A paper by Schaefer et al. (2009) gives an interesting new view of interhemispheric glacier fluctuations during the Holocene, through the use of a high resolution 10BE chronology and historical moraine records.
Natural climate variability during the interglacial conditions of the Holocene is a fundamental baseline for evaluating the anthropogenic impact on global climate today. As has been explored previously in this blog, by knowing past natural variability, we can better identify the effects of synchronous global climate change at present. Studies in the past few years have challenged the traditional view of a stable Holocene climate. These more recent studies of Northern Hemisphere paleoclimatic data show that cooling temperatures, accompanied by millennial-scale variations (leading to Medieval Warm Period (MWP) / Little Ice Age (LIA) oscillations) occurred through the Holocene. As can be seen, terrestrial paleoclimatic data for the Northern Hemisphere had been widely used. However, sources of this data are much sparser in the Southern Hemisphere and it therefore remains unclear whether the south followed the north in this cooling trend and whether the same MWP/LIA oscillation was observed globally.
The authors of this paper aim to investigate these interhemispheric linkages, by seeing if Holocene glacier advances in New Zealand relate to those seen in the Northern Hemisphere. The terminal moraines deposited by each glacier advance in the Southern Alps of NZ was dated and used to look for any north/south pattern and therefore if global or regional climate signals dominated during the Holocene.
Whilst many previous chronological studies have suggested that the most recent cool interval in New Zealand came at the termination of the northern LIA (c.1850), more recent studies using dendrochronology have instead suggested that the glacial maximum in the southern hemisphere is notably older than the termination of the northern LIA. This problem of obtaining reliable ages has inevitably impacted on the ability of studies to give records of glaciers in a global context. Therefore, this study applies cosmogenic isotope dating, which has a much higher resolution than radiocarbon dating for example, and which can also be applied to more recent deposits. Using 10Be exposure dating, a total of 74 boulders were sampled on the Mueller Glacier Holocene moraines, which offered the most complete sequence of moraines. These moraine exposure ages were then interpreted as being representative of a terminal moraine and therefore the end of a glacial event. Overall, the authors noted a strong positive correlation between the exposure ages given by the cosmogenic isotopes and the historic ages of the moraines. As well as this, the paper notes that these records also matched well to a radiocarbon chronology using buried wood found within several lateral moraines. With both 10Be and 14C records indicating the maximum extents of each glacial advance, a minimum of 15 different pulses of advance and retreat were identified since the mid-Holocene. Using past tree-ring data as well, all of the data showed a clear record of summer temperature changes and accompanying glacier fluctuations during the late Holocene.
With the widely-supported high resolution record of glacier fluctuations in the Southern Alps of New Zealand, the study compared the Holocene moraine record to Northern Hemisphere records. From this, three very clear and succinct conclusions were drawn. Firstly, noticeable interhemispheric differences were seen in the timing of maximum ice extent. The NZ glaciers were at their greatest extent c.6500 years from present; whilst in the Northern Hemisphere, this point came much more recently during the LIA. The second conclusion was that during the northern warm periods, glacial advances were still being observed in NZ, advancing beyond 19th century terminus. Thirdly, the greatest similarity between the Northern Hemisphere and NZ records was seen between 300 and 700 C.E. (the Dark Ages), with broad similarities seen during the past 700 years (LIA) as well. The study notes that the similarities during the LIA consisted of multiple glacier advances, followed by a termination c.1850. However, despite these broad similarities; whereas the northern glacier record is dominated by LIA-maximum terminal moraines (under 400 years old), the NZ glaciers of this study proved to have reached their maximum extent earlier than this. For example, the terminal moraine of the Mueller glacier is 570 years old and thus deposited much earlier than the moraines of the Northern Hemisphere. The authors also note that this pattern of broad similarity, with conflicting smaller detail, is true for the past 150 years as well.
Figure 1. Timing of NZ glacial fluctuations with N. Hemisphere glacial records (Schaefer et al., 2009)
The results of this study showed neither interhemispheric synchrony of mid to late-Holocene climate fluctuations, or of any kind of interhemispheric asynchrony. Accordingly, the authors view any kind of global driving mechanism as unsatisfactory in explaining Holocene glacier dynamics. They also consider two other hypotheses which try to deal with regional climate differences. Firstly, that changing strength of deepwater production between the north and the south can explain interhemispheric imbalance. This is rejected for its inability to explain regional differences instead of just hemispheric differences in Holocene climate. A second alternative hypothesis regarding solar forcing of regional temperatures isn’t dismissed, and the authors state that good correlation exists between the solar record and the NZ moraine chronology used in this study. However, as the authors state very clearly:
“...we suggest that regional ocean-atmosphere oscillations may account for the observed glacier fluctuation pattern... the Interdecadal Pacific Oscillation (IPO) has been an important influence on glacier behaviour in New Zealand over the past few decades... These changes are well reflected in New Zealand’s glacier length fluctuations.”
Evidently, the authors propose a number of different hypotheses for the regional glacier dynamics seen in NZ. Whilst one clear regional influence is put forward in the form of the IPO, the suggestion of solar forcing as well perhaps suggests the author’s belief in more than one forcing factor on regional temperature. However the summary of the findings remains to the point:
“...our study shows that mid- to late Holocene glacier fluctuations were neither in phase nor strictly antiphased between the hemispheres, and therefore it is likely that regional driving or amplifying mechanisms have been an important influence on climate.”
This study has shown clearly that through the use of high precision exposure ages for Holocene moraines, a much more accurate chronology has been produced to show interhemispheric differences in glacial extents. Whilst a definitive answer hasn’t yet been given; the issue of what drove these regional Holocene climate variations is much clearer as a result of a greatly improved knowledge of southern hemisphere glacier fluctuations.
All of this helps us to greater understand what influenced glaciers in the past and how for the future, we may be able to apply our more complex knowledge of regional and hemispheric influences to better predict future levels of retreat across the globe.
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