In a new report in Science, scientists present results matching neither the hypothesis of inter-hemispheric synchrony during the mid to late Holocene nor rhythmic asynchrony, downplaying the importance of global driving mechanisms. It seems that several glacier advances occurred in New Zealand during classic northern warm periods. Understanding the relationship between climate changes in the northern and southern hemispheres during the Holocene, along with their causes, remains a major problem of climate science.

Many anthropogenic global warming proponents have pointed to differences between recent warm and cold periods in the northern and southern hemispheres as an indication that such periods—the Little Ice Age and the Medieval Warm Period for example—were strictly regional variations and did not represent global climate trends. This new study, led by Joerg M. Schaefer, indicate that regional driving or amplifying mechanisms have been an important influence on climate in the past. They suggest that atmospheric circulation changes in the southwest Pacific were one important factor in forcing high-frequency Holocene glacier fluctuations in New Zealand.

Part of the problem with tracking past changes in climate is the sparsity of data for the southern hemisphere. Records for the northern hemisphere are extensive and quite well documented but much of the data for the southern part of our planet comes from Antarctic ice core. Since Antarctica is isolated from much of the fluctuation in more northerly regions by wind and water currents it is hard to judge what was going on in the southern hemisphere as a whole. This new paper tries to address that relationship in the following manner:

Natural climate variability during the interglacial conditions of the Holocene (the past 11,500 years) is a fundamental baseline for evaluating the anthropogenic impact on global climate. Recent studies using marine and terrestrial samples from the North Atlantic region and the tropics challenge the traditional view of a stable Holocene climate. Northern Hemisphere paleoclimate records suggest that temperatures cooled through the Holocene and that this long-term trend was overprinted by millennial-scale variations (4), culminating in the Medieval Warm Period (MWP)/Little Ice Age (LIA) oscillation (5, 6). Terrestrial paleoclimate data are sparse in the Southern Hemisphere, and it remains unclear whether the northern trend of progressive cooling was followed in the south and whether northern millennial-scale climate changes, including the MWP/LIA oscillation, were globally extensive. We explored this problem via the question: Were Holocene glacier advances in New Zealand generally coeval with those in the Northern Hemisphere?

The study's results, as is so often the case in scientific research, do not resolve the debate over inter-hemispheric climate linkage. Focusing on the Mueller Glacier Holocene moraines, with complementary samples from the moraine sequences of the Hooker and Tasman glaciers, the research team has constructed a new, more detailed climate history for the southern hemisphere out side of the Antarctic region. Their findings are shown in the figure below.

The top graph shows fluctuations of two index glaciers in the Swiss Alps, the Great Aletsch Glacier and the Gorner Glacier, reconstructed from historical accounts, tree rings and radiocarbon data from fossil wood by G. C. Wiles, et al. The middle graph shows the glacier advances in coastal Alaska and the Canadian Rockies by Wiles and B. H. Luckman, respectively. The bottom graph shows the timing of Holocene glacier fluctuations near Mount Cook in New Zealand’s Southern Alps, together with published ¹⁴C ages on soils buried by Mount Cook glacier expansion events over the past 4000 years, and a tree ring reconstruction of austral summer temperature in New Zealand over the past 1100 years, compared with glacier fluctuations in the Northern Hemisphere from the top part of the graph.

The probability plots at the bottom are summary curves of all individual ¹⁰Be boulder ages from each moraine dated in New Zealand. The blue bars show the arithmetic means of the moraine age. The ¹⁰Be ages are from Mueller Glacier moraines, except for the 1650-year moraine (Tasman Glacier) and 1370- and 1020-year moraines (Hooker Glacier).

Moraines are deposits of rock and soil picked up and transported by glaciers as they advance. Moraine material is transported by a glaciers and eventually deposited by melting when the ice retreats. By dating these deposits researchers can estimate when a glacier reached a point of maximum advance during the past. From the study data three main conclusions can be drawn.

First, there is a notable difference in the timing of maximum ice extents between the two hemispheres. The Mount Cook glaciers were further advanced about 6500 years ago than at any subsequent time. In contrast, most Northern Hemisphere glaciers reached their greatest Holocene extents during the Little Ice Age (LIA), around 1300 to 1860AD.

Second, several New Zealand glaciers advanced beyond their 19^th century maximums during northern warm periods. In the north, these periods were characterized by diminished northern glaciers, such as between 7500 and 5500 years ago in the Swiss Alps and Scandinavia, during the Bronze Age Optimum (1500 to 900 BC), during the Roman Age Optimum (200 BC to 300 AD), and during the MWP (800. to 1300 AD).

Third, the greatest coherency between the Mount Cook and Northern Hemisphere records was during the Dark Ages (300 to 700 AD), and broad similarities were apparent during the LIA (roughly the past 700 years), with multiple glacier advances followed by a general termination commencing in the mid- to late 19^th century. However, northern Holocene moraine sequences are dominated by the LIA-maximum terminal moraine less than 400 years old—typically mid-19^th century in the Swiss Alps and mid-18^th century in Scandinavia. By comparison, the most prominent moraine of the past millennium at Mueller Glacier is about 570 years old and is followed inboard by several smaller moraines. This pattern of broad consistency but differing detail of glacier behavior has continued over the past 150 years.

The climate history indicated by these data are somewhat different than data collected in other locations and by other means. The decreasing extent of New Zealand Holocene glaciers does not agree with proxy based studies of nearby mean-annual sea surface temperature, which show progressive cooling of the same period. On the other hand, melt-layer records in West Antarctic ice that indicate increasing Holocene summer maximum temperatures are in closer agreement with New Zealand glacier behavior.

What does all this mean? The paper summarizes it this way:

Our results are in accord neither with the hypothesis of interhemispheric synchrony of mid- to late Holocene climate change nor with a rhythmic asynchrony, downplaying the importance of global driving mechanisms. This includes solar irradiation changes translating quasi-linearly into near-surface climate. However, recent studies show that climate models driven by solar changes can induce regionally distinct temperature changes, and indeed the Mount Cook moraine chronology shows some similarities to the solar record. Alternatively, variations in the strength of deep-water production between the north and the south have been proposed to explain interhemispheric incoherencies in Holocene climate. But this mechanism predicts strictly antiphased glacier behavior in north and south and it is not obvious how such large-scale ocean variations may account for contemporary regional contrasts, as exhibited by the general advance of glaciers in New Zealand and southernmost South America in unison with the retreat of glaciers on the Antarctic Peninsula and northern Patagonia.

So neither of the previous competing models is correct: the northern and southern hemispheres are neither locked in sync nor do they alternate warm and cold periods. Once again simplistic assumptions do not suffice—Earth's climate is far too complicated for such simple explanations. Remember that the next time some pundit or proselytizer tries to tell you climate is controlled by carbon dioxide levels; they too are wrong.

Be safe, enjoy the interglacial and stay skeptical.