Climate Change News

Scientists Find the Point of No Return for Antarctic Ice Cap

Varying amounts of glacial debris in a core of ancient sediment show the ice cover grew and shrank until airborne carbon dioxide levels fell below 600 parts per million, spurring steady growth.


The Antarctic ice cap can withstand only so much carbon dioxide in the atmosphere, and scientists might have found this limit. In a new paper published today in Science, researchers present the first physical evidence that conditions about 33 million years ago may have led the vast southern ice sheet to become a massive, stable block.

A million or so years before then, the Earth was much hotter than it is today, and atmospheric carbon dioxide (CO2) concentrations exceeded 750 parts per million (ppm)—much higher than they are today. But when CO2 concentrations declined to 600 ppm, the southern ice cap became much more long-lived.

“What we see now is that the strengthening of the ice sheet works only with CO2 levels below 600 ppm,” said Simone Galeotti, a paleoclimatologist at the University of Urbino, Italy, and lead author on the paper. He added that if we reach those levels again by continuing to pump greenhouse gases into the atmosphere, humans could sentence the ice sheet to an inevitable collapse.

Icehouse Planet

When the Antarctic ice cap started forming about 35 million years ago, it was a tumultuous time in Earth’s past during which a lot of carbon, mainly in the form of carbon dioxide, quickly left the atmosphere. Among several factors contributing to this carbon shift, the Antarctic continent peeled away from South America and Australia, forming the Southern Ocean. Now unencumbered by land, strong westerly winds were free to blow around Antarctica, pushing water northward and pulling water from great depths.

Upwelling of this deep, nutrient-rich water led to a flurry of biological activity in the Southern Ocean for the first time. Blooms of phytoplankton pulled CO2 from the atmosphere as organisms made their shells. As the organisms died, this carbon became trapped in the ocean.

This sinking of carbon into the ocean helped to plunge the Earth into an “icehouse” phase—as opposed to its previous “greenhouse” phase—and glaciers on Antarctica bloomed.

Clues in Ocean Sediment

To investigate the behavior of Antarctica’s ancestral ice sheet, the researchers turned to a sediment core bored into the bottom of the Ross Sea 16 years ago. The core contains many layers, and the isotopic composition of these layers reflects Earth’s natural heating and cooling phases. Known as Milankovitch cycles, those phases result from changes in Earth’s orbit and the tilt of its rotational axis.

The core also contains evidence of ice advance and retreat, both in the types of sediment present and in patterns within the layered material indicating what sort of erosion took place at the time. In the layers corresponding to ice advance, the researchers found larger pieces of sediment, or clasts, that the growing ice transported. In the layers corresponding to ice retreat, the researchers found more marine sediment and other types of features typical of a continental shelf lacking ice above.

The researchers compared the layers in the sediment core with an already existing data set of CO2 concentrations taken from ancient plankton shells in marine sediment so they could pinpoint the state of the ice sheet as CO2 levels changed. When CO2 concentrations were above 750 ppm, around 35 million years ago, some glaciers likely existed at high elevations but they didn’t extend to the ocean. But when CO2 concentrations fell between 750 and 600 ppm, the fledgling ice sheet was unstable and highly responsive to the Milankovitch cycles, extending and retreating over 20,000- to 40,000-year cycles, the team reported.

When the CO2 concentration reached 600 ppm 32.8 million years ago, the number of cycles of advance and retreat dropped, implying the existence of a widespread, stable ice sheet that was less responsive to Milankovitch cycles and that extended and retreated on 100,000- to 400,000-year timescales.

“When CO2 dropped below 600 ppm is when we see first evidence of the ice sheet expanding and growing into the ocean and onto the continental shelf,” said coauthor Timothy Naish, a glaciologist and director of the Antarctic Research Centre at the Victoria University of Wellington in New Zealand.

“This paper strengthens our understanding that CO2 affects climate including ice and sea level, that smaller ice masses respond more rapidly, and that large ice-sheet changes affect other aspects of the climate and in turn influence themselves,” said Richard Alley, a glaciologist at the Pennsylvania State University in University Park, who wasn’t involved in the new research.

Antarctica’s Future

The current concentration of atmospheric CO2 sits well below this threshold; it recently surpassed 400 ppm. Although recent research has found signs that the West Antarctic ice sheet is already in irreversible decline, today’s research reveals that the entire ice sheet could revert back to its unstable roots if the 600-ppm threshold is crossed once again—like replaying the ice sheet’s evolution backward through time.

Models run by the Intergovernmental Panel on Climate Change show that this threshold could be reached by the end of the century. However, Naish emphasized that even if we were to reach this crucial threshold, it would be thousands of years before the ice sheet melted completely. Although many other greenhouse gases contribute to warming, he added, they would contribute little to this potential deterioration of the Antarctic ice cap because they remain so briefly in the atmosphere compared with CO2, which can last for centuries.

