Dueling Studies Show Antarctic Uncertainty

May 6, 2021

Two new studies showing that if you are uncertain about what is exactly going on in Antarctica, and what it means for long term sea level, you are pretty much keeping up with the science.

Washington Post:

Scientists struggling to understand the threat of sea level rise on a warming Earth found Wednesday that amid lingering uncertainty, this much is clear: Meeting the goals of the Paris climate agreement remains humanity’s best hope for preserving current coastlines in the 21st century.

At the same time, they diverged over the risks posed by the biggest wild card, the Antarctic ice sheet, which contains by far the most ice on the planet and holds the potential to unleash tens of feet of sea level rise.

Ice losses from Antarctica have been accelerating in recent years, and research suggests that in warm periods in the Earth’s past (similar to the one that humanity is now fueling), the ice sheet shed a great deal of its mass. But a central issue is how fast that could occur this time around and whether today’s computer simulations can adequately capture what will really happen, especially during the lifetimes of people currently living.

Two studies published Wednesday in the journal Nature underscore how the answers to that complex scientific puzzle remain unsettled.

In one study, a group of 84 international experts using hundreds of simulations found a relatively muted Antarctic response as the climate warms in coming decades. That’s largely because a rise in snow falling on the ice sheet could substantially offset the loss of ice to the ocean at the continent’s perimeter, the study says. Only a minority of models, the scientists noted, produced a more alarming response.

(I spoke to British Antarctic Survey expert Jonathan Bamber on this question in 2017)

Meanwhile, in a second study, a smaller group of experts published a model that included an additional process known as “marine ice-cliff instability” that factors into the potential for faster collapses of ice from large glaciers perched against the ocean. This research found that a startling amount of sea level rise could result if global warming reaches about three degrees Celsius (5.4 degrees Fahrenheit) above preindustrial levels. The world already has exceeded one degree Celsius of warming.

(see top for my talk with Rob DeConto, one of these current authors, from 2017 also)

At this point, the authors found, ice losses could greatly accelerate beginning in the second half of this century and extending well beyond it, with particularly rapid sea level rise in the 2100s and 2200s.

“For managing coastal flooding, we still need to stay really flexible because we haven’t pinned down that uncertainty in future sea level rise,” said Tamsin Edwards, an expert on the Antarctic ice sheet at King’s College London and lead author of the first study. “We need to be able to adapt to a wide range.”

Rob DeConto, a researcher at the University of Massachusetts at Amherst, who led the latter study with scientists at institutions in the United States, the United Kingdom, Canada and China, agreed that the future remains unwritten, and said what happens will depend in part on decisions humans are making now.

“We need to worry about the next century if we do let emissions stay unchecked,” he said, adding that a failure to combat climate change could result in “globally catastrophic levels of sea level rise where you’re talking about remapping the global coastline.”

The two studies suggest a difficult task for the United Nations Intergovernmental Panel on Climate Change (IPCC), the official expert body whose sixth sweeping report on our understanding of climate change is expected later this year. Those findings will inform the policies that many world leaders pursue in the years ahead.

The treatment of sea level rise by the IPCC has been a persistent source of controversy. In the last report in 2013, for instance, authors provided a “likely” range for sea level rise by the year 2100, with a high-end number of just under a meter in a world with very high greenhouse gas emissions. But the report also noted that Antarctica could upend what is currently considered “likely.”

“Only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century,” the body noted then.

“The major uncertainty in global sea level rise is what the Antarctic ice sheet is going to do,” Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University who led a 2019 IPCC report chapter on the sea level rise, said in an interview. “These papers are a strong reminder that our understanding of the future of the ice sheet — our ability to estimate what its contribution to sea level rise is going to be — remains quite uncertain.”

That’s okay, he added. Science often means only incremental discovery and consensus. “Understanding of it doesn’t happen overnight,” Oppenheimer said. “People shouldn’t expect miracles; this is slow work.”

Another noted climate expert, who also was not involved in either study, agreed.

“Taken together, these studies by a large collection of the world’s top ice sheet experts show once again that ice sheet instability remains a wild card of climate change risk,” said Stefan Rahmstorf, an oceanographer at the Potsdam Institute for Climate Impact Research in Germany.

