16. August 2010

Menschen im Gespräch: Teil IV



copyright Dr. Fowler

Dr. Anthony Fowler

Dr. Fowler is a Senior Lecturer at the School of Environment at The University of Auckland (Biography). His research activities over the last two decades have covered three fields: Palaeoclimatology, "Future climate change and climate change impact assessment and Hydrology" and water resource planning. Dr. Fowler has published numerous papers in well known peer-reviewed journals like The Holocene or the Journal of Quaternary Science

I am looking forward to get some new informations on longtime climate reconstructions for the Southern Hemisphere.


W.v.B.: Dr. Fowler, let's start with some questions on the occurence of the Medieval Warm Period in Australia and New Zealand.

During the last decade, several papers on longtime climate reconstructions for the Southern Hemisphere have been published. If we take a look on New Zealand, we can refer to the papers "Speleothem master chronologies: combined Holocene 18O and 13C records from the North Island of New Zealand and their palaeoenvironmental interpretation" (Abstract here: Speleothem master chronologies ...) and of course "Speleothem stable isotope records interpreted within a multi-proxy framework and implications for New Zealand palaeoclimate reconstruction" (Abstract here: Speleothem stable isotope ... ). On the other hand, Arrigo, Cook and Palmer found evidence for a MWP in an "Oroko Swamp silver pine tree-ring chronology", back in the year 2002, but also point to the fact that we must bear the following in mind:
The occurence of the Medieval Warm Period (MWP) in the Southern Hemisphere is uncertain because of the paucity of well-dated, high resolution paleo-temperature records covering the past 1,000 years.

Cook, E. et al.: Evidence for a 'Medieval Warm Period' in a 1,100 year tree-ring reconstruction of past austral summer temperatures in New Zealand, in:

(downloadable [pdf-format, 370 KB] here: Evidence for a 'Medieval Warm Period' ...)

What do you think, has there been done new research since 2002; briefly and concisely: What ist he "state of art" on tree-ring climate reconstruction for Australia and New Zealand? In the wider sense: Is there new evidence for the existence of a MWP in general for Australia and New Zealand, including other proxy data?

Dr. Fowler: From a local tree-ring perspective the Oroko Swamp study has not really been superseded and the cautious conclusions of Cook et al remain appropriate. Most recent New Zealand (NZ) work has focused on kauri tree rings in the context of ENSO reconstructions, rather that temperature. Our kauri chronology goes back almost 4000 years, but the species is not suitable as a temperature proxy. Those NZ species that are temperature sensitive do tend to show cooler conditions prior to the 20th century, but conclusions about whether that cooling was from an earlier warmer period largely rest with Cook and colleagues working in Tasmania and the NZ South Island west coast, and on other proxy data. We are currently assembling and synthesising the available evidence and, yes, there is evidence that NZ was somewhat warmer towards the beginning of the millennium, but not necessarily synchronised with the Northern Hemisphere. We are also seeing some evidence of a fairly abrupt transition into the period known as the Little Ice Age. It is worth noted here though that NZ is highly sensitive to changes in atmospheric circulation changes and that generalising to the scale of the Southern Hemisphere is decidedly problematic.

W.v.B.: If we cast a glance at South America, we will find new evidence for the existence of a MWP in this part of the world. In several papers, data from stratigraphy, pollen analysis, data from marine sediment cores, data from ice cores and other proxy data has become available and could be evaluated. Villalba et al. noted in the paper "Long-term multy-proxy climate reconstructions and dynamics in South America (LOTRED-SA): State of the art and perspectives", published in: Palaeogeography, Palaeoclimatology, Palaeoecology, 281 (2009) 175 (downloadable [pdf-format, 220 KB] here: Long-term multy-proxy ...):
The review and research articles in this special issue cover a variety of archives (historical documents, tree rings, ice cores, glacier fluctuations and lacustrine sediments) and suggest that, at least south of ca. 18°S. the spatial and temporal coverage of the data is adequate to develop synoptic multi-proxy reconstructions for the past ca. 400 years. Although additonal high-quality proxy records are still needed to resolve the finer temporal and spatial structure of past climate variations, the currently available data do provide a consistent picture of climate fluctuations at a large scale.

In my opinion, this is a step forward. What is your opinion on the evidence for a MWP in South America and the Antarctica - in short for the Southern Hemisphere in general?

Dr. Fowler: There is some great work coming out of South America and I note with interest evidence of abrupt transitions that bear some similarity to what we are seeing in NZ. As for the Southern Hemisphere as a whole - I think that the definitive synthesis work has yet to be done (and probably the data are not up to it), so I am reserving judgement!

W.v.B.: Dr. Fowler, is there any doubt about the accuracy of the statement, the MWP was at least a phenomenon that occured in the Northern Hemisphere? 

Dr. Fowler: I am fairly convinced by the evidence for a North Atlantic focus for what has been called the MWP, but am also intrigued by the differences in timing of warm periods across the hemisphere. So, the answer is yes, at least in places and, in an average sense, yes probably for the hemisphere as a whole. 

W.v.B.: Would you finally please summarize your opinion on the existence of a MWP (hemispheric and global). 

Dr. Fowler: NH local - I'm convinced. NH overall - yes, in an average sense (but with regional caveats). SH overall - uncertain, but probably warmer than the LIA, and probably more subdued than the NH. Global - I'm not sure it makes sense to combine