.4 m,” they reported. “When global sea level was close to its current level (≥-10 m), the millennial average rate of global sea level rise is very likely to have exceeded 5.6 m kyr-1 but is unlikely to have exceeded 9.2 m kyr-1.”
That report estimated polar temperatures were 3–5 °C warmer than today, toward the high-end of the IPCC's estimated range for the next century. What is most interesting in the paper by Dutton & Lambeck is that under conditions very close to today's the seas were much higher. This, of course, implies that there would be nothing unnatural or unprecedented about the oceans rising several meters before the climate begins its long, slow slide back into glacial continuations.
Despite reasonable expectations of higher sea levels, a number of scientists, politicians and other self-appointed experts prance about the world stage proclaiming an incipient flood, the likes of which have not been seen since Noah. Unlike Noah's flood, which is attributed to divine retribution, this new diluvian onslaught is of man's own making—one of the outcomes of anthropogenic global warming. There is little difference between such dire warnings and those of ancient biblical prophets, save that modern secularism had replaced God as the motive force behind the impending disaster and substituted mankind. It would seem that even those who have denounced religion still feel the atavistic need to blame acts of nature on some supernatural entity.
Strangely, study after study have found inconclusive evidence at best for a rapid, dramatic increase in current sea levels. What's worse, even after accounting for glacial melting and other factors, the current slow rise in sea levels is actually too rapid. This has sent environmentalists and climate scientists scurrying to find a reason the oceans are not rising at the expected rate. In a news article by Amanda Mascraelli, “Source found for missing water in sea-level rise,” published in Nature, it is claimed that the unaccounted for rise is also due to human activity:
During the latter half of the twentieth century, global sea level rose by about 1.8 millimetres per year, according to data from tide gauges. The combined contribution from heating of the oceans, which makes the water expand, along with melting of ice caps and glaciers, is estimated to be 1.1 millimetres per year, which leaves some 0.7 millimetres per year unaccounted for. This gap has been considered an important missing piece of the puzzle in estimates for past and current sea-level changes and for projections of future rises.
Recall that R. E. Kopp et al. estimated the rate of rise during the Eemian was 5.6 to 9.2 millimeters per year, so the modern 1.8 is actually quite slow. The source of the missing water, according to another study by Yadu N. Pokhrelis et al., is human water usage. According to their report in Nature Geoscience, “We find that, together, unsustainable groundwater use, artificial reservoir water impoundment, climate-driven changes in terrestrial water storage and the loss of water from closed basins have contributed a sea-level rise of about 0.77 mm yr-1 between 1961 and 2003, about 42% of the observed sea-level rise.”
Estimates of various contributions to observed sea level change.
The authors note that the unsustainable use of groundwater represents the largest contribution, something that certainly needs to be addressed in the near future. “I didn’t expect that terrestrial water storage had such a big impact on sea level,” said Taikan Oki, a global hydrologist at the University of Tokyo and co-author of the paper. “I didn’t expect that human extraction of groundwater would matter so much.” What he doesn't point out is that, if humans are indeed responsible for 42% of current sea level rise, this means global warming (whatever its source) has an even weaker impact on current fluctuation than previously thought. It appears that the AGW link to rising sea levels is about half a strong as the IPCC alarmists' claim.
The Dutton & Lambeck paper points out the seminal difference between the Eemian world and today: “The difference between ESL at +4 and ESL at +9 m higher than present is important: The former can be largely accounted for through thermal expansion of seawater, loss of mountain glaciers, and partial loss of the Greenland ice sheet, but higher levels require a contribution from Antarctica.” And as any glaciologist will tell you, large masses of glacial ice like the Greenland ice sheet and those covering Antarctica take a long time to melt.
Regular readers of The Resilient Earth website will note that I recently reported on the climate during the mid-Miocene and how we might reasonably expect things to grow warmer in the future when the Pleistocine ice age eventually ends. That era of ice free Arctic seas and forested Antarctic shores had temperatures high as 11°C higher than today. Even under those conditions, however, it would take thousands of years to melt the existing polar ice sheets.
That brings up another point, usually omitted by the prognosticators of doom: do they really think the prospective victims of sea level rise are just going to sit while the waters rise around them? Whether it is the inundation of coastal cities like Venice and New Orleans, or the submergence of low lying countries like the Netherlands or Bangladesh, the change will come slowly. There will be time for dikes and levies to be strengthened, time for orderly relocation of populations. People will respond and adapt, that is what our species does—that is why H. sapiens is a success.
Surely someone would have seen this coming?
What caused the extra ice sheet melting during the Eemian? There were no fossil fuel burning humans around to conveniently blame the extra balmy climate on. Dutton & Lambeck conclude that it must have been a slightly higher rate of solar irradiance that caused the dramatic difference in sea levels the last time around. Bottom line: humans have precious little to do with the waxing and waning of glaciers and hence, not much impact on sea levels. While climate change alarmists attempt to frighten the public with images of flooded cities and other horrors, it is really the Sun that dictates climate change here on Earth.
Be safe, enjoy the interglacial and stay skeptical.