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Did an ice sheet collapse 120,000 years ago pushing sea levels up to 9m higher than today?

Posted By Joanne Nova On August 19, 2013 @ 3:33 am In Global Warming | Comments Disabled

Proving that nature can outdo anything humans have done, a new paper shows that sea-levels off Western Australia may have risen as high as 9 m above the current level during the last warm period over a hundred thousand years ago. The authors (O’Leary et al) conclude that seas were 3-4 m higher for most of the last warm period (known as the Eemian) but towards the end of the period a large sudden rise occurred. They suggest that an ice shelf collapsed in Antarctica  or Greenland or both, causing a 5m rise (17 feet).

The point of the paper was this double spiked shape of the sea level rise during the last warm interglacial known as the Eemian.

The Age interviewed O’Leary who said “he was confident that the 17-foot jump happened in less than a thousand years – how much less, he cannot be sure.”

Figure 3 j Relative sea-level curve for Western Australia. A
geomorphically defined palaeoMSL datum of C2:5m 120 kyr ago (Fig. 1c) anchors a predicted relative sea-level curve at Red Bluff, which includes a GIA signal based on the test calculation (see Methods) plus the following ESL history: ESL jumps from 0 to 3.4m between 127.5 kyr and 127 kyr agoand remains at this level until 120 kyr ago; and 120 kyr ago, ESL jumps 6m over 1 kyr. Dashed green line is an inferred sea-level curve based on a minimum coral palaeodepth (solid bar above circle) of 0.4m below palaeoMSL. This palaeodepth calculation is applicable only to highest in situ corals, as corals of the same age found at lower elevations will have a known water depth of at least up to the height of the coral above it. Arrows indicate potential for greater palaeodepth range.

Note the graph runs “backwards” and the oldest dates are on the right. (Which annoys me since we read left to right, so dates ought to run left to right…).

In the last 130,000 years sea levels have ranged from 120m below the current level to 9 m higher. For thousands of years they’ve stayed higher than today’s level –  long enough for whole coral reefs to form, die and be left high and dry. Those reefs are now long gone and fragmented. Meanwhile our sea-levels are rising at 2mm or even 3mm a year (but only if you believe the highly adjusted data) and we’re whipped into a panic.

Tipping Points in Ice Sheets?

Before we push the anxiety button on tipping points, we ought to remember that the Vostok Ice Cores tell us Antarctica was over 2 degrees warmer during the early Eemian, and Greenland was as much as 8 degrees warmer. And if seas were 3 -4 m higher before the ice sheet collapsed, that rather suggests we have a way to go before we reach that precarious state…

The late big rise apparently occurred at 118.1 ±1.4 thousand years ago. We don’t really know if that rise occurred in a decade or over 3,000 years. We do know that the cooling that came after that was bad news for the coral reef off Western Australia that was left high and dry after the water receded. Otherwise, corals, fish, and people around the world have somehow survived massive shifts in temperature and sea-level that had nothing to do with humankind.

Perhaps there are tipping points in Antarctic and Greenland shelves, and we would want to avoid triggering them (assuming we could).  But things were also warmer 7,000 years ago in the Holocene and the large ice sheets appeared to have managed just fine. That’s another point suggesting we have some safety margin. Plus, there’s no reason to believe that 2 degrees of warming is coming anytime soon, because the amplified exaggerations of the climate models are known to be wrong.

Curtin University Press Release

“… after mapping and surveying the Last Interglacial fossil shorelines from Augusta to Exmouth, my team and I were able to show that these ancient shorelines outcropped at similar elevations along this entire length of coastline and therefore have not been affected by tectonic movement.”

Dr O’Leary said the team then applied an isostatic model correcting for crustal deformation caused by glacial meltwater loading of the Indian Ocean basin.

“This research shows that even a modest rise of a couple of degrees centigrade in global temperatures could result in a significant rise in global sea levels, and will also help improve our understanding of the sensitivity Greenland and Antarctic ice sheets under higher global temperatures,” Dr O’Leary said.

This graph below captured my attention purely because of the large variations on display. It covers the last 180,000 years of sea level change at Red Bluff, WA. (From the supplementary file.) No there doesn’t appear to be a late Eemian surge visible here (I’m not sure why), and yes, this is from a model (so caveats apply). Red Bluff is at the North end of the map below.

Figure S2: Predicted relative sea-level (RSL) change (in meters) at Red Bluff, Western Australia (24.014S; 187 113.456E) due to GIA based on the two glacial cycle ice history and viscoelastic Earth model discussed in 188 the text and Methods Section.

This map shows the places they sampled along the West Australian coast.

The sites span nearly 2000 km of coastline on SW Western Australia

This paper spends a lot of time discussing GIA and using simulations, of which I remain somewhat skeptical, so I’ll refrain from drawing any conclusions other than the generic ones above: that we are not dangerously close to a tipping point, and that huge variations — far larger than we face today — have occurred before.

Ht to the Hockeyschtick

REFERENCE

Michael J. O’Leary*, Paul J. Hearty,William G. Thompson, Maureen E. Raymo, Jerry X. Mitrovica
and Jody M.Webster (2013) Ice sheet collapse following a prolonged period of stable sea level during the last interglacial, Nature Geoscience

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