The New Zealand subantarctic islands are home to the southernmost-growing trees in the southwest Pacific, a region of global importance climatically and ecologically but for which there is a very little observational data. In sheltered locations, some Dracophyllum have been shown to reach more than four metres in height, with their growth strongly influenced by temperature. By measuring the thickness of annual tree rings, Dracophyllum offers the possibility of developing a natural weather record back to the 19th century to better understand past, present and future change.
Dracophyllum trees on Campbell Island (52˚S)
The research program
The Southern Ocean plays a fundamental role in global climate but suffers from a dearth of observational data. As the Australasian Antarctic Expedition 2013-2014 we have developed the first annually-resolved temperature record using trees from subantarctic southwest Pacific (52˚-54˚S) to extend the climate record back to 1870. With modeling we show today’s high climate variability became established in the ~1940s and likely driven by an atmospheric signal that originated in the tropical Pacific. Our results suggest that the influence of contemporary equatorial Pacific temperatures may now be a permanent feature across the mid- to high latitudes of the Southern Hemisphere, with substantial impacts on seabird and mammal populations.Download
Here we report new insights into global climate-carbon dynamics during and after the so-called Little Ice Age, a period of marked cooling accompanied by glacial advance across the Northern Hemisphere between 1250 to 1775. During this cooling, a marked and sustained decrease in atmospheric carbon dioxide levels has been reported from numerous Antarctic ice core records. The source of this change in carbon dioxide has been keenly debated over the last two decades. Here we use a network of tree ring series across the mid-latitudes of the Southern Hemisphere. Because the Southern Hemisphere atmosphere is relatively old when compared to the north due to the upwelling of old carbon from the Southern Ocean, the radioactive content of the tree rings provide a sensitive measure of ocean circulation over time. Our results suggest that changes in the Northern Hemisphere were the cause of the carbon dioxide decrease during the Little Ice Age.Download
On the subantarctic islands, peat exposures show Dracophyllum once grew above present day tree line. Here we find there was a major collapse in the altitudinal limit of growth between approximately 2000 and 1000 years ago suggesting westerly winds were stronger at this time.Download