“They show there is still large uncertainty how the ice sheets will respond to further global heating — and uncertainty is not our friend here. We know from Earth’s history that warming can lead to rapid ice sheet decay and large sea level rise.”

To understand why the Antarctic matters so much, and why the ambiguity surrounding it matters, it helps to break down the sea level rise that the Earth will see in the year 2100 into its main components.

The study Wednesday by Edwards and her colleagues, finds that mountain glaciers and Greenland are indeed very closely tied to temperature, meaning that cutting emissions and curtailing the Earth’s warming has a powerful effect. Sea level rise from these two key contributors could be held to around 3½ inches by the year 2100 if the world manages to drastically limit warming in line with the most ambitious aims of the Paris agreement.

But then there is the question of Antarctica.

Here, models in Edwards’s study found an average of about 1½ inches of sea level rise by 2100 — even in cases in which the world unleashed relatively high levels of greenhouse gas emissions. This result, which assumes large amounts of additional snowfall blanketing Antarctica and substantially offsetting the melting of ice by the oceans, would seem to diverge from what we know about the Earth’s past, when parts of Antarctica appear to have collapsed during ancient warm periods. But it could be that such collapses simply take more than a century to actually occur, even with warm temperatures and high greenhouse gas levels in the atmosphere.

Still, a few models included in Edwards’s study produced more extreme results, including very high sea level rise from the Antarctic even with quite low emissions. In those cases, this rise averaged an additional eight inches or more by the year 2100, just from the Antarctic.

None of these models included a process known as the “marine ice-cliff instability,” which some experts believe is already underway in parts of Greenland and that could someday affect the Antarctic. It refers to the idea that larger and thicker glaciers can become chronically unstable once their ice shelves shatter and crumble into the ocean, leaving behind tall and fragile cliffs overlooking the ocean.

These are “potentially game-changing processes in a warming world, with a warmer ocean surrounding Antarctica, and a warmer atmosphere,” said DeConto.

The study, which does include this instability, suggests an Antarctic trigger may exist somewhere around three degrees Celsius of warming. Holding warming to the Paris agreement targets of “well below” two degrees Celsius, compared with preindustrial levels, generally only produced a few inches of sea level rise from Antarctica by 2100 in his simulations. But higher levels of warming began to generate much bigger losses in some simulations, leading to dramatic ice retreat in the region of West Antarctica.

“These results demonstrate the possibility that rapid and unstoppable sea level rise from Antarctica will be triggered if Paris agreement targets are exceeded,” the study states.

Experts evaluating the complex picture presented by the two studies homed in on one underlying conclusion. That is, even if we don’t understand the full extent of Antarctica’s vulnerability or how easily it can be triggered, the world would be wise to reduce emissions as sharply as possibly, and as soon as possible.

“All we know is we are heading into dangerous territory,” Oppenheimer said, “and we ought to back off.”

8 Responses to “Dueling Studies Show Antarctic Uncertainty”

  1. grindupbaker Says:

    “group of 84 international experts using hundreds of simulations …. a smaller group of experts published a model”. So some definite disagreement. And then there’s also a “climate change scientist” “Paul Beckwith” who states confidently to his acolytes that the ice sheets will lose 2,520,000 billion tonnes of ice (net mass reduction) over the next 50 years. So even more disagreement. You’all guess which scientist type the public studies with and hangs on their every word and random thought. What a Gong Show.

  2. grindupbaker Says:

