Reality quells fears of catastrophic ice shelf melt

Beneath the Ross Ice Shelf – click to enlarge

I’ve been informed of significant new research that discovered Antarctic ice shelf melting—constantly referred to as “catastrophic” in warmster commentaries—is considerably less dramatic than we were led to expect—quelle surprise. It even involved a New Zealand scientist but so far this is the only media coverage.

Dr Oliver Marsh, Gateway Antarctica

These results will have significant implications for long-term predictions of sea level rise long past the end of the century, probably reducing them from imagined scores of metres to a few centimetres. The mechanisms, on being observed, turn out to be completely different from initial guesses—which were mostly informed, we imagine, by wide-eyed horror at human climatic interference and a readiness to find it everywhere.

In reality, there’s no evidence that significant atmospheric warming of human origin is finding its way to the underside of any Antarctic ice shelves.

Kiwi scientist

The paper, Ocean Stratification and Low Melt Rates at the Ross Ice Shelf Grounding Zone, was published in Journal of Geophysical Research: Oceans on 25 September. The Kiwi researcher is Dr Oliver J. Marsh, of Gateway Antarctica, University of Canterbury, Christchurch.

Abstract

Ocean-driven melting of ice shelves is a primary mechanism for ice loss from Antarctica. However, due to the difficulty in accessing the sub-ice shelf ocean cavity, the relationship between ice shelf melting and ocean conditions is poorly understood, particularly near the grounding zone, where the ice transitions from grounded to floating. We present the first borehole oceanographic observations from the grounding zone of the Ross Ice Shelf, Antarctica’s largest ice shelf by area. Contrary to predictions that tidal currents near grounding zones mix the water column, we found that Ross Ice Shelf waters were vertically stratified. Current velocities at mid-depth in the ocean cavity did not change significantly over measurement periods at two different parts of the tidal cycle. The observed stratification resulted in low melt rates near this portion of the grounding zone, inferred from phase-sensitive radar observations. These melt rates were generally <10 cm/year, which is lower than average for the Ross Ice Shelf (~20 cm/year). Melt rates may be higher at portions of the grounding zone that experience higher subglacial discharge or stronger tidal mixing. Stratification in the cavity at the borehole site was prone to diffusive convection as a result of ice shelf melting. Since diffusive convection influences vertical heat and salt fluxes differently than shear-driven turbulence, this process may affect ice shelf melting and merits further consideration in ocean models of sub-ice shelf circulation.

Plain language version

Ice shelf melting is an important player in ice loss from the Antarctic Ice Sheet, affecting sea level rise. Ice shelf melting is controlled by ocean properties and processes, but sparse observations of the sub-ice shelf ocean cavity limit our understanding of these controls and thus limit our ability to predict sea level rise. This study presents rare ocean observations deep below the largest ice shelf by area, the Ross Ice Shelf, far from the open ocean. The observed ocean setting is surprisingly quiescent, and waters are cold, around 2 °C. This study also presents new, highly localized ice shelf melting measurements at the site that show that these ocean conditions lead to slow ice shelf melting of only centimeters per year. These observations reveal the ways in which the Ross Ice Shelf contrasts with rapidly melting ice shelves affected by warmer seawater elsewhere in West Antarctica. Thus, they add nuance to our scientific understanding of ice-ocean interactions around the Antarctic continent.

And just for me and others like me, here’s Ice Melting Rates for Dummies from a learned friend:

Ice melting rates at the grounding zone of the Ross Ice Shelf were considerably lower than expected because the observed conditions were not the same as modelled conditions.

Commentary

Science finds the truth, whatever our misconceptions.

History of “catastrophe”

This brief historical review is a bit long, but if you leave it out you’ll be no worse off—certainly I say little 😕 —but if, given time on your hands over summer, you should indulge, you might find it sickeningly fascinating. These items confirm the term “catastrophic” has been applied to predicted events in Antarctica long past the point of brain-washing, though it’s a tiny sample.

1995

Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse (1995, Blanchon & Shaw). Elevations and ages of drowned Acropora palmata reefs from the Caribbean-Atlantic region document three catastrophic, metre-scale sea-level-rise events during the last deglaciation … Such dramatic evidence of catastrophic climate and sea-level change during deglaciation has potentially disastrous implications for the future, especially as the stability of remaining ice sheets—such as west Antarctica—is in question.

