Ancient Sea-levels Rewrite Ice Age Transitions
There have been a rash of bogus reports in the news media about rapidly rising sea-levels supposedly caused by global warming. Sea-levels are notoriously hard to measure on a global basis since land also rises and sinks due to tectonic activity. With historical records mostly unreliable how can we tell if current conditions are normal for Earth 14,000 years after a deglaciation? A new report, based on calcium growths in caves on the Mediterranean island of Mallorca, says that sea levels around 81,000 years ago were higher than today. Higher sea-levels imply less glacial ice and warmer temperatures than today as well. Even more interesting is that this occurred during a warm period called marine isotope stage (MIS) 5a, which was more than 30,000 years after the Eemian interglacial ended and glaciation had resumed. This could mean that current theories about how ice age glacial periods start are wrong.
Scientists have long made a strong connection between Earth's climate and sea-levels. Warmer climate means less glacial ice and higher ocean water levels. This means that during glacial periods, often called ice ages, sea-levels drop because more water is trapped in glacial ice. Scientists know from paleoclimate records that the end of a glacial period occurs quickly and is accompanied by wild swings in climatic conditions. But because the buildup of large amounts of glacial ice depends on precipitation it was thought that descent into a new glacial period was a more gradual affair. A report in Science, entitled “Sea-Level Highstand 81,000 Years Ago in Mallorca,” authored by Jeffrey A. Dorale and colleagues, not only claims that sea-levels were higher than modern levels during ages past but that there have been significant swings in sea levels after the last interglacial warm period ended.
Dorale et al. base their report on data taken from speleothems—stalactites and stalagmites—found in caves on the island of Mallorca, off the Mediterranean coast of Spain. By looking at encrustation markings on the subterranean speleothems the researchers were able to measure sea-levels over +80,000 years ago. Their methodology is described in the following quote from the paper:
Large fluctuations in global sea level occurred throughout the last interglacial/glacial cycle, but the precise magnitudes of some of these fluctuations are subjects of energetic debate. The eustatic (ice equivalent) sea-level height of the marine isotope stage (MIS) 5a highstand event is among the more controversial of these sea-level variations, with estimates ranging widely from +3 to –30 m relative to modern sea level. Along the coast of Mallorca in the western Mediterranean, caves exist that provide an extraordinary setting for capturing past sea-level changes. The caves formed by the mixing of fresh water and seawater in the coastal phreatic zone and contain numerous speleothems (such as stalactites and stalagmites) that formed in early Quaternary time when the caves were air-filled chambers. Throughout the Middle and Late Quaternary, the caves were repeatedly flooded by glacioeustatic sea-level oscillations. The water level of each flooding event was recorded by a distinct encrustation of calcite or aragonite over existing speleothems and along cave walls.
Marine isotope stages (MIS), or marine oxygen-isotope stages, are alternating warm and cool periods in Earth's climate history. Sometimes referred to in older literature as oxygen isotope stages, their existence was deduced from oxygen isotope data collected from ice-core data and deep sea core samples. The marine isotopic record shows about 50 climate cycles going back over the past 2.5 million years. In 1955, Cesare Emiliani, the founder of paleoceanography, divided oxygen isotope records from deep-sea cores collected in the Caribbean into stages. These stages were numbered with 1 being the most recent. As more detailed data became available, Marine Isotope Stage 5 (MIS5) was divided into five sub-stages (a-e), with the Eemian Interglacial being widely associated with MIS5e (see “Marine Isotope Substage 5e and the Eemian Interglacial”).
Types of speleothem deposits.
Speleothems, from the Greek for “cave deposit,” are mineral deposits formed in caves by the evaporation of mineral-rich water, their presence implying that the Mallorcan caves were not always filled with water to the depth they are today. Stalactites and stalagmites formed during times when sea-levels were low and the caves were air-filled chambers. Over time, the caves were repeatedly flooded by changes in sea-level, with the water leaving a record of each flood as encrustations of calcite on speleothems formed during earlier low water periods.
In all, the researchers collected six speleothem encrustations from five different caves along the eastern and southern coast of Mallorca. All the sampled caves were within a horizontal distance of 250 m (800 ft) of the coast. In the past, the local water table and consequently the level of water in the caves rose and fell with sea-level changes. These changes in water level left tell tale marks on the cave interiors. Encrustations were sampled from two distinct heights located 1.2 to 1.5 (3.9 to 4.9 ft) and 2.6 m ±5 cm (8.45 ft ±2 in) above present sea-level.
