Greenland’s Demise – Some Clarification

August 17, 2020

A new paper about Greenland has been released with headlines about “Greenland’s point of no return”.
We are definitely in trouble, but…don’t wave the white flag just yet.

Paleo Climate expert Stefan Rahmstorf and NASA’s Gavin Schmidt have a number of useful qualifiers – a lot to digest here, so I’m going to bookmark this one and keep as a reference:

Stefan Rahmstorf on Twitter:

News about the death of the Greenland Ice Sheet is greatly exaggerated. What the new paper shows is an observed increase in solid ice discharge into the ocean, which has been at a higher rate than before during the past 15 years. The authors basically say: if this continues, ……then the Greenland Ice Sheet will continue to lose mass. 

But due to that, the ice front retreats from the ocean, as nicely shown e.g. in Andy Aschwanden’s detailed simulations (Above).

And when the ice loses contact with the ocean, the ice discharge stops.

The tipping point for losing the Greenland Ice Sheet altogether therefore depends on surface melt permanently exceeding snowfall and does not involve solid ice discharge. Whether this real tipping point of Greenland has been crossed, we do not know.

Thus, Greenland ice loss is a real concern – already today it contributes a lot to sea level rise. And Greenland does have a tipping point where the ice sheet becomes doomed to total loss. But the new paper by King et al. does not say anything about whether this has been crossed.

Gavin Schmidt on Twitter:

A lot of interest in the Greenland ice loss study from King et al. 

Dynamic ice loss from the Greenland Ice Sheet driven by sustained glacier retreat
The Greenland Ice Sheet is losing mass at accelerated rates in the 21st century, making it the largest single contributor to rising sea levels. Faster flow of outlet glaciers has substantially contri…https://www.nature.com/articles/s43247-020-0001-2

To be clear, this is not good news. Greenland ice sheet is demonstrably out of balance with current temperatures. But it doesn’t (necessarily) presage the collapse of the whole thing.

This is an analysis of 35 years of data, not a modeling study, and so while it can do a good job at attributing the current rates of loss to dynamic responses of the ice sheet, it says nothing about where the process would end up in the future under any plausible scenario.
Nonetheless, it is a very valid question (and subject of much research) to ask at what point the Greenland ice sheet is unviable.

From the Pliocene records, we know that a global mean of ~3ºC above the pre-industrial does not seem to be compatible with a substantial GIS.From the history of past interglacials, with differing temperature changes, the ice sheet responded to different extents: 

A low climate threshold for south Greenland Ice Sheet demise during the Late PleistoceneUnderstanding how warmer climates affected Greenland in the past helps in determining how future warming will impact it. The Greenland Ice Sheet (GIS) has retreated during recent interglacials, sugge…

https://www.pnas.org/content/117/1/190

From the last interglacial, we suspect that an Arctic warming of > 6ºC is compatible with a loss of ~1/3 of the GIS (ie. ~2m of global sea level rise). (fig from royalsocietypublishing.org/doi/pdf/10.109…)

Image

A low climate threshold for south Greenland Ice Sheet demise during the Late PleistoceneUnderstanding how warmer climates affected Greenland in the past helps in determining how future warming will impact it. The Greenland Ice Sheet (GIS) has retreated during recent interglacials, sugge…https://www.pnas.org/content/117/1/190

Note that the polar amplification differs between the Pliocene and LIG (Last Inter-Glacial) because of the importance of the orbital forcings in the latter case.

So what do models suggest for the sensitivity of the ice sheet as a whole? History suggests there’s unlikely to be a single ‘tipping point’, but rather a series of non-linear steps to any new state.

There’s a summary of this in the IPCC sp.
rep: ipcc.ch/srocc/ (ch 2+3)
This modeling study looked in detail at the thresholds involved, suggesting that the threshold for complete loss is somewhere between 1º and 3ºC above pre-industrial (best estimate was ~1.6ºC); 

Multistability and critical thresholds of the Greenland ice sheet

A comprehensive stability analysis shows that the critical global temperature rise that leads to collapse of the Greenland ice sheet is only 1–2 °C above the pre-industrial climate state, which is si…https://www.nature.com/articles/nclimate1449

But that was a single (relatively coarse) model and we are still waiting on more comprehensive models to weigh in. Note that to get Arctic temps comparable to the LIG (when 1/3 of the ice sheet was lost), we’d expect global mean changes of ~2 to 3ºC.

None of this should suggest complacency, and indeed there is a lot more work to be done (and please link to any papers I missed below), but the current observations don’t on their own imply the inevitability of Greenland collapse – and certainly not this century.

Example of coverage:

CNN:

Greenland’s ice sheet has melted to a point of no return, and efforts to slow global warming will not stop it from disintegrating. That’s according to a new study by researchers at Ohio State University.

“The ice sheet is now in this new dynamic state, where even if we went back to a climate that was more like what we had 20 or 30 years ago, we would still be pretty quickly losing mass,”

Ian Howat, co-author of the study and a professor at Ohio State University, said. Greenland’s ice sheet dumps more than 280 billion metric tons of melting ice into the ocean each year, making it the greatest single contributor to global sea level rise, according to Michalea King, the lead author of the study and researcher at Ohio State University.

