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Monday, September 3, 2012

Stop-and-go deglaciation, Nature Geoscience editorial, doi: 10.1038/ngeo1574

(2012) 585; doi: 10.1038/ngeo1574; published online August 31, 2012

Past transitions from glacial to interglacial climates have not been smooth. It would be wise to prepare for similarly sudden episodes of ice loss in future climate changes.

Only when climate records at centennial to millennial scales became available did the complexity of glacial terminations become apparent. Early low-resolution palaeoclimate reconstructions suggested a rapid and relentless meltdown. But at a closer look, an interhemispheric asynchrony emerged. Warming began in Antarctica around 18,000 years ago, which was interrupted by an abrupt reversal about 14,500 years ago. However, this southern cooling was countered by warming in the Northern Hemisphere until it too was interrupted by 1,000 years of cooling. Full interglacial conditions in both hemispheres were only reached after 11,700 years ago.
The long and complicated transition from full glacial conditions to early interglacial conditions (and beyond) are the subject of a Progress Article and a Review on pages 601 and607 of this issue. Of particular interest is the transfer of water from vast continental ice sheets to the world's oceans, which led to a 120-metre rise in sea level.
Post-glacial sea-level rise followed an uneven trajectory. Against a background of a gradually rising waters, several distinct meltwater pulses, presumably from sudden partial ice-sheet collapses, pushed sea level up tens of metres within a few centuries. Synthesizing work on the past two glacial terminations, Carlson and Winsor (607) argue that rapid disintegration events are a hallmark of ice sheets that reach the ocean, whereas ice sheets that terminate on land have melted more steadily as more solar radiation reached them.
Törnqvist and Hijma (601) review studies of later, decimetre-scale rises that have become possible thanks to increasingly precise measurements. About 8,500 and 8,200 years ago, two rapid jumps in sea level have been linked to the final drainage of Lake Agassiz in northern North America, a lake of glacial meltwater on the margin of one of the largest ice sheets. The analysis suggests that such injections of fresh water into the ocean were not only the consequence of warming, but also a necessary condition for abrupt climate change associated with changes in the ocean circulation.
It is tempting, in the face of rising temperatures, to draw comparisons between the warming at the end of glacial periods and the projected melting of the remaining ice sheets on Greenland and Antarctica. The underlying causes of the deglaciation — subtle changes in incoming solar radiation as the Earth's orbit around the Sun evolved — were far slower than the changes in radiative forcing that are attributable to anthropogenic greenhouse gas emissions, and direct comparisons cannot be made. But in view of the climatic complications during the last deglaciation, the response to future climate change might be far from gradual.

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