Ice sheet, shelves. Photo: Chris Rudge
Global mean temperatures are rising and are forecast to do so for many decades or centuries, if greenhouse gas emissions are not significantly and rapidly reduced. The world’s ice sheets are sensitive to these changes and, as the largest reservoir of freshwater on Earth, there is significant potential for the Antarctic ice sheets to affect future sea levels and disrupt global ocean circulation.
To determine ice sheet response to warming, we examine environmental records of how the Antarctic ice sheets and surrounding ocean have changed in the past. Three recent publications (below) contextualize present ice sheet dynamics and aid our ability to predict future changes under emissions scenarios.
From new sea surface temperature reconstructions from the Ross Sea and offshore Adelie Land, the team demonstrated the strong connection between atmospheric carbon dioxide concentrations and Antarctic ice sheet volume during the Cenozoic Era, but with tectonic-driven differences in sensitivity between the late Oligocene and Miocene.
The results are consistent with the concept of a threshold response at atmospheric CO2 values of ~400 ppm. Above this threshold, ocean warming at Antarctica’s margin greatly exacerbates marine ice-sheet retreat into interior subglacial basins, with profound consequences for global sea level.
Sedimentological and palaeomagnetic records from a 6.21-metre-long sediment core spanning the last 1.1 million years tracks past ice sheet changes in the Ross Sea region of Antarctica. The team showed that the advance and retreat of the West Antarctic Ice Sheet is primarily paced by 41,000-year-long obliquity cycles until at least 400,000 years ago.
These cycles suggest that high-latitude solar insolation influenced Southern Ocean heat uptake and continued to be the main pacemaker of Antarctic glaciations well into the late Pleistocene. This study reconciles the historical mismatch between the inferred glacial cycles and the solar insolation record.
Past climate changes in the McMurdo Dry Valleys were investigated using geochemical analyses of mid-to-late Miocene-aged soils. This time period is useful for understanding long-term climate responses to modern CO2 levels. At some point from the mid-to-late Miocene, the regional climate became especially cool and hyper-arid. But this new reconstruction indicates that high elevations of the McMurdo Dry Valleys experienced warm and wet climatic intervals later than expected.
This suggests that the McMurdo Dry Valleys may be more susceptible to climate change than anticipated, possibly becoming warmer and wetter in the future.
The first reconstruction of ice surface lowering at Byrd Glacier, the largest outlet glacier of the Transantarctic Mountains, was developed by dating the time of surface exposure of glacial erratic cobbles. The results show a rapid pulse of ice sheet thinning ~8,000 years ago. This reconstruction was compared with ice sheet model experiments to better identify key processes responsible for ice sheet change since the Last Glacial Maximum and assess model skill. The model experiments that best match the thinning profiles show higher basal friction. This finding gives us important context for future projections of ice sheet change in the Ross Sea region.
Millennial-scale cycles (thousands of years) are extensively documented in paleoclimate proxies and linked to complex Earth system feedbacks associated with dynamic behaviour of large continental-scale ice sheets at both poles. From ANDRILL Core 2A, the team identified millennial-scale variability in the East Antarctic ice sheet in the early Miocene, a time characterized by warmer climates, higher atmospheric pCO2, and no large Northern Hemisphere ice sheets. These patterns likely arise from nonlinear interactions between the atmosphere, ocean, and ice that appear to have characterized Earth’s cryosphere for at least the past 20 My.
Ross Ice Shelf Photo: Nicola Holmes