Swamp kauri reveals ancient Antarctic melting's link to rising temperatures

Wednesday, 13 September 2017

Researchers on a swamp kauri. An ancient kauri pulled from a peat swamp near Dargaville provided vital clues to help explain global weather 30,000 years ago.

Scientists have used dating data from a Northland swamp kauri to show rapid melting of Antarctic ice about 30,000 years ago was linked to an abrupt rise in temperature in the North Atlantic region.

The focus is now on trying to understand what would cause the Antarctic ice sheet to retreat so dramatically. It is known there have been major collapses of the Antarctic ice sheet in the past.

A trillion-tonne iceberg, one of the largest ever recorded, has snapped off the West Antarctic ice shelf, scientists who have monitored the growing crack for years said.

Researchers say the work helps explain a 'seesaw' relationship known to have happened during the last glacial period, which started about 110,000 years ago. Ice cores from Greenland and Antarctica have shown that at times when it warmed in the north it cooled in the south and vice versa.

New research published on Wednesday uncovered a sequence of events that included a major retreat of Antarctic ice, a period of 400 years when the surface of the Southern Ocean cooled, and a spike in temperatures in the North Atlantic.

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To try to understand what was going on, researchers used a climate model to simulate the release of large volumes of freshwater into the Southern Ocean.

Writing in The Conversation, study lead author earth sciences and climate change professor Chris Turney, from the University of New South Wales, said all the model simulations showed the same thing.

'Regardless of the amount of freshwater released into the Southern Ocean, the surface waters of the tropical Pacific nevertheless warmed, causing changes to wind patterns that in turn triggered the North Atlantic to warm too,' he said.

'Regardless of how it happened, it looks like melting ice in the south can drive abrupt global change, something of which we should be aware in a future warmer world.'

The kauri log that played a key role in the study was the remains of a tree that lived between 29,000 and 31,000 years ago. It was pulled from a peat swamp near Dargaville, and accurate dating showed it was alive during a short Dansgaard-Oeschger (D-O) warming event, during which temperatures in the Northern Hemispherer would have risen.

Turney said Greenland ice cores showed there had been 25 major D-O events during the past 90,000 years. The abrupt, massive, millennial-scale swings in temperature across the North Atlantic region happened too quickly to have been caused by Earth's slowly changing orbit around the Sun.

'The scale of these swings is staggering: in some cases temperatures rose by 16C in just a few decades or even years,' Turney said.

He is a co-leader of The Ancient Kauri Project, which is using a technique called dendroclimatology to analyse the tree rings of swamp kauri pulled from swamps in northern New Zealand.That is providing a year-by-year record of past climate, with many of the logs extracted from peat bogs are between 12,000 and 55,000 years old.

Radiocarbon dating the ancient kauri wood was providing unparalleled new insights into how Earth's carbon cycle behaved during periods of abrupt and extreme climate change, the project website said. That information was crucial for understanding the future of the planet.

Nowhere else on Earth was there such an extensive collection of ancient wood.

The new research, published in Nature Communications, found that at the same time as the Antarctic ice retreated about 30,000 years ago, rain-bearing Pacific trade winds over tropical northeast Australia also collapsed.

What researchers did not find was any evidence of change in global ocean circulation. That's significant because it is thought the bipolar seesaw - warming in the north and cooling in the south - is linked to the shut down of deep ocean currents.