Thursday, May 10, 2012

North and South contrast

During the last week we have been confronted by two papers published in Nature and Science relating changes in the cryosphere and global sea-level. Their conclusions point to opposite directions and the attention they have found in the media is quite unbalanced.
Yesterday I was surprised that the German News in the prime time Tageschsau put up a story about a new paper in Nature authored by some our colleagues at the Alfred-Wegener-Institute for Polar research in Bremerhaven. The story was a couple of minutes long and did not include many details about the study itself. It did referred to the main conclusion, namely that sea-level rise in the next decades could rapidly accelerate if ocean currents around the ice-shelf in the Weddel Sea in Antarctica change in a certain fashion, and showed two interviews with reserachers from AWI. The news this morning in the radio also refereed to this new paper, but they had a more lengthy story, and it was only then clear to me that these results are based on model simulations entirely, something that apparently is considered to be too complicated to be understood by the general public.

The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line through submarine glacial troughs and causing high rates of melting at the deep ice-shelf bases3,4. The interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice-mass loss. Here we show that a redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century would lead to increased movement of warm waters into the deep southern ice-shelf cavity. Water temperatures in the cavity would increase by more than 2 degrees Celsius and boost average basal melting from 0.2 metres, or 82 billion tonnes, per year to almost 4 metres, or 1,600 billion tonnes, per year. Our results, which are based on the output of a coupled ice–ocean model forced by a range of atmospheric outputs from the HadCM35 climate model, suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern Weddell Sea due to thinning of the formerly consolidated sea-ice cover. The projected ice loss at the base of the Filchner–Ronne Ice Shelf represents 80 per cent of the present Antarctic surface mass balance6. Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise.

In stark contrast, the media attention to a paper published in the last week issue of Science has been much more limited. The authors here had analysed ten years of observations of Greenland glaciers and concluded that:

Earlier observations on several of Greenland’s outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200+) of Greenland’s major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf–terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below proposed upper bounds.

The authors estimate that on present rates of acceleration of glacier melting, the dynamical contribution (i.e. by ice sliding,  not surface melting ) of the  Greenland contribution to global sea-level rise by 2100 will be about 9 cm. Surprisingly (?), I have not seen any reference to this study in the non-scientific media, although Nature this week does refer to the article published by its competitor. The dyxnamical contribution of polar ice-sheets was considered to be the largest source of uncertainty in the Fourth IPCC Report and compelled some studies that tried to use statistical methods to try to estimate this potentially large contribution (e.g, Rahmstorf 2008). The much quoted estimation 2 meter sea-level rise by 2100 has been justified by  the contribution of the dynamics of polar ice sheets.


Marcel Crok said...

Hi Eduardo,
thanks for posting this. The conclusions of the Moon et al paper were even more remarkable:

"Finally, our observations have implications for recent work on sea level rise. Earlier research used a kinematic approach to estimate upper bounds of 0.8 to 2.0 m for 21st-century sea level rise. In Greenland, this work assumed ice-sheet–wide doubling of glacier speeds (low-end scenario) or an order of magnitude increase in speeds (high-end scenario) from 2000 to 2010. Our wide sampling of actual 2000 to 2010 changes shows that glacier acceleration across the ice sheet remains far below these estimates, suggesting that sea level rise associated with Greenland glacier dynamics remains well below the low-end scenario (9.3 cm by 2100) at present. Continued acceleration, however, may cause sea level rise to approach the low-end limit by this century’s end. Our sampling of a large population of glaciers, many of which have sustained considerable thinning and retreat, suggests little potential for the type of widespread extreme (i.e., order of magnitude) acceleration represented in the high-end scenario (46.7 cm by 2100). Our result is consistent with findings from recent numerical flow models."

So the observations are well below the low end scenario of 9 cm!
Let's see indeed how many newspapers will bring this very good news.

Anonymous said...


Nach all den Jahren im immer noch zu kalten heidnischen Norden solltest Du lieber nicht vergessen, die Sau des Tages nicht so einfach rauslassen, und am besten nur brav die Tagesschau gucken :-)


eduardo said...
This comment has been removed by the author.
Vinny Burgoo said...

Eduardo, the Moon et al paper was covered by The Guardian, The Register and Reuters, all of which misrepresented it in various ways. The fairest coverage was, for once, in The Guardian.

A day or so after the Moon et al paper came out, David 'Grist' Roberts did a one-off Tweet about a year-old study of Greenland glaciers that had come to more alarming conclusions. One might almost think he feared a celebration of the good news and was trying to pre-empt it.

There's also a strange symmetry linking the Moon et al paper to James Hansen via Real Climate, but it's late. Time for bed.

Anonymous said...

In Germany the Moon et al. paper was covered by SPIEGEL,


stan said...

"Just because the papers were in Nature and Science doesn't necessarily mean they are wrong."

Anonymous said...

@ Eduardo

Real climate seems to be unhappy about media coverage of Moon et al., too. Today appeared a guest post of Moon at RealClimate:

eduardo said...

Thank you to all your links. I might indeed have overseen some comments on the media on the Moon et al paper.

Concerning the post on realclimate, I would generally agree except for one sentence:
'But neither does this new work contradict any of the previous estimates of future sea level rise, such as that of Vermeer and Rahmstorf.'

I do think that it contradicts projections of 1.5 meters of sea level rise by 2100. Realclimate suggests that the Moon et al paper are provisional results, based on 10 years of data. This is true, and they may be proved wrong in further studies. But as they stand now, they are not compatible with a 1.5 or higher sea-level rise in the next 90 years