21 July 2014

The Lord of the Rings: how dendrochronology leads to a better understanding of climate dynamics

This blog post was written by postgraduate student Stefan Unterrader as part of the course, Research Methods in Ecology (Ecol608). Stefan revisits a Lincoln University research area that looks at historical climate data taken from tree rings in 2010.

Previously... on the global climate change show - It is well understood that we are facing ongoing warming of our planet's climate. Whether or not we are responsible for this process (and we are), rising temperatures around the globe have been observed for at least the last couple of decades. Although our behaviour is recognized today as a significant influencing factor for Earth's climate, it is not the only driver for today's changing climate. To get a grip on these drivers scientists often use little helpers, such as tree rings, that carry astounding potential for this task. Richard Duncan, a former professor at Lincoln University, and Pavla Fenwick, a former Lincoln PhD student, are among those scientists who are hot on the scent of such internal climate drivers using tree-ring dates from a native New Zealand tree, the pink pine. Their team of dendrochronologists - scientists who date tree-rings - pursue a promising lead which links internal climate variation to regional temperature patterns and try to explain phenomena that do not match the general global warming trend.

Residents of the Arctic Circle are concerned about rising
temperatures (Photo by Payton Chung). Tree rings can help
to better understand who's behind global warming and
how to mitigate its consequences.

Climate Change in a nutshell: Greenhouse gas concentrations were on the rise throughout the 20th century and global temperature mainly followed this worrying trend. But the devil is in the details: local and even regional temperature patterns around the globe had the cheek to behave differently. And as if this isn't enough, some cooling periods in the northern hemisphere have coincided with some rapid increases of temperature in New Zealand during the same period  (for instance, from 1940-1975). So what’s behind all this? Have New Zealanders been polluting the air to such an extent that they generate their own little hot spot? Well, chances are that humankind isn't alone being in the driver's seat to this mess. 

Climate proxy: extracting tree rings
 from a slice of wood (Norway spruce) and
plotting its biological growth-curve.
(Figure from the author's thesis.)

Back to the Future: In general, global climate is known to be driven by both natural and anthropogenic external forcing as well as by internal dynamics. In order to get a glimpse of Earth's climatic behaviour climatologists can use what are known as climate proxies: preserved physical characteristics of the Earth's past and present climate. Such proxy data allow for a reconstruction of climatic conditions from periods where no instrumental or historical records are available. Tree-rings can be used as such proxies because some trees, such as conifers growing in a temperate climate, will build up one tree-ring each year to grow in both size and rigidity. Based on adequate temperatures and moisture the duration of each growing season will determine the widths of tree rings: high temperatures will generally allow for wider tree-rings, and vice versa. And since many NZ tree species are climatically sensitive enough to temperature for this to be seen in their tree ring widths, we can use their tree-ring sequences as a substitution for instrumental climate records. This is actually quite useful as instrumental coverage here in New Zealand only goes back to about 1850. 

The hunt for understanding internal climate variability: Because of its isolated location and exposure to considerably high mountain ranges, New Zealand's climate is highly sensitive to variations in atmospheric circulation, the movement of air and thermal energy across the globe. With its native forests that contain many long-lived tree species, NZ is the perfect dendrochronology-lab for investigating changes in such climate circulation patterns. In their 2010 study, Duncan and his colleagues established a tree-ring chronology (a timeline based on a great number of connected, dated tree-ring samples) for Halocarpus biformis, a tree species endemic to NZ and commonly known as pink pine. Duncan's team was able to gather samples whose tree-rings were strongly influenced by temperature and significantly corresponded to instrumental temperature data in NZ. Based on this chronology they could reconstruct the mean annual temperature in NZ since the 14th century. Apart from verifying NZ's instrumental records the authors also observed several departures from a global trend of rising temperatures. On top of that, the behaviour of NZ temperatures is not in line with northern hemisphere temperatures. In fact, they appear to be sometimes directly out of phase with those in the northern hemisphere.
Who's in charge? Differences in climate patterns between the northern and southern hemispheres have typically been explained by the spatial variation of "radiative forcing": the effects of solar radiation, volcanoes and aerosols on Earth's climate, which all vary both in space and time. In contrast to such a global forcing, scientists identified phases when the southern hemisphere began to warm more rapidly while parts of the northern hemisphere experienced cooling temperatures. Since previous studies already identified regional climate drivers as strongly influencing large-scale temperature variations across the globe, Richard Duncan and his team took a closer look on the main modes of such internal variation: the Interdecal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO) which can both be derived by measuring the sea surface temperature.

Pink pines on Waharoa Saddle, Arthur's Pass National Park,
New Zealand (Photo kindly provided by Chris Morse).
One Pine to rule them all:  Through comparing the variations in the Pacific and Atlantic Oscillation patterns with their pink pine tree-ring chronology, Duncan and his colleagues were able to link New Zealand's temperatures to these internal oscillations for the past 550 years. At the same time they identified time intervals where NZ temperatures follow northern hemisphere temperatures and therefore a global warming trend more closely. To make a long story short, these opposing climate states (in-phase or out-of-phase with global warming) recur on a more or less regular basis and can hardly be explained by us polluting the air alone. While our influence on Earth's climate is beyond doubt, this study reinforces previous research where natural climate variation on a more regional scale still plays a powerful role in controlling New Zealand's temperatures! The story of this NZ pink pine chronology not only shows how valuable tree-rings can be for reconstructing past temperatures but also reminds us that the term "global warming" may only tell parts of the story.  

...to be continued: The study by Duncan and his colleagues is not the last piece of work in this area. The University of Auckland's dendrochronology-lab, for example, is primarily investigating kauri trees and has built up kauri tree-ring records for the last several thousand years. Tree-ring records stretching over such long time scales have been related to El NiƱo/Southern Oscillation  (ENSO) activity which is another key climate driver for New Zealand and likely to be more dominant in the decades to come. A current overview of what has already been done with tree-rings and other climate proxies on the southern hemisphere was given by Neukom and Gergis shortly after the release of the pink pine study. And there's definitely more to come - I wouldn't wonder if tree rings will stay a key indicator for understanding New Zealand's past and future climate. 

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