Editor's commentsThs paper represents the first attempt at creating a continuous record of ice-volume variability (sea level change) throughout the last glacial cycle which is independent of assumptions about the orbital insolation forcing of climate or about glaciological processes. The authors conclude that polar climate and ice-volume changes are closely coupled to within centuries. The analysis hints that Antarctic climate change leads global ice-volume change by several centuries.

During the past few million years, variability in global ice volume (sea level) has been one of the main feedback mechanisms in climate change. Highly resolved and continuous sea-level records are essential for quantifying ice-volume changes. A reliable timing of past ice-volume changes, relative to polar climate change, has so far been impossible because available sea-level records either were dated by using orbital tuning or ice-core timescales, or were discontinuous in time.

Reconstructing sea level for the past 150'000 year

The authors resolve these issues for the past 150,000 years using a novel approach to provide a detailed chronology to the continuous and highly resolved record of Red Sea relative sea-level. They are able to directly link the delta oxygen-18 record of eastern Mediterranean surface waters and that of caves on bordering land masses downwind of this highly evaporative sea, specifically the record from stalactites in Soreq Cave in Israel dated using uranium-thorium isotope dating.

To reconstruct the new RSL chronology involved two stages. First, the authors built an age model for eastern Mediterranean sediments (core LC21) by correlating its delta oxygen-18 record with the Soreq Cave delta oxygen-18 record over the interval 40–160  thousand years bfore the present. In addition for recent times 0-40'000 years ago they used 14 radiocarbon datings and two well-documented tephra (volcanic eruptions). Next, they transferrd the new LC21 age model to the RSL record between 22,000 and 150,000 years ago using the delta oxygen-18 record.

Comparison with other sea level reoconstructions and the Antarctica and Greenland climate record

The author compare their new RSL sea-level curve with other records of sea level and the polar climate record. The new RSL record agrees well—within uncertainties—with coral sea-level benchmarks. In general, the new sea-level record agrees well with the major climate transitions recorded in Greenland and Antarctic ice cores from the European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (EDML), Dome C (EDC), and North Greenland Ice-core Project (NGRIP).

Figure 2 Confidence intervals of 95% for the RSL data (light grey) and probability maximum (dark grey) are superimposed on:
a, Red Sea RSL data with new chronology (orange crosses; black line = moving Gaussian filter);
b, coral sea-level data
c, global benthic plankton (pink);
d, benthic plankton from marine core MD95-2042 (purple);
e, EPICA Dronning Maud Land (EDML) (blue) and EPICA Dome C (EDC) (orange)
f, NGRIP (green)

The structure and amplitude of Antarctic climate variations agree well with the record of ice-volume fluctuations. The Antarctic–RSL relationship is most tenuous at about 95–115  thousand years ago, at which RSL instead agrees better with Greenland climate fluctuations. The timing of ice-volume changes is generally found to be close to that of Greenland climate variability. The timing and structure of large-scale sea-level variability reflects a global signature of climate changes recorded in both Antarctic and Greenland ice cores.

The authors find that ice-volume lags of 100–400 and 200–400 years produce the best correlations with Antarctic and Greenland climate changes, respectively. The authors infer that Greenland climate closely tracks or is directly coupled with ice-volume changes, whereas Antarctic climate variability may precede ice-volume changes by up to 700 years.

Figure c, RSL most probable (grey shading), RSL data (blue crosses) and rate of change of RSL (red) with 95% confidence interval (pink shading). Rates of sea-level change of +12 and −8 m  per thousand years are indicated (dashed lines). Red arrows mark peaks in sea-level rises of more than 12 m  per thousand years.

The new RSL chronology reveals that rates of sea-level rise reached at least 1.2 m per century during all major phases of ice-volume reduction. Rates of sea-level lowering rarely exceeded 0.8 m per century.

Nature, 491, 744–747 (29 November 2012)