Achi news desk-
As the climate warms and Antarctica’s glaciers and ice sheets melt, the resulting rise in sea level has the potential to displace hundreds of millions of people worldwide by the end of this century.
A key uncertainty about how much and how fast the seas will rise is whether currently “stable” parts of the West Antarctic Ice Sheet may become “unstable”.
One such region is West Antarctica’s Siple Coast, where rivers of ice flow off the continent and drain into the ocean.
Journal of Geophysical Research, CC BY-SA
This ice flow is slowed by the Ross Ice Shelf, a floating mass of ice nearly the size of Spain, which holds back the land-based ice. Compared to other ice shelves in West Antarctica, the Ross Ice Shelf has very little melting at its base because the ocean underneath is very cold.
Although this region has been stable in the last few decades, recent research suggests that this was not always the case. Radiocarbon dating of sediments under the ice sheet tells us that it retreated hundreds of kilometers around 7,000 years ago, and then advanced again to its current position within the last 2,000 years.
Finding out why this happened can help us better predict how the ice sheet will change in the future. In our new research, we test two main hypotheses.
Read more:
What an ocean hidden beneath Antarctic ice reveals about our planet’s future climate
Testing scenarios
Scientists have considered two possible explanations for this retreat and movement of ice sheets in the past. The first is related to the Earth’s crust below the ice sheet.
As an ice sheet shrinks, the change in ice mass causes the Earth’s crust to slowly rise in response. At the same time, and counterintuitively, sea level falls near the ice because the gravitational pull between the ice sheet and ocean water weakens.
As the ice sheet has thinned and retreated since the last ice age, crustal uplift and falling sea levels in the region may have redistributed floating ice, causing the ice sheets to advance.
AGU, CC BY-SA
The other hypothesis is that the behavior of the ice sheets may be a result of changes in the ocean. When the surface of the ocean freezes, forming sea ice, it expels salt into the water layers below. This cold salt water is heavier and mixes deep into the ocean, including under the Ross Ice Shelf. This prevents the warm ocean currents from melting the ice.
AGU, CC BY-SA
Sea floor sediments and ice cores tell us that this deep mixing was weaker in the past when the ice sheet was retreating. This means that warm ocean currents could have flowed under the ice shelf and melted the ice. Mixing increased as the ice sheet advanced.
We test both of these ideas with computer model simulations of ice sheet flow and Earth’s crustal and sea surface responses to ice sheet changes with varying ocean temperatures.
Because the rate of crustal uplift depends on the viscosity (viscosity) of the underlying mantle, we ran simulations within ranges estimated for West Antarctica. A viscous mantle means slower crustal uplift as the ice sheet thins.
The simulations that best matched the geological record had a viscous mantle and a warmer ocean as the ice sheet retreated. In these simulations, the ice sheet retreats faster as the ocean warms.
When the ocean cools, the simulated ice sheet moves to its current position. This means that changes in ocean temperature best explain the behavior of past ice sheets, but the rate of crustal uplift also affects how sensitive the ice sheet is to the ocean.
Veronika MedunaCC BY-SA
What does this mean for climate policy today?
Much attention has been paid to recent studies which show that glacial melting may be irreversible in some parts of West Antarctica, such as the Amundsen Sea basin.
In the context of such studies, policy debates hinge on whether we should focus on adapting to rising seas rather than cutting greenhouse gas emissions. If the ice sheet is already melting, are we too late for mitigation?
Read more:
We can still stop the collapse of the West Antarctic ice sheet – if we act quickly to keep future warming in check
Our study suggests that it is premature to abandon mitigation measures.
Global climate models run under high emissions scenarios show less sea ice formation and deep ocean mixing. This could lead to the same cold-to-warm ocean switch that caused the retreat of vast ice sheets thousands of years ago.
For West Antarctica’s Siple Coast, it is best to prevent this ocean warming from happening in the first place, which is still possible if we choose a low emissions future.
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The post West Antarctica’s ice sheet was smaller thousands of years ago – here’s why this matters today – The Conversation appeared first on Canada News Media.
