What’s behind this story and why this publication is highly questionable.
By Bianca K.
Last week a newly published paper (http://www.ingentaconnect.com/content/igsoc/jog/pre-prints/content-ings_jog_15j071) stated that the Antarctic Ice Sheet is actually gaining and not loosing ice. A statement that is quite the opposite to pretty much every other scientific study on this topic.
No need to mention that this sparked a new debate and released a media storm.
Wait, what? Antarctica is gaining ice? So climate change is a lie?
Unfortunately, this paper is a very good example on how to spot doubtful science, especially when published by a chief scientist at NASA (surely a NASA chief scientist is right, or is he?). Being an Antarctic scientist myself and working on a very similar issue as the authors, I will try to bring across the main issues about this study.
A brief summary about the paper:
The study uses satellite altimetry (http://www.cmar.csiro.au/sealevel/sl_meas_sat_alt.html) observations that provide us with information about surface elevation changes over time. This allows us to calculate changes in the ice sheet within a chosen time period.
There are different altimetry missions (http://www.mdpi.com/2072-4292/6/3/2255) available and two of them have been used in this study: the European Remote Sensing Satellites ERS-1 and ERS-2, operating between 1992-2001, and NASA’s ICESat mission covering the period 2003-2009.
While both satellites measure the same thing, they work slightly different. The European satellites use a radar (http://www.altimetry.info/radar-altimetry-tutorial/how-altimetry-works/) altimeter, which is sent down to Earth, reflects off the surface and travels back to the satellite. NASA’s ICESat (https://directory.eoportal.org/web/eoportal/satellite-missions/i/icesat) satellite uses a laser pulses that sends down a beam of light. The time it takes for these pulses to return to the satellite is measured and consequently converted into a distance. Over time these measurements allow us to obtain information about changes in surface height, if the measured distance between the Earth’s surface and the satellite differs.
However, the observations have to be corrected for other factors. One is the uplift of the underlying bedrock (https://en.wikipedia.org/wiki/Post-glacial_rebound), a process that is known to occur in Antarctica but remains largely uncertain in terms of the rate at which the bedrock is moving, as there are not enough observations due to the constant ice cover of the continent.
The other factor is due to the compaction of snow (https://oncirculation.com/2013/07/23/the-journey-of-a-snowflake-from-snow-to-ice/). The actual glacier ice of the Antarctic Ice Sheet is covered by a layer of snow (usually called firn), which can be up to ~100m thick in some regions. Every time snow falls it is added at the surface. However, due to changes in temperature and loading weight (new snow that falls on top of previously accumulated snow) the consistency of the snow changes over time, a process that transforms snow into glacier ice. Because of these changes the thickness of the snow layer actually changes, as the snow gets more and more compacted. As a result the surface elevation decreases but the ice sheet hasn’t actually lost any ice.
While scientists know that snow compaction occurs, it varies across the ice sheet and, once again, introduces large uncertainties due to its complex physical processes.
While the authors of this paper considered both processes (bedrock motion and snow compaction), they only use one bedrock model, and further assume that their way of modelling snow compaction is correct. However, there are several models available that estimate both bedrock motion and snow compaction rates, usually showing different results. To overcome uncertainties that arise due to such model discrepancies, it is useful (and often done) to use various models in your study to compare the outcome, depending on the different results of the employed models.
But lets assume what they do is correct and their results are accurate, there are still more issues within their research: They use two satellite altimetry missions to cover the period between 1992-2008.
We are currently at the end of year 2015. What happened to the 7 years between now and then?
Satellite missions and observations improve every year; surely there are more datasets available?
For example, the ERS-1/-2 satellites have a follow on mission called ENVISAT, which began its observations in 2002 and operated until 2012. The ICESat mission actually provided observations until October 2009, and in 2010 another altimetry mission called CryoSat-2 was launched, which is operating very successfully since then.
And besides all those available altimetry missions, the GRACE (https://oncirculation.com/2012/08/29/the-other-face-of-grace/) mission that monitors changes in the Earth’s gravity field (including ice mass changes), has successfully operated since 2002, providing important insights into ice mass changes. All these mission observations have not been included in their study, although they are available and have been an important part of many other studies (http://www.sciencemag.org/content/338/6111/1183.abstract) in the past years, indicating an increase in Antarctic ice mass loss.
I think if the paper had said something similar to “our findings suggest that the current ice loss of the Antarctic Ice Sheet is smaller than previously assumed” and “more research is needed to validate our findings” it would be a good and very interesting paper. But by saying that Antarctica is gaining ice, in contrast to every other study published, it has scientists up in arms and simply transmits the wrong message.
For example, the paper does state that, “while ice mass loss in West Antarctica continues to accelerate, mass gain in East Antarctica is slowing down, closing the gap to offset mass loss”. Obviously this is not mentioned anywhere in the news, as the statement about Antarctica gaining ice and, subsequently, the question whether climate change is still real, makes for much better news!
“I don’t think there will be enough snowfall increase to offset these losses.”
But what does it mean for our current climate if Antarctica indeed does not contribute to current sea-level changes? Sea-level is rising (http://www.star.nesdis.noaa.gov/sod/lsa/SeaLevelRise/) and if Antarctica does not contribute it is even more worrisome as this means other parts in the world are contributing more to sea-level than previously thought.
You can read more here (http://news.nationalgeographic.com/2015/11/151103-antarctic-ice-growing-shrinking-glaciers-climate-change/?utm_source=Facebook&utm_medium=Social&utm_content=link_fb20151103news-antarctic&utm_campaign=Content&sf14902654=1) and here (http://www.carbonbrief.org/qa-is-antarctica-gaining-or-losing-ice)