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.

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