While the Arctic pack ice has continued to melt, the Antarctic pack ice tended rather to expand between 1979 and 2015, before its extent was also drastically reduced in 2016 and 2017. A surprising expansion phenomenon in the era of global warming!
While at present the recent variation in the Arctic pack ice can be explained with a good degree of confidence, that is far from being the case for the Antarctic pack ice. Moreover, most of the models that allow the pack ice to be studied and the phenomena that occur there to be better understood are much less realistic in the Antarctic Ocean than in the Arctic Ocean. “A research project financed by the FNRS was set up in order to study the biases of these models. The idea is to have models as reliable as those for the Arctic,” explains Olivier Lecomte, a researcher at the Georges Lemaître Centre for Earth and Climate Research (TECLIM).
Two very different ice packs
Moreover, the phenomena that govern the two ice packs are very different! On the one hand, the Arctic pack ice develops surrounded by continents, while the Antarctic pack ice develops around the continent; it is therefore not limited by the coasts. “In addition, the Antarctic pack ice is composed for the most part of seasonal ice dependant on the winter freezes and summer thaws, while the Arctic pack ice includes more multi-year or perennial ice which can survive the summer thaw and so can become thicker in the course of several winters. But this ice is melting year by year due to global warming.” Another notable difference: the Arctic pack ice is melting, while the Antarctic pack ice is expanding. This is a phenomenon that especially interested the researcher.
A key feature: oceanic heat input
Aside from the hypotheses of a change in the winds, the ocean currents or the hydrological cycle of the Antarctic Ocean, one of the most serious approaches to explaining such a process involves the supply of oceanic heat at the surface, which is much more significant in the Antarctic Ocean than in the Arctic Ocean. Depending on the quantities of ice present each year, the density of the ocean at the surface changes due to the salt released during the periods of ice formation and the fresh water released when it melts. The result is that the seawater, more or less heavy at the surface, sometimes sinks easily toward the bottom, sometimes not, which affects the vertical heat exchanges1. “If a perturbation enhances ice formation for one or more years, the mechanism we are stressing can lead to less vertical mixing in the longer term and so less heat input from the intermediate layers of the ocean located at depths of 100 to 200m, which are naturally a bit warmer (~0°C), toward the surface at the freezing temperature of seawater (~-2°C),” Olivier Lecomte elaborates.
The heat that is no longer available at the surface to make the ice melt in the summer or limit its growth in the winter, due to the progressive decline in vertical exchanges, is thus trapped at a depth. The consequence is warming at a depth, cooling of the ocean at the surface and an accumulation of sea ice. This is called ocean-ice positive feedback, as the mechanism loops back on itself by amplifying the starting perturbation. “This is a natural phenomenon; it just needs an initial perturbation of any origin to become established.”
Demonstrating this phenomenon
Nevertheless, ocean-ice positive feedback was only a theoretical model up to now. This is why Olivier Lecomte and his colleagues went in search of physical evidence of its existence. “To do this, we studied the variations in ocean temperatures at different depths using measurements made during oceanographic expeditions or by drifting buoys over thirty years in the Ross Sea. It paid off! We found in fact a link between the temperature at approximately 125 m depth, which has increased over the last decades in the Ross Sea, and the quantity of surface ice. Ocean-ice feedback is therefore a valid hypothesis to explain the expansion of the Antarctic pack ice over the period 1979-2015,” says the researcher, whose results are published in the journal Nature Communications, with enthusiasm.
A conciliatory discovery...
Thanks to this work, the explanation of expansion of the Antarctic pack ice by the natural variability of the ocean-sea ice system is supported by a quantified physical process for the first time. To date, it had only been confirmed statistically, which does not provide a precise explanation of the nature of the phenomenon. “Furthermore, it also allows the other hypotheses that had been evoked to be taken into account. In fact, positive feedback requires an initial perturbation that can take the form of a change in precipitation, wind or especially low temperatures one winter, etc. From this point of view, the study is an important additional step toward an overall explanation of the expansion of the Antarctic pack ice until 2015, compatible with all the other hypotheses related to climate change.”
…and compatible with the 2016 observations
Finally, there is another lesson to be learned from this work: if the feedback is reversed and the heat trapped at a depth is released to the surface, sufficient energy would then be available to melt the surplus ice formed over the last decades. “It’s all the more interesting since our publication comes at a time when the extent of the Antarctic pack ice is in the process of beating its record minimum annual coverage for almost 40 years. In fact, after expanding for almost 36 years, the surface area of the Antarctic pack ice has diminished drastically between 2016 and 2017. The future will tell whether a particular event in the inter-annual variability of the extent, within a continuing trend toward increase, is involved, or whether a longer-term inversion is involved, but the reversibility of the mechanism proposed could also explain this type of situation,” concludes Olivier Lecomte.
Elise Dubuisson
1To understand these mechanisms in greater detail, the following educational documents in particular can be consulted:
Explanatory video (in English) on the role of fresh water in ocean dynamics
Popular article on the same topic
A glance at Olivier Lecomte's bio
2005-2008 Hydrographic and Oceanographic Engineer (ENSTA Bretagne, Brest, France)
2007-2008 Master in Oceanic and Atmospheric Physics (UBO, Brest, France)
2008-2014 Research Assistant (UCL)
Sept.-Nov. 2012 Field expedition on the East Antarctic pack ice (SIPEX2)
2014 Doctor of Science (UCL)
2014-2016 Postdoctoral Staff (UCL/ELIC, Earth and Life Institute)
Since nov. 2016 Postdoctoral Staff (UCL/ELIC) and teaching aide in the Student Support Service (UCL/AIDE)