At the 600-ppm threshold, Naish continued, we start to commit to continent-wide ice loss that “we can’t stop.”

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2016), Scientists find the point of no return for Antarctic ice cap, Eos, 97, doi:10.1029/2016EO047929. Published on 10 March 2016.

© 2016. The authors. CC BY-NC 3.0
  • VooDude

    “The Antarctic ice cap can withstand only so much carbon dioxide in the atmosphere, and scientists might have found this limit”

    Late last year: ”❝The good news is that Antarctica is not currently contributing to sea level rise, but is taking 0.23 millimeters per year away,❞ Zwally said.”

    ”Mass changes of the Antarctic ice sheet … Satellite (ICESat) data (2003–08) show mass gains from snow accumulation exceeded discharge losses by 82 ± 25 Gt a–1, reducing global sea-level rise by 0.23 mm a–1. European Remote-sensing Satellite (ERS) data (1992–2001) give a similar gain of 112 ± 61 Gt a–1. …”

    Zwally, H. Jay, et al. 2015 “Mass gains of the Antarctic ice sheet exceed losses.” Journal of Glaciology

    Antarctica is a really big place. It is divided up into the (huge) East Antarctica, the (small) West Antarctica, and, depending upon whom is speaking, “the Antarctic Peninsula” (which is in the west, anyway).

    The HUGE Eastern portion has been accumulating ice, perhaps enough that it absorbs all the ice loss in the Western and the Peninsula portions:

    “… the Antarctic ice sheet [actually] showed a net gain of 112 billion tons [Gt] of ice a year from 1992 to 2001. That net gain slowed to [an increase of only] 82 billion tons [Gt] of ice per year between 2003 and 2008. … Our main disagreement is for East Antarctica and the interior of West Antarctica – there, we see an ice gain that exceeds the losses in the other areas.”

    ”Mass changes of the Antarctic ice sheet … Satellite (ICESat) data (2003–08) show mass gains from snow accumulation exceeded discharge losses by 82 ± 25 Gt a–1, reducing global sea-level rise by 0.23 mm a–1. European Remote-sensing Satellite (ERS) data (1992–2001) give a similar gain of 112 ± 61 Gt a–1. …”

    Zwally, H. Jay, et al. 2015 “Mass gains of the Antarctic ice sheet exceed losses.” Journal of Glaciology

    Thomas 2015: ”… 300  year records of snow accumulation from two ice cores drilled in Ellsworth Land, West Antarctica. The records show a dramatic increase in snow accumulation during the twentieth century, ”

    ”In the Antarctic Peninsula models reveal an upward trend in regional precipitation since 1979 [Lenaerts et al., 2012; van den Broeke et al., 2006], an increase in elevation (1992–2003) [Davis et al., 2005], and an increase in ice core derived snow accumulation [Thomas et al., 2008]. Conversely, in West Antarctica no trend in either measured or modeled snow accumulation is observed between 1980 and 2009 on Thwaites Glacier [Medley et al., 2013], while in central West Antarctica observed and simulated records show a negative trend in accumulation rates during this period [Burgener et al., 2013].”

    E. R. Thomas, J. S. Hosking, R. R. Tuckwell, R. A. Warren, and E. C. Ludlow 2015 “Twentieth century increase in snowfall in coastal West Antarctica” Geophysical Research Letters

    Callens 2015: ”For the Antarctic ice sheet, … we compute the mass budget of major outlet glaciers in the eastern Dronning Maud Land sector of the Antarctic ice sheet … This approach is an improvement on previous studies, as the ice thickness is measured, … In line with the general thickening of the ice sheet over this sector, we estimate the regional mass balance in this area at +3.15 ± 8.23 Gt/a according to the most recent SMB model results.”

    ”A significant issue of mass change estimation is that none of the methods presently used are free from significant errors, and all rely on either models or approximations (Shepherd and others, 2012).”

    ”Satellite gravimetry [GRACE] and altimetry (e.g. Gunter and others, 2009) measure the absolute mass change, but rely on a glacial isostatic adjustment (GIA) model, while altimetry also suffers uncertainty, due to the densification process … these methods struggle to provide good estimates, because a small error in the GIA model will introduce large relative errors in the results (Hanna and others, 2013)..”

    ”Lenaerts and others (2012) compare several datasets and identify a discrepancy up to 15%, which is >300 Gt/a for the whole Antarctic ice sheet.”

    ”According to the latest model and thickness measurements near the grounding line, this part of Antarctica gains 3.15 Gt ice/a. However, given the relatively large uncertainties and discrepancies in the SMB, this value needs to be treated with caution.”

    Callens, Denis, et al. 2015 “Mass balance of the Sør Rondane glacial system, East Antarctica.” Annals of Glaciology

    Shoen 2014: “… an overall positive trend in SMB over the whole continent.”

    “We conclude that there was no statistically significant net loss or gain in the seven year period.”