Relative sea-level change at any particular location reflects not only changes in global ice volume but also the response of Earth to changes in surface loading in the form of surface deformation, referred to as glacial isostatic adjustment (GIA). The authors propose that Mallorca’s location in the western Mediterranean is unusual with regard to GIA, being fairly close to a predicted zone of neutral GIA to the west. Their conclusion is that Mallorca occupies a narrow transition zone between regions of emergence and submergence in the Mediterranean basin, where sea-level nearly follows the eustatic curve. In other words, the observed sea-level change at Mallorca is not affected significantly by changes other than the changes caused by variation in glacial ice volume.
The researchers reached the conclusion that western Mediterranean relative sea level was ~1 m (~3.25 ft) above modern sea-level ~81,000 years ago during marine isotope stage 5a. “Although our findings seemingly conflict with the eustatic sea-level curve of far-field sites, they corroborate an alternative view that MIS 5a was at least as ice-free as the present, and they challenge the prevailing view of MIS 5 sea-level history and certain facets of ice-age theory,” the authors state. Variation in sea level and Insolation levels is shown in the figure below, taken from the article.
Fig. 2 Comparison between the Mallorca and other sea-level estimates. Dorale et al.
There are two conclusions from this work that stand out. The first is that sea-levels were higher by at least 3 feet during the last interglacial and even during one 4000 year long warm period after the termination of interglacial conditions. This implies that an additional rise in sea-levels in the future would not be an indication of abnormal global warming, as claimed by some climate change alarmists. The fact is, recent sea-levels have been remarkably stable. According to the new speleothem data “mean sea level has remained stable on Mallorca for the past ~2800 years.” The researchers cite other, corroborating data:
We therefore consider the simple interpretation of our data that eustatic sea level during MIS 5a stood around +1 m relative to present sea level, implying less ice on Earth 81,000 years ago than today. Although this interpretation conflicts with the generally accepted eustatic sea-level curve based on the far-field sites of Barbados and New Guinea, it is consistent with a number of other estimates from around the world, including those from the Bahamas, the U.S. Atlantic Coastal Plain, Bermuda, Cayman Islands, and California.
Indeed, in 2006, Morag K. Coyne, Brian Jonesa, and Derek Ford reported a study of the Ironshore Formation on the western part of Grand Cayman. They found formations dating to MIS 5 that indicated sea-levels of 2 to 6 m above modern levels and that “the highstands associated with MIS 5e, 5c, and 5a were at similar elevations.” All those who are “alarmed” at the melting of glaciers during the current interglacial should take note: there were higher sea-levels and less glacial ice during the last interglacial and human activity had nothing to do with it.
The second interesting point is that the conventional wisdom regarding onset of a glacial period, that it is a slow and gradual affair, is quite possibly wrong. Because of the relationship between sea-level and continental ice volume, an accurate sea-level history has been sought by scientists interested in ice-age cycles and their underlying causes. This study has important implications for currently accepted relationships among glacial ice volume, historic CO2 levels and global temperature change. In the words of the authors:
Ice-age theory has long assumed gradual ice buildup and more rapid ice melting in the generally accepted model of the ~100-ky cycle of glaciation. Instead, the emerging body of evidence suggests that both melting and accumulation can be very rapid during discrete intervals of time when specific conditions prevail. Furthermore, the 100-ky model of glaciation has always faced the problem that although the deep-sea δ18O record is dominated by a 100-ky cycle, northern high-latitude summer insolation has negligible power in this band. Our data from Mallorca and data from other sites around the world indicate the possibility that eustatic sea level was near modern levels at ~80 ka. If this is true, the 100-ky cycle so universally accepted as the main rhythm of the Middle and Late Quaternary glaciations, in fact, applies rather poorly to ice growth and decay, but much better to carbon dioxide, methane, and temperatures recorded by polar ice.
Other researchers have hinted that termination of the last interglacial took place rapidly. In 1980, John T. Hollin reported in Nature that “substage 5c was essentially interglacial, and was terminated by a catastrophic cooling.” Add corroborating data from others including Dorale et al. and the evidence seems to be mounting that the currently held view of interglacial terminations is wrong.
I myself have written of the difference between deglaciations, which are known to be rapid, wild transitions, and gradual interglacial terminations. Here is evidence that glaciation can also be a roller coaster ride, with periods of rapid freezing and melting. Furthermore, it may mean that climate scientists have read far too much into ice-core records of CO2 and other atmospheric gases. If these findings prove accurate they reinforce something that every scientist and non-scientist should always keep in mind—conventional wisdom is often wrong and science is never settled.
Be safe, enjoy the interglacial and stay skeptical.
Mallorca, Spain. Nice work if you can get it.