How much does Antarctica contribute to sea level rise? (And how should that be communicated?)

There’s an interesting new paper in Nature (King et al.) this week that looks at how much the Antarctic continental ice contributes to sea level changes. It initially caught my eye as it uses data from the GRACE satellites, which are very cool! They are twin satellites that can detect tiny changes in the distance between one another. These distance changes are driven by changes in the gravity field so it is then possible to work out how that relates to changes in mass at Earth’s surface.

King et al. aren’t the first to use GRACE to look at Antarctic mass change but they have used a new model of the way ice sheets affect the Earth’s surface. When this new model is used, you get quite a low number for the contribution of Antarctic mass loss to global sea level: 0.19mm ± 0.05mm (this is less than half of previous GRACE estimates of Antarctic mass loss to global sea level).

The first result I found for global average sea level rise for 1993-2009 was: 3.3 ± 0.4 mm per year (thanks wikipedia!) so you can see that it is a small contribution.

Anyway, I tweeted a link to this paper from my @Antarctic_news twitter account and then noticed a story about the paper in the Sydney Morning Herald and tweeted a link to that as well.

Someone quickly pointed out that the headline in the SMH was wrong – it said Antarctica was contributing 1mm to global sea level when it should be less than that (0.19mm ± 0.05mm). It turned out that Ben Cubby, who wrote the SMH article, had already noticed the mistake (and our tweets) and the headline was corrected by the next day. This is why the article has a rather clumsy headline now!

But the chat on twitter didn’t end there. Quite a few tweets were exchanged between myself, Ben and Barry Woods, who felt that Ben should have said ~0.2mm per year in his article rather than “less than a millimetre per year”, which is what he did say (and was probably why the sub-editor made a mistake with the headline).

Personally, I feel that either (~0.2mm or less than 1mm) would have been ok so tried to defend Ben’s choice of words. Both options sound quite small and, without the context of average global sea level change (which I doubt many people hold in their head), the more accurate figure doesn’t really add much. Moreover, the full passage that includes the “less than a millimetre per year” bit gives some important qualitative information that does contextualise the result:

While the continent contains enough frozen water to raise global sea levels by 59 metres should it ever all melt, the findings show it is currently contributing less than a millimetre per year. Professor King said the findings showed that sea levels had already been rising faster than they had for centuries without much extra water from the Antarctic ice sheet.

That last bit, which I’ve emboldened, seems to convey that the Antarctic contribution is small in comparison to global changes without using either of the numbers (i.e. 0.19mm ± 0.05mm and 3.3 ± 0.4 mm per year).

Someone else suggested that is was in the interest of “environmental activists” to maximise the contribution of Antarctica to sea level rise but I’m not sure that even makes sense: the view from King et al. seems even more worrying i.e. sea levels are rising without a large contribution from Antarctica.

So what was the point of this blog post? Maybe it was so that I could articulate my thoughts without twitter’s 140 character limit but I was also wondering what other people thought about how to communicate findings like this. Should journalists always use the figures straight from papers or are phrases like “less than a millimetre” ok if they make the article more accessible?

King MA, Bingham RJ, Moore P, Whitehouse PL, Bentley MJ, & Milne GA (2012). Lower satellite-gravimetry estimates of Antarctic sea-level contribution. Nature PMID: 23086145

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11 Responses to “How much does Antarctica contribute to sea level rise? (And how should that be communicated?)”

  1. omnologos Says:

    Hello Andy. I stepped into those exchanges too. I still find it baffling that a +/-0.05 uncertainty would accompany a 0.19 value.

    Call me old-fashioned by if the uncertainty is on the second decimal, the “9” is meaningless. I hope we agree on this matter.

    Also, if we take wikipedia at face value, since 0.2<0.4 the most meaningful interpretation of King et al is that the contribution by Antarctica to sea level changes is zero (=too small to matter).

    • andyrussell Says:

      I don’t think you can just throw an arbitrary rule at this sort of error calculation. In the paper the error is calculated for mass loss and then converted into sea level rise. I think that this depends on the precision of the models that calculate the isostatic stuff. So you’d have to look there to decide if the 2 significant figures are justified.

      As for the second point, yes, it would seem Antarctic mass loss contributed very little to global sea level change over the period of the study.

  2. Barry Woods Says:

    My thoughts were why put a figure that is ambiguous and subject to error (ie is it, 0.9mm, 0.8mm, 0.2 mm etc, who knows), the original headline got generated, because a sub editor saw a <1mm and rounded it up to 1mm, 'close enough' perhaps he thought.
    But 5x the actual figure..

    I think the public can understnd 0.19mm +-0.05mm, ie like a tolerance (but I suggested ~0.2mm as being close enough for the purpose of the article), if that was too 'complcated' for the public. But felt the reponses not to do this very odd and even acusatory.

    Also a criticism of the media (generally), is their failing to link to papers or press releases, which is so easy to do now. (some say, in light of press releases, because of obvious churnalism, not in this case)

    No doubt, Ben Cubby thinks I'm the pedantic person from hell now, but from my perspective, his responses looked odd as well. why would it be so hard to replace the <1mm in the text, with either suggestion, at the very least to prevent, a sub editor someone else running an inaccurate headline.

    But my motives were questioned, and many other odd responses made.. I only found out the correct, the actual paper(very interesting) and the actual figures, because another academic tweeted to me the links… any reader would be oblivious and have no easy means (ie a url) to look for themselves.

    I thanked them for a headline change, but remain very puzzled, why not make the simple obvious change in the text, not least because the way it had originally been presented, produced an error in the original headline.

    which if you google the original phrase, has been propgated through out the media. Pauls response was perhaps more cynical (but accurate?) than mine for the reasoning not to.

    I had thought to Storfy all the tweets, but haven't got the energy.

    • andyrussell Says:

      My view, as I said above, is that ~0.2mm means just as much as “less than 1mm” without the global context. And I think that that global context was given in the quote.

      I’d also like to see links to papers in newspaper articles and Ben’s response to that did seem a bit weak (along the lines of “I didn’t have time”). But that problem is across most online media, isn’t it? The BBC has only recently started doing it (if it does it all).

  3. Barry Woods Says:

    One thing I disagree with, the actual paper shows that the Antartic a contribution, was LESS than previous estimates of Antartica contribution, which I think is also materilally important. This seems lost in the article, in fact a casual reader would not know, that in fact it was ‘less than we thought’ and might get an opposite impression

    media putting url’s to papers, abtracts and press releases would be really helpful (where available), with very little effort required

  4. John Russell (@JohnRussell40) Says:

    “Less than X”, ‘more than X’, ‘about X’ is the terminology people are used to hearing in normal speech and writing — as well as the ‘up to X’ (usually followed by an exclamation mark!) preferred by advertisers. They’re all correct, if at times weasly. Using ‘+or-‘ or ‘~’ means nothing to most people and certainly the advertising authorities would not permit those in an advert, on the grounds that they might be confusing.

    I agree wholeheartedly that all references to scientific research in newspapers should be accompanied by a link to the actual paper — or to a more detailed discussion of a paper’s contents, which includes the link.

  5. Hengist McStone (@hengistmcstone) Says:

    In the lifetime of a human being that’ll be approx 13.3 mm of Antarctic SLR. I think what the skeptics are driving at is that we dont need to worry about this one . That’s a value judgment from the parochial POV of our lifetimes but over a long enough period of time small amounts will still add up to large sea level rises.

  6. omnologos Says:

    I just realised, we’re talking about hundreds of microns. Perhaps there is something more worthwhile to talk about, Hengist?

  7. Graeme Says:

    Andy…do you have any updates on this?

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