Posts Tagged ‘sea level’

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

October 26, 2012

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