    Schoen, Nana, et al. 2014 “Spatio-temporal modelling of Antarctic mass balance from multi-satellite observations.” EGU General Assembly Conference Abstracts

  • VooDude

    “…forcing relocation of villages that have existed in the same spot for over 1,000 years…” Post details. Which village?

    • VooDude

      ”…slow-moving environmental catastrophe undercutting houses and boardwalks within the Yup’ik village of Newtok.”

      ”Unfortunately, widespread melting of permafrost has not occurred, and Newtok’s problem, while exceptionally dire, didn’t occur due to warming climate.”

      ”Like many Alaska Natives, the people of Newtok once lived a semi-nomadic life, moving from summer fish camp to seal-hunting camp to winter digs.”

      ”Where they never lived during spring, summer and fall was at the site of the village’s present location, along the Newtok River north of the wide, swift and bank-eating Ninglick River. Areas like this — near fresh water on tundra — were always used as winter camps, when the tundra froze as solid as concrete. During the season of melt, … the people moved to drier slopes and rocky beaches, closer to herring fishing and seal hunting. …

      A half century ago, someone at BIA sent a barge with materials for a school. Where the barge could offload, the village site got anchored. More permanent facilities followed. As the decades passed, a locale suitable for winter camp became a year-round American townsite, complete with infrastructure that now faces destruction from mostly natural causes.”

      Yup’ik village of Newtok.

      ”Barrow, Alaska … America’s northernmost city (pop. 4,500), … at at the junction of the Chukchi and Beaufort seas.”

      ”The city’s small wooden homes were built on pilings to keep them from melting the permafrost, which would cause them to sink.”

  • Alberto Enriquez

    The question is: will the accelerating climate change we’ve already set in motion drive us inevitably toward ever higher levels of atmospheric CO2 and methane––and past the tipping point toward unstable climates or severe global warming?
    We are already seeing several factors accelerating us toward that outcome. Methane is coming out of the permafrost and the deep oceans at an accelerating rate; the oceans are becoming dramatically more acid (30%!) reducing the ability of marine life to survive and to sequester carbon long-term as carbonate; the increased melting of the polar ice cap is greatly increasing the amount of heat captured by the Arctic; and all of these systems represent positive feedbacks toward greater warming.
    In plain language, it’s not about where we’re at. It’s about whether we can still turn back.

    • rocdoctom


    • VooDude

      No, the increased melting of the polar ice caps is not causing much… The earth does not heat up as a result of Arctic sea-ice melting. Sure, it warms the upper layer, and inhibits ice formation, but it isn’t what you suggest.

      The earth, in a basic concept, heats up at and near the equator, and transports that heat to the poles, via ocean currents and air currents. At any moment, the warm waters that were brought to the Arctic, in currents like the “Gulf Stream” or Florida currents, contains enough heat to melt the Arctic Sea Ice. It doesn’t, much of the time, because the warm ocean waters are quite salty, and the fresher water nearer the top is butt-cold, which insulates the sea ice from the warmth below the thermohalocline.

      When the sea-ice in the Arctic is gone, it improves the heat transfer from the ocean to the atmosphere by two orders of magnitude … 100X … this vents heat to space.

      NSIDC: “Less ice also contributes to higher air temperatures by allowing transfer of heat from the relatively warmer ocean.

      ”… the release of heat to the atmosphere from the open water is up to 100 times greater than the heat conducted through the ice.”

      Zwally, H. Jay, et al. Antarctic sea ice, 1973-1976: Satellite passive-microwave observations. No. NASA-SP-459. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON DC, 1983.

      • VooDude

        EOS prevents me from commenting further…

  • john n-g

    “Models run by the Intergovernmental Panel on Climate Change show that this threshold could be reached by the end of the century.”

    This is incorrect. Future CO2 concentration is fundamentally an input parameter to the climate models, and is only an output parameter to the extent that model-input emissions have been tuned to produce future CO2 concentrations that approximately coincide to the Representative Concentration Pathways. A correct statement would be:

    “Many scenarios of future global development, including most scenarios without CO2 mitigation, show that this threshold would be reached by the end of the century.”

    • VooDude

      “…Future CO2 concentration is fundamentally an input parameter …”
      No, it isn’t. Models can run that way, but, for the IPCC, they are set at a given “scenario”.

      Most models’ simulations (during concentration-driven scenarios) do not include any feedback, from CO2 fertilization of plants, or changes of carbon stored in the oceans, since the atmospheric concentration of CO2 is defined, and fixed, by the scenario.

      ”Technically, there is no carbon cycle feedback in concentration-driven simulations [CMIP5], since changes in the amount of carbon stored in the ocean and on land do not influence the atmospheric CO2 concentration. ”

      ”…none of the models considered here, implement a sensitivity of biological production to increasing carbon availability …”

      Schwinger, Jörg, et al. 2014 “Non-linearity of ocean carbon cycle feedbacks in CMIP5 earth system models.” Journal of Climate