    Washington Post: By Chris Mooney “Antarctic ice loss has tripled in a decade. If that continues, we are in serious trouble”. Well yes, absolutely, because at 2100 AD that is going to lead to the loss of 11,809,800 billion tonnes of ice per year and causing 32.8 metres (107 feet 7 inches) per year of globally-averaged sea level rise (SLR) because 3**10 = 59,049. In fact, the entire present Antarctic ice sheet is going to be lost into the ocean 3.5 times over the deacde 2090-2100, which is very troubling when planning coastal defences.
    Note to Washington Post: and Chris Mooney:
    If Antarctica is losing 1 billion tonnes per year for a decade and next decade it’s losing 200 billion tonnes per year then your headline is “Antarctic ice loss is 200 times as fast as 10 years ago. If that continues, we are in serious trouble”
    If Antarctica wasn’t losing any ice and next decade it’s losing 1 billion tonnes per year then your headline is “Antarctic ice loss is infinitely faster than last decade. If that continues, we are in serious trouble”
    If it starts with a negligible rate of change in quantity and then the rate increases then, obviously, it’s always going to be “N times as fast as it was a shoprt while ago !!!! Breaking news !!!” where N is a large number.
    Once the change in quantity has become sizeable then from then on . errrrr …. not so much of a large N ratio you pile of scam artists.
    Exact same thing done with Greenland “7 times as fast as before” you pile of scam artists.
    This constant playing with statistics in order to provide mis-leading dis-information rather than presenting the facts clearly, to inform, is sickening.
    It’s supposed to be the coal/oil shills, the opposing team, who pull this crap. They might have copyrighted the procedure.
    There’s a “climate change scientist” “Paul Beckwith” who does this exact same dis-information crap on the exact same topic.
    Was Greenman a journalist for while way back when and does he understand their over-arching need to make wealth and their minor interest in presenting science with any degree of rigor and ethic whatsoever ?
    Sheesh but the need for stupid, pointless, un-informative clickbait is endlessly sickening. Washington Post eh.

    • redskylite Says:

      Tried to look at Chris Mooney’s Washington Post article that you refer to from 2018/06/13 – pity, alas like the New York Times, the Washington Post is now behind a pay wall, I do not want to subscribe so I can’t comment on his piece. Agree that the news media should stick to mainline science and not pounce on sensationalism, always regarded The Washington Post and Chris Mooney as one of the fairer media outlets. I have a very high regard for Stefan Rahmstorf and the Potsdam Institute for Climate Impact Research in Germany. Basing estimates on modelling will always be contentious and no replacement to fieldwork and actual observations.

      What is for sure we need to curtail our industrial CO2 and Methane emissions, for the future of island dwellers, low lying coastal dwellers, and to preserve our precious marine stocks. Don’t need a model to tell us that.

      “Catastrophic Sea-Level Rise from Antarctic Melting is Possible with Severe Global Warming ”


    • grindupbaker Says:

      OK so I typed “deacde 2090-2100” when it S.B. decade 2110-2120 for the next 100 years because I got so annoyed with all this “Now with 17 times as many raisins !” “Now 9 times faster than before ! ” “Now 14 times as cheap as last week !” stuff that I totally forgot what year and Millenium it is now. That’ll happen.

    • rhymeswithgoalie Says:

      I found your complaint largely incoherent.
      Is it about scientific findings?
      Is it about media coverage?
      Is it about not clarifying that melt rates “don’t grow to the sky” so to speak?
      Do they need to spend more time describing Punctuated Equilibria?
      Is it about wording of headlines?
      Do you share my dismay of attempts to soft-pedal the information by always adding “but it’s not too late”?

  3. Brent Jensen-Schmidt Says:

    Interesting stuff with practical application to planning. Then again it melts, whether fast or slow, and by what mechanism, is academic until proven. Should one be skeptical, what is an acceptable level of risk? Considering the consequences, no level is ‘acceptable’. Unto the breach dear friends. HAGO

  4. neilrieck Says:

    This scientific paper from Denmark has not yet been published but is still shocking to me: https://os.copernicus.org/articles/17/181/2021/

    Why? The first line of the abstract reads “Recent assessments from the Intergovernmental Panel on Climate Change (IPCC) imply that global mean sea level is unlikely to rise more than about 1.1 meters within this century but will increase further beyond 2100”.

    This statement infers that one meter (39 inches) is within the scope of possibility. Let’s say that the actual rise will be have of this value. I do not need to mention that ANY rise is vertical but beaches are sloped. This means that the damage from flooding and storm surges will increase much sooner. While it is true that science-deniers may dismiss this warning, insurance companies will not.

    • rhymeswithgoalie Says:

      I think there’s a major communications problem associated with “global mean sea-level rise”, because, while it is an intellectually interesting scientific metric, sea level only matters as a local measurement.

      Here is a subset of the types of local conditions that affect sea-level rise:

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