2010

The Warming of Antarctica: A Citadel of Ice Begins to Melt (22 November, 2010, Yale Environment 360). If the warming continues, we are eventually going to get to the point where sea ice won’t form any more, and that would be catastrophic to the system.

2011

The measured and the catastrophic: glacial and ice sheet melt (3 May 2011, SciencePoles). And are we still talking about that possibility? Yes we are, but I never talk about it without trying to put a time scale on it. I think it is hard to see West Antarctica deglaciating entirely on anything less than a 500-year time scale. For example, in the past, people have waved around big numbers such as a five metre sea level rise by 2100, as a result of a “catastrophic collapse” of the West Antarctic Ice Sheet. In my view, this would not be possible, and there is hardly any evidence to support this particular scenario. But this is not to say that more modest levels of sea level rise are not to be taken seriously, or indeed do not have very serious implications for low lying regions and their populations.

Is Antarctica headed for a catastrophic meltdown? New evidence of ancient climate change may hold clues (aired 28 December, 2011, on PBS). Almost three miles of ice buries most of Antarctica, cloaking a continent half again as large as the United States. But when an Antarctic ice shelf the size of Manhattan collapsed in less than a month in 2002, it shocked scientists and raised the alarming possibility that Antarctica may be headed for a meltdown. Even a 10 percent loss of Antarctica’s ice would cause catastrophic flooding of coastal cities unlike any seen before in human history.

2013

Active Volcano Found Under Antarctic Ice: Eruption Could Raise Sea Levels (19 November, 2013, National Geographic). “We’re not talking about an eruption causing the ice sheet to melt and cause catastrophic sea-level rise,” Lough told National Geographic. “This volcanic complex has been operating for millions of years … There have been past eruptions of this system and the ice has survived for millions of years, [so] future eruptions alone will not cause the ice sheet to fail.” However, the heat from the volcano could increase melting at the base of the glacier and meltwater could act like a lubricant that makes the overlying ice flow out to sea faster. Global sea levels could rise by a small amount as a result. [Thanks, I’m not worried. Why did he say catastrophic?]

2014

5 Myths About Antarctic Melt (13 May, 2014, LiveScience). News that the catastrophic collapse of the West Antarctic ice sheet is already under way, like other big reports about the southernmost continent, spur chatter and questions about what’s really happening at the bottom of the world.

Catastrophic collapse of Antarctic ice sheet now underway, say scientists (13 May, 2014, Christian Science Monitor). The biggest glaciers in West Antarctica are hemorrhaging ice without any way to stem the loss, according to two independent studies. The unstoppable retreat is the likely start of a long-feared domino effect that could cause the entire ice sheet to melt, whether or not greenhouse gas emissions decline. [I especially liked: “One study tracked the region’s biggest glaciers for 40 years, and concluded from direct observations that the ice is unstoppable.” They’re on to it, I thought. They know not to stand in the way of millions of tons of ice. ALSO: “‘One of the most striking features is they have been reacting almost simultaneously,’ Rignot said. ‘We do think this is related to climate warming.'” Yes, the question is, reacting to what? But it’s a stretch, Dr Rignot, to say simultaneity is a reaction to climate.]

2015

Why scientists are so worried about the ice shelves of Antarctica (12 October, 2015, The Washington Post) “Increases in air temperature, and surface melt and ponding, has led to the abrupt and catastrophic collapse of a number of ice shelves,” says Trusel, lead author of the study, which was published Monday in Nature Geoscience.

2016

Antarctic Ice Sheet Melt Could Cause Catastrophic Sea Rises (31 March, 2016, Environmental Monitor). Already melting in response to global warming, the Antarctic ice sheet may collapse if greenhouse gases aren’t curbed, scientists warn in a release from Victoria University of Wellington. If the ice sheet collapses, the sea level could rise by 60 meters, researchers found.

2017

‘Catastrophic collapse’ of West Antarctic ice sheet could raise global sea levels by three metres, warns scientist (13 July, 2017, The Independent). Climate change and the hole in the ozone layer could cause “a catastrophic collapse” of the vast amount of ice on west Antarctica, raising sea levels by 3.3 metres, a leading scientist has warned.

2018

Antarctic ice melting faster than ever, studies show (13 June, 2018, The Guardian). Ice in the Antarctic is melting at a record-breaking rate and the subsequent sea rises could have catastrophic consequences for cities around the world, according to two new studies.

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