 The ice loss has been so massive in recent years, she said, that it has caused a measurable change in the gravitational field over Greenland.

Ice melting in Greenland contributes more than a millimeter rise to sea level every year, and that’s likely to get worse. Sea levels are projected to rise by more than 3 feet by the end of the century, wiping away beaches and coastal properties.Coastal states like Florida, and low-lying island nations are particularly vulnerable.

Just 3 feet of sea level rise could put large areas of coastline underwater. Forty percent of the US population resides in coastal areas that are vulnerable to sea level rise. 

“There’s a lot of places, like in Florida especially, where one meter alone would cover a lot of existing land areas,” King said. “And that’s exacerbated when you get storms and hurricanes and things like that, that then cause extra surge on top of a higher baseline.”

As Eric Rignot has said more than once, it may not be time yet for coastal dwellers to run for the hills, – but they might want to start walking…

UPDATE:

9 Responses to “Greenland’s Demise – Some Clarification”

  1. J4Zonian Says:

    We’ve been hearing for years now that the point of irreversible loss of Greenland’s ice sheet was 1.6°C above pre-industrial temp. But we’re almost certainly going to reach that in a decade or less and we’re already more than 1° over. There’s essentially no hope of not passing 1.6 in the lifetimes of most people now alive. I don’t understand why scientists are talking about this as if it’s something we still have a choice about, rather than something we just have to accept, adapt to, and look beyond to see what’s even worse that we still do have a choice about.

    • greenman3610 Says:

      we know it’s out there somewhere, and not far away.
      agree we need to talk about adaptation.

      • rhymeswithgoalie Says:

        I consciously take on the label asshole by frequently pointing out that we need to talk about abandonment. Too many people are still thinking in terms of sea walls and Venice-like megaprojects that attract corruption. I expect a few might be cost-effective, but we all know of local politics which have been too naive, overoptimistic or corrupt to rely on to save communities from the onslaught of uncaring natural forces.

        • dumboldguy Says:

          Beat me to it—-“adaptation” is a feel-good bit of wishful thinking in nearly all cases, and just puts off the inevitable. “Adapt” to SLR in South Florida? LOL

  2. redskylite Says:

    And the Antarctic is (slowly in human terms) following suite. . .

    “And when the ice loses contact with the ocean, the ice discharge stops.

    The tipping point for losing the Greenland Ice Sheet altogether therefore depends on surface melt permanently exceeding snowfall and does not involve solid ice discharge. Whether this real tipping point of Greenland has been crossed, we do not know.”

    ================================================

    “Antarctica’s Ice Shelves Have Lost Millions of Metric Tons of Ice

    Ocean water is melting them from the bottom up, causing them to lose mass faster than they can refreeze.

    That’s according to a new study analyzing satellite data from 1994 to 2018. The results were published yesterday in the journal Nature Geoscience.

    That spells bad news for the hundreds of glaciers spread out along the Antarctic coastline.

    Ice shelves are ledges of ice that jut out from the edge of the continent into the ocean. They help to keep glaciers stable, holding them in place.

    As ice shelves melt, they become thinner, weaker and more likely to break. When this happens, they can unleash streams of ice from the glaciers behind them, raising global sea levels.

    https://www.scientificamerican.com/article/antarcticas-ice-shelves-have-lost-millions-of-metric-tons-of-ice/

    • redskylite Says:

      Just published in the “International Journal of Climatology.”

      “New Research Reveals Effect of Global Warming on Greenland Ice Melt and Consequences for 21st Century Sea Level Rise

      The estimates, which scientists warn are “conservative” given the powerful effects of changes in weather systems and possible ways of accelerating ice loss, are broadly consistent with recent predictions reported by the Intergovernmental Panel on Climate Change.

      https://www.lincoln.ac.uk/news/2020/08/1629.asp

      • neilrieck Says:

        I wish these authors were a little more careful in what they post. For example, the first paragraph mentions 10-cm by the end of the year. I am guestimating that this is the amount that will be contributed by the Greenland melt because most scientists agree that the total sea-level rise in the previous century was 2-mm/year or or 20-cm/year or 8-inches/century whilst the current rate (as measured by satellite RADAR) is 3.3-mm/year or 33-cm/year or 13-inches per century.

        https://climate.nasa.gov/vital-signs/sea-level/

        • redskylite Says:

          you wrote:”the first paragraph mentions 10-cm by the end of the year.”
          ===================
          First paragraph reads as follows:

          “New analysis of almost 30 years’ worth of scientific data on the melting of the Greenland Ice Sheet predicts global sea level rise of at least 10 centimetres by the end of the 21st Century if global warming trends continue.”

          I know it feels like 2099 with covid and all but it’s only 2020

  3. rhymeswithgoalie Says:

    AFAICT, this discussion is about average global (or arctic) air temperatures near the planet surface. What about the low albedo issues (i.e., “dark snow”) that accelerate heat retention in direct contact with the snow and ice?


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