January 8, 2007

Antarctic Ice Shelf Melt: Remember the Holocene!

Filed under: Antarctic, Polar

The recent climate change literature contains a great deal of evidence in support of the idea that the high latitudes will experience the greatest atmospheric warming. One of the most rapidly warming regions is the Antarctic Peninsula, and it is no surprise that warming here shortens the spatial and temporal extents of snow cover, glaciers, and sea ice. Ice shelves are not immune from the effects of warming, and rather dramatic ice shelf recession has occurred over periods as short as days and weeks as apparent thresholds in the drivers of melt are surpassed. The break-up of the Larsen-B ice shelf of the Antarctic Peninsula in 2002 received a great deal of worldwide attention, as it was believed that the Larsen-B had remained intact for thousands of years. The volume of glacial melt has prompted some climate change alarmists to push the panic button on global sea level rise. At the front of this crowd is Al Gore, who loves to show images and video footage of falling glacial ice and computer generated representations of inundated coastal areas while claiming that the recent global warming is unprecedented. Such images are meant to generate shock, fear, and a desire to place blame. There is little doubt that warming has indeed occurred across parts of Antarctica over the last few decades. However, let’s consider the possibility that a significant portion of the warming may be natural, and that regions, such as the Antarctic Peninsula, are likely to have experienced as warm or warmer conditions in their climate history, before human emission of greenhouse gases.

Earlier this year in an article in Quaternary Science Reviews, Carole Pudsey of the British Antarctic Survey and three of her colleagues reported on their investigation of the Holocene history of the northern Larsen Ice Shelf of the Antarctic Peninsula. Their work, “Ice shelf history from petrographic foraminiferal evidence, Northeast Antarctic Peninsula,” aimed at determining if the recent retreat of the ice shelf is unique to the current climate or if it has previously occurred on a millennial time scale. Pudsey and her colleagues explain that small ice shelves and their associated glaciers are possible indicators of climate and oceanographic change during the Holocene. They further note that ice cores from the Antarctic Peninsula extend back only a few hundred years and their temperature records do not match each other or instrumental records. The researchers conclude that the environmental record for the Holocene must be obtained from sediments on the continental shelf. Most importantly, Pudsey and her colleagues note that small Antarctic ice shelves have varied due to natural climate forcing during the Holocene, “which suggests that the recent decay may not result entirely from anthropogenic climate perturbations.”

Pudsey et al. obtained sediment cores from the area of the former Larsen-A ice shelf and the nearby continental shelf to provide a detailed Holocene record of the presence or absence of ice shelves. Their data indicate “widespread ice shelf breakup in the mid-Holocene”, which was in accord with earlier findings (Pudsey and Evans 2001) of an adjacent ice shelf receding during the same period but followed by a colder climate that allowed the shelf to re-form. Pudsey and her colleagues note that the breakup and subsequent re-formation likely took centuries to complete. They therefore note that “the maximum ice shelf limit may date only from the Little Ice Age.” The researchers also examined benthic foraminifera for evidence of the presence or absence of the ice shelf and found a “deglaciation signal in Prince Gustav Channel” that further suggested the occurrence of a mid-Holocene ice shelf breakup and the clear indication of a “mid-Holocene warm period.”

The work of Carole Pudsey and her colleagues contributes to a growing body of literature that makes clear the idea that the greatest extent of the Larsen ice shelf during the current interglacial period occurred only a few hundred years ago. The ice shelves that have recently disintegrated were likely created at about that same time, meaning that previously they did not exist. The recurring conclusion is that the recent global warming may not be unprecedented, and that a significant portion of the warming may be natural.

The message here is that although images of glacial disintegration are alarming, events such as these may have happened with or without human contributions of carbon dioxide to the atmosphere. Quite possibly, hundreds of years from now humans may be hearing about ice shelf growth on the Antarctic Peninsula and questioning what it is that their society has done to cause such a climate “anomaly.”

References:

Pudsey, C.J., Murray, J.W., Appleby, P., and Evans, J. 2006. Ice shelf history from
petrographic foraminiferal evidence, Northeast Antarctic Peninsula. Quaternary Science Reviews, 25, 2357-2379.

Pudsey, C.J. and Evans, J. 2001. First survey of Antarctic sub-ice shelf sediments reveals
mid-Holocene ice shelf retreat. Geology, 29, 787-790.




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