Little Ice Age: Started In The Southern Hemisphere?

  • Date: 07/09/12

That the Little Ice Age (LIA) – a cooling centred on the 17th Century – took place is beyond doubt. What is questioned however is its spatial extent and its cause. Reading the last document produced by the IPCC on the subject of the LIA one is left in no doubt that it thinks it was a mainly European event.

It states: Evidence from mountain glaciers does suggest increased glaciation in a number of widely spread regions outside Europe prior to the 20th century, including Alaska, New Zealand and Patagonia. However, the timing of maximum glacial advances in these regions differs considerably, suggesting that they may represent largely independent regional climate changes, not a globally-synchronous increased glaciation … hemispherically, the “Little Ice Age” can only be considered as a modest cooling of the Northern Hemisphere during this period of less than 1deg C relative to late 20th century levels.

I note that 1 deg C is a “modest cooling” in this context and that such a description is seldom used to describe the smaller amount of recent warming seen in the instrumental global temperature record. Also note that the most used reference of the strong regional variations in the timing, magnitude and character of the LIA is Jones and Mann 2004. So it is that the LIA is typically considered to be a Northern Hemisphere climate phenomenon characterised by alpine glacial advances and relatively cool temperatures observed between 15th to mid-19th centuries. There are signs however that the IPCC will have to reevaluate its stance on the extent of the LIA.

The cause of the LIA has been the subject of much debate, with explanations ranging from increased volcanism, reduced solar irradiance and ocean circulation changes. Paleoclimate records suggest that the LIA is the most recent cooling event of a series that punctuated the Holocene. Such apparent oscillatory behavior in Holocene climate has led to speculation about what role the thermohaline circulation of the world’s oceans may have played in instigating or amplifying these climate changes. No consensus has been reached on this issue and a key concern is the spatial bias toward Northern Hemisphere proxy and meteorological records. An important tool to understand the LIA must therefore be the acquisition and interpretation of Southern Hemisphere paleoclimate records, but few are currently available.

Some years ago scientists compared studies of West Antarctic ice cores to the Greenland Ice Sheet Project suggesting a synchronous global Little Ice Age. An ocean sediment core from the Bransfield Basin in the Antarctic Peninsula displays centennial events that the researchers attributed to the Little Ice Age and Medieval Warm Period (MWP) noting “other unexplained climatic events comparable in duration and amplitude to the LIA and MWP events also appear.” The Siple Dome in Antarctica also shows a climate episode whose start is coincident with that of the LIA in the North Atlantic. The Siple Dome ice core also contains its highest rate of melt layers between 1550 and 1700, due to it has been suggested to warm summers during the LIA.

An impressive new paper and Ph.D thesis by Rachael Rhodes formerly of the Victoria University of Wellington, New Zealand looks at the Little Ice Age climate and oceanic conditions of the Ross Sea, Antarctica from a coastal ice core record.

Antarctic ice cores are valuable archives of past climate and only recently have been obtained with adequate temporal resolution to investigate any traces of the LIA in Antarctica. They are beginning to suggest that Antarctica did experience the LIA. If this is the case then the LIA cannot be due to fluctuations in the thermohaline circulation of the world’s oceans.

Rhodes at al 2012 obtained subannual glaciochemical records from a coastal ice core at a place called the Mount Erebus Saddle (MES) in the south-western Ross Sea. The portion of the LIA captured in the MES ice core (1446-1850 AD) shows enhanced lithophile (microorganisms that can live in tiny cracks and pores in rocks) element concentrations and a rapid decrease in lithophile element concentrations between 1848 and 1850 AD – the end of the LIA. The MES stable isotope record suggests that the Ross Sea region experienced 1.6 ± 1.4 deg C cooler average temperatures during the LIA in comparison to the last 150 yr.

The ice core record currently dates back to 1446 AD but curiously there is no sign that the onset of the LIA is captured. Rhodes points out that a frequently cited date for the onset of the LIA in the Northern Hemisphere is 1450 AD but estimates vary.

In addition recent research from the Antarctic Dry Valleys suggest that the onset of the LIA was actually earlier in the Southern Hemisphere. If verified such findings could stand our ideas about the LIA on its head.


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