Yesterday Ed Caryl showed us that the Greenland ice core shows there’s nothing unusual going on with our climate around Greenland. Today geologist Sebastian Lüning and Prof. Fritz Vahrenholt tells us the same for the other end of the Earth.
By Dr. Sebastian Lüning, Prof. Fritz Vahrenholt
[Translated/edited by P Gosselin]
Antarctic sea ice has truly surprised science. Contradicting the models, it has continuously grown since 1979 during a time that the climate models were only able to find scenarios of receding ice.
Today we would like to take a look back before the satellite time. How did the South Pole develop during the time when satellites were unable to continuously and completely monitor sea ice movement?
Information can be gathered for example from old satellite photos made during the pioneering phases of satellites. On 29 August 2014 the University of Colorado Boulder reported on an amazing discovery of old Nimbus photos:
“And the Antarctic blew us away,” he said. In 1964, sea ice extent in the Antarctic was the largest ever recorded, according to Nimbus image analysis. Two years later, there was a record low for sea ice in the Antarctic, and in 1969 Nimbus imagery, sea ice appears to have reached its maximum extent earliest on record.”
In 1964 Antarctic sea ice was hugely expanded, while to the contrary in 1966 it retreated massively. And in 1969 the sea ice had returned once again close to record high levels. This is an enormous amount of natural variability.
A team of scientists led by Tingting Fan used the premise of growing sea ice since 1979 as a reason for investigating the climatic conditions in the southern ocean. Here scientists found that the oceans had cooled over the previous 35 years, which fits well with the notion of expanding sea ice. During the 1950-1978 period, on the other hand, the southern ocean warmed up. This was the basis for a long-term ice retreat during that phase. The paper appeared in the April 2014 Geophysical Research Letters. The abstract writes:
Recent Antarctic sea ice trends in the context of Southern Ocean surface climate variations since 1950
This study compares the distribution of surface climate trends over the Southern Ocean in austral summer between 1979–2011 and 1950–1978, using a wide variety of data sets including uninterpolated gridded marine archives, land station data, reanalysis, and satellite products. Apart from the Antarctic Peninsula and adjacent regions, sea surface temperatures and surface air temperatures decreased during 1979–2011, consistent with the expansion of Antarctic sea ice. In contrast, the Southern Ocean and coastal Antarctica warmed during 1950–1978. Sea level pressure (SLP) and zonal wind trends provide additional evidence for a sign reversal between the two periods, with cooling (warming) accompanied by stronger (weaker) westerlies and lower (higher) SLP at polar latitudes in the early (late) period. Such physically consistent trends across a range of independently measured parameters provide robust evidence for multidecadal climate variability over the Southern Ocean and place the recent Antarctic sea ice trends into a broader context.”
Already in November 2013 a group led by Loïc Barbara published a reconstruction of the sea ice in the area of Antarctic Peninsula in the journal of Quaternary Science Reviews. Between 1935-1950 the ice receded, and after that there is no recognizable trend. Instead the sea ice fluctuated back and forth over years and decades. The paper’s abstract follows:
Diatoms and biomarkers evidence for major changes in sea ice conditions prior the instrumental period in Antarctic Peninsula
The Antarctic Peninsula (AP) has been identified as one of the most rapidly warming region on Earth. Satellite monitoring currently allows for a detailed understanding of the relationship between sea ice extent and duration and atmospheric and oceanic circulations in this region. However, our knowledge on ocean–ice–atmosphere interactions is still relatively poor for the period extending beyond the last 30 years. Here, we describe environmental conditions in Northwestern and Northeastern Antarctic Peninsula areas over the last century using diatom census counts and diatom specific biomarkers (HBIs) in two marine sediment multicores (MTC-38C and -18A, respectively). Diatom census counts and HBIs show abrupt changes between 1935 and 1950, marked by ocean warming and sea ice retreat in both sides of the AP. Since 1950, inferred environmental conditions do not provide evidence for any trend related to the recent warming but demonstrate a pronounced variability on pluri-annual to decadal time scale. We propose that multi-decadal sea ice variations over the last century are forced by the recent warming, while the annual-to-decadal variability is mainly governed by synoptic and regional wind fields in relation with the position and intensity of the atmospheric low-pressure trough around the AP. However, the positive shift of the SAM since the last two decades cannot explain the regional trend observed in this study, probably due to the effect of local processes on the response of our biological proxies.”
In May 2014 a team led by Kate Sinclair published a reconstruction of sea ice from the Ross Sea in the Geophysical Research Letters. Between 1880 and 1950 the ice was apparently stable. From 1950-1990 the ice receded, varied beginning in 1993, but took on an increasing trend, which continues today. The abstract:
Twentieth century sea-ice trends in the Ross Sea from a high-resolution, coastal ice-core record
We present the first proxy record of sea-ice area (SIA) in the Ross Sea, Antarctica, from a 130 year coastal ice-core record. High-resolution deuterium excess data show prevailing stable SIA from the 1880s until the 1950s, a 2–5% reduction from the mid-1950s to the early-1990s, and a 5% increase after 1993. Additional support for this reconstruction is derived from ice-core methanesulphonic acid concentrations and whaling records. While SIA has continued to decline around much of the West Antarctic coastline since the 1950s, concurrent with increasing air and ocean temperatures, the underlying trend is masked in the Ross Sea by a switch to positive SIA anomalies since the early-1990s. This increase is associated with a strengthening of southerly winds and the enhanced northward advection of sea ice.”
We conclude our look at the Antarctic sea ice with an anecdote that appeared in the December 2014 in the Geophysical Research Letters. In the paper authors Jeff Ridley and Helene Hewett claimed that the sea ice trend in the Anatarctic indeed would be irreversible as a result of climate warming. That would of course mean that the increase of the last 35 years would never be reversed. Yet, to the contrary, Arctic sea ice trends are supposed to be reversible. Apparently the authors are anticipating an increase in north polar sea ice. Absolutely curious. Here’s the abstract of the paper:
A mechanism for lack of sea ice reversibility in the Southern Ocean
We find evidence that ocean processes during global warming may result in irreversible changes to the Antarctic sea ice, whereas the Arctic sea ice changes appear to be reversible. Increased forcing gives rise to strong heat uptake in the Southern Ocean, and existing pathways provide an increased transport of heat to the Weddell Sea. As atmospheric concentrations of CO2 are returned to preindustrial levels, the Antarctic ice extent at first recovers, but a rapid change in the position of the an ocean front in the South Atlantic maintains the heat transport into the Weddell Sea. A cooling surface initiates deep convection, accessing the stored heat, resulting in a substantial loss of sea ice, which has not recovered after a further 150 years at preindustrial CO2.”