Kiwi scientists on the Ross Ice Shelf in the summer of 2017/18
Scientists expect to make better predictions about the impact of climate change on Antarctic ice sheets, after modelling the 1000km retreat of the Ross Ice Shelf during the past 20,000 years.
The work will help predict how stable the Antarctic ice sheets will be in a warming world, and their impact on global sea level rise.
The Ross Ice Shelf is the largest area of floating ice in Antarctica and is thought to play an integral role in buttressing – or holding back – ice sheets, which are over land.
Projections suggest thinning of the Ross shelf, as it melts from underneath, will lead to an increase in Antarctic contributions to sea levels.
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“If the ice shelf is thinner, then ice from on land flows faster.” Dan Lowry, from Victoria University of Wellington’s Antarctic Research Centre, said. “And that’s what contributes to sea level rise, when ice moves from on land to the ocean.”
An article discussing simulations of the Ross shelf retreat during the past 20,000 years, or so, carried out by Lowry and colleagues, was published on Thursday in the journalScience Advances.
At the peak of the last ice age, global sea level is estimated to have been 128 – 134 metres lower than it is now. Recent estimates put the contribution from Antarctica to the increase in sea levels since then at 7.5 – 13.6m.
Around 20,000 years ago, the Antarctic ice sheet was nearly at the edge of the continental shelf and was connected to the ground. But as the ice retreated it started to float on top of the ocean, Lowry said.
Lowry and his colleagues argue that while atmospheric conditions controlled the timing of ice shelf retreat in the early stages of the melt, during the Holocene period, which started about 11,000 years ago, the retreat was mostly due to changes in ocean temperature.
Most of what was known about what happened in the past came from dating of rock samples, Lowry said. “They are giving a pretty incomplete picture of what happened so we use these simulations to kind of fill in those gaps, and test the mechanisms that would have caused that retreat to happen,” he said.
“We can change ocean temperatures, we can change air temperatures, and we can see how that ice sheet changes in response to it.”
In what is believed to be the first underwater footage of its species captured in the area, a minke whale exhales at the surface in the icy Ross Sea.
Results from the simulations were then checked against the geological data that was available. “We use those geological records that come from rock samples and from radiocarbon ages of shells in sediment from the Ross Sea, and we can compare our simulation to those records to see if they line up,” Lowry said.
“I think what we show is that through the history of this ice sheet the ocean was a key factor in controlling ice sheet behaviour. So going into the future that is the process we need to get right, in those models that we use to predict how these ice sheets will change,” he said.
“It’s challenging to say exactly how ocean temperatures under the Ross Ice Shelf are going to change into the future. It depends on all kinds of things, but mainly how circulation changes.” That depended on such things as winds, melt water from the ice sheet, and changes to the sea ice.
“All of these things control the flow of ocean water under the ice shelf. We know it’s sensitive if that temperature increases but don’t know how much that temperature will increase into the future.”
Work was under way on building a new climate system model that integrated the new information.