An international team including Te Herenga Waka—Victoria University of Wellington and GNS Science researchers has found there was significantly more ice on Antarctica during the Early Miocene—around 16-18 million years ago—than was previously thought.
The paper examines data from the International Ocean Discovery Program and concludes the West Antarctic Ice Sheet was at times much larger than today during the early Miocene epoch—a time in the past when atmospheric carbon dioxide levels were similar to those we expect in the coming decades under low to mid-range emissions pathways.
Previously, scientists had assumed climate in the early Miocene was too warm to support a large West Antarctic Ice Sheet and struggled to understand how changes in the East Antarctic Ice Sheet could have contributed to the 60-metre variations in sea level revealed in geological records from locations around the world.
The research team drilled into sediments in the Ross Sea, Antarctica, to find layers that corresponded to the coldest and warmest periods of the Miocene.
They found evidence of material deposited by the West Antarctic Ice Sheet at locations that are presently covered by ocean, showing the ice sheet grew much larger than it is today during the coldest periods and it retreated again during warm episodes.
This was only possible as more of the land surface beneath the West Antarctic Ice Sheet was above sea level in the past, and it is easier to grow ice sheets on mountains on land than to grow ice sheets that sit below sea level. Over millions of years, glaciers subsequently eroded West Antarctic below sea level, making ice sheets in this region more vulnerable to melting caused by oceanic warming.
This study confirms the West Antarctic Ice Sheet today is very vulnerable to oceanic and atmospheric warming and cooling, and could raise sea level a significant amount in the future as our planet warms.
“Our observations from the past help inform predictions of how the West Antarctic Ice Sheet, which is considered particularly vulnerable to rapid ice mass loss today, will respond under various future warming scenarios,” lead author Jim Marschalek from Imperial College London said.
Co-author Associate Professor Robert McKay, director of the Antarctic Research Centre at Te Herenga Waka—Victoria University of Wellington, was co-chief scientist on the International Ocean Discovery Program expedition that collected the cores.
“The good news here is that prior to our study, we were concerned that ice sheet models were underestimating loss of ice on land and associated sea level rise in these warmer climates, and therefore could also be underrepresenting ice loss in the projections for future warming,” Dr McKay said.
“The geological information now matches the modelling, and gives us more confidence that the models are capturing the response of both the East Antarctic Ice Sheet and West Antarctic Ice Sheet in the past.”
"One of the key ice sheet models used for our Miocene experiments is also used to project future ice sheet contribution to global sea level,” co-author Associate Professor Richard Levy of GNS Science said.
“This particular model reveals a critical ‘tipping point’ that we are rapidly approaching if we don’t reduce emissions, causing sea level to rise well over a metre by 2100 and much more in the centuries to come.
“Keeping future warming below two degrees, and ideally to 1.5 degrees, is the target to aim for to prevent this,” Dr Levy said.