Ice Ages & CO2, Part II – Rising Sea-levels in Tahiti

Having reported that scientists did not find CO2 responsible for a change in the duration of ice age glacial periods 700,00 years ago, another new report takes a look at the conditions around the last interglacial warm period and our own Holocene warming. Using corals from the south seas paradise of Tahiti to track sea-level changes, researchers probed the mechanisms driving Earth's climate between glacial and interglacial states. Almost as an after thought they added that there is no longer any doubt: changes in sea-level drive changes in CO2, not the other way around.

In a paper appearing in the May 29, 2009, issue of Science titled “Penultimate Deglacial Sea-Level Timing from Uranium/Thorium Dating of Tahitian Corals,” Alex L. Thomas, et al., investigate the events that occur during a deglaciation, the change from a glacial period to an interglacial. Using uranium/thorium dating of fossil corals from around the South Pacific island of Tahiti and other locations, they reconstructed the rise and fall of sea-levels during periods of climate change. What they found was that the last period of change—the one resulting in the warm period we are now enjoying—behaved pretty much as scientists expected with regard to orbitally induced insolation changes. The previous transition was another matter however. Quoting from the paper's abstract:

The timing of sea-level change provides important constraints on the mechanisms driving Earth’s climate between glacial and interglacial states. Fossil corals constrain the timing of past sea level by their suitability for dating and their growth position close to sea level. The coral-derived age for the last deglaciation is consistent with climate change forced by Northern Hemisphere summer insolation (NHI), but the timing of the penultimate deglaciation is more controversial. We found, by means of uranium/thorium dating of fossil corals, that sea level during the penultimate deglaciation had risen to ~85 meters below the present sea level by 137,000 years ago, and that it fluctuated on a millennial time scale during deglaciation. This indicates that the penultimate deglaciation occurred earlier with respect to NHI than the last deglacial, beginning when NHI was at a minimum.

Besides the very fine investigative work figuring out the timing of events 135,000 years ago, there are two ancillary observations of interest here. One comes directly from the text and the other from observation. The observation is this: even when scientists think they have a good idea of how something works they are very often wrong. Perhaps more correctly, they are often only partially right. In this case, prevailing theory was correct for one event—the onset of the Holocene warming—but not correct for the preceding warming.

No glaciers formed on Tahiti during the last ice age.

The island of Tahiti Nui, part of French Polynesia, is located in the tropical South Pacific and is about as far away from from glacial ice sheets as one can get. Sea-level change at Tahiti during a deglaciation is caused by the addition of water to the oceans from melting glaciers rather than by other physical effects of ice mass removal. Glaciers, unlike pack ice, rest on continental land masses, which are themselves floating buoyantly on top of the upper mantel. Removal of the not inconsiderable weight of a glacier can cause the land it resided upon to rise significantly, sometimes faster than the rise in sea-level caused by the addition of meltwater. Since Tahiti has no glaciers to melt, and is known to be subsiding (sinking) at a fixed rate of 10 inches (0.25 m) per 1000 years, it is a good location to measure historical sea-level variation.

Using 234U/238U ratios of Tahitian corals and corals from other locations, along with δ18O-based reconstructions of sea level from the Red Sea, a detailed reconstruction of sea-level changes was created. According to Milankovitch's theory, variations in Earth's orbit and axial inclination cause changes in seasonal and hemispheric insolation, which in turn result in glacial-interglacial cycles. Based on this it was assumed that the rate of ice sheet collapse should be greatest when Northern Hemisphere summer insolation (NHI) was at a maximum. This suggests that peak melting would occur around 129,000 years ago, but that is not what the researchers found.

So here is the place where science's previous best guess fails. Tahitian coral data show that sea-level was clearly rising before 129,000 years ago, indicating that calculating peak melting based only on orbital cycles may be incorrect by up to 7000 years (see the figure above). According to the authors: “It is clear that the phasing of the penultimate deglacial was very different from that of the last deglacial, and that tuning approaches assuming a constant phasing are inappropriate if millennial-scale accuracy is required.” Clearly, the importance of NHI to the onset of deglaciation is not as expected.

The sea-level rise during the previous warming can be compared with the rise in atmospheric CO2. Here the CO2 reconstruction is based on matching N2/O2 variations to local insolation at the Dome Fuji ice core site by Kawamura et al. Interestingly, writing in Nature, Kawamura stated: “Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations.”

According to Thomas, et al: “This comparison indicates no resolvable difference in timing between sea level and CO2. This is in disagreement with a sea-level lag of 4000 years previously inferred with the use of ice-core atmospheric δ18O as a lagged response to sea-level change. This discrepancy suggests that atmospheric δ18O is not a reliable indicator of sea level.” Those results, along with some reconstructions of CO2 levels are shown in the figure below, also taken from the article.

The second point I took away from this paper came from a brief statement toward the end. That statement is: “The apparent synchroneity of sea level and CO2 change at this deglacial [and the slight sea-level lag during the last deglaciation] means that mechanisms involving sea-level changes as drivers of CO2 change are no longer falsified by timing constraints, as had previously been suggested.” In other words, sea-level changes drive CO2 level changes, not the other way round.

This paper has helped reinforce my evaluation of climate science as young and incomplete in many ways. While the Croll-Milankovitch cycles continue to be identified as the primary instigator of ice age glacial-interglacial transitions, how they do it is still very much up in the air. The results reported in Hönisch et al. (see Change In Ice Ages Not Caused By CO2), along with the almost off hand remark regarding CO2 and sea-level in this paper, also support the conclusion that carbon dioxide is simply not a primary driver of climate change. Not that any climate scientists who support the IPCC conclusions will admit that. They continue to shout “CO2 causes dangerous global warming” even as they provide evidence proving CO2 isn't up to the job.

Be safe, enjoy the interglacial and stay skeptical.

Not a bad place to do research.