I am starting a new category, for when nothing exciting has happened to our group and/or when relevant papers are out. This week in particular I have no excuse, for I’ve discovered three such papers since I last wrote, all three using satellite data to obtain measurements from remote regions: Greenland, Antarctica, and the bottom of the Atlantic Ocean. Note that I am not at all linked to any of the people involved – I just want to talk about these papers because I like talking!
Mouginot et al. (2015), Fast retreat of Zachariae Isstrøm, northeast Greenland
This paper is quite depressing honestly. Zachariae Isstrøm is a marine terminated glacier: like Petermann, from the grounding line onwards, it floats over the ocean. Using 40 years of satellite and radar measurements, the authors found that after having been stable for about 25 years, the glacier started going into trouble in the early 2000s, and has seriously accelerated since 2012. Retreat of the ice front, large calving events, quadrupled (!) rate of grounding line retreat, tripled surface ice velocity… And did I mention the glacier 1.1m sea level rise equivalent?
This paper has a special interest to me due to the variety of data it uses: the topography is obtained using ‘airborne gravity inversion’; the position of the ice front and the surface velocity using Landsat; the location of the grounding line using satellite radar interferometry; and the ice thickness using radar and LiDAR. I am personally about to start working with this kind of data, and I’ve just signed up for the MOOC ‘Monitoring Climate from Space’organised by ESA. I’ll obviously review it for you, dear reader!
Zwally et al. (2015), Mass gains of the Antarctic ice sheet exceed losses
Well, everything is in the title. In agreement with everyone else, the authors find that the ice sheets on the coast of West Antarctica and in the Antarctic Peninsula are losing ice. However, these losses are compensated by even stronger increases from the East Antarctic ice sheet and other parts of the West Antarctic one. Also, contrary to previous studies, they do not find that the ice increase is due to an increase in snowfall, but rather by a ‘dynamic thickening […] due to a deficiency in ice flow‘ for this part of Antarctica. They don’t seem to explain why the ice is not flowing as much as it did though.
Of course, that got the climate deniers very excited. Just search for the press release of this article to get a taster, and also learn that a UFO crashed in Antarctica three years ago (here). More seriously, to me this paper simply demonstrates once more how little we know about Antarctica. Also, other scientists quickly pointed out two weaknesses in the methods since its publication:
– the uncertainties are very large compared to the magnitude of the signal, so it is not clear whether the author are actually capturing any significant trend in East Antarctica;
– their study, using the same data for the same part of the world, does not find the same values as other previous studies – maybe their retrieval algorithm (i.e. the code that they wrote to translate satellite images into snow thickness) is not working correctly.
You can find a more detailed version of the scientific discussion here.
I don’t know if it’s becaused I used it during the Master internship that introduced me to oceanography, or if it’s because it has the same name as the UEA supercomputer that suffered a lot during my PhD, but I like GRACE. Its two satellites measure changes in the Earth gravity field, hence changes in mass (of water in our case). That is, GRACE can see water move. And the authors of this final paper just understood how to isolate the change of mass that corresponds to the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is driven by deep water formation in the Labrador and Nordic seas, which is expected to slow down as Greenland melts next to it (I suspect I’ve already told you about that…). And as the AMOC controls the European climate among other things, we are quite concerned about its fate and need to know what it is doing.
This paper does not really teach us anything new, but it proves that measuring the AMOC from space is feasible! And that is amazing because:
– most satellite-based methods work only for the surface of the ocean, whereas here they measured a deep current;
– these results are in very good agreement with in-situ measurements from the RAPID-array; this network of buoys is a great success, but its logistics are much more complex (hence risky) than the use of satellites.
So in summary, satellites are now reliable enough and have been above us for long enough to use them for climate sciences, i.e. assessing changes. However, one needs to be careful when processing satellite data, especially if they suddenly find results that totally disagree with everyone else. It could well be an amazing breakthrough, but errors happen too (remember the neutrino story?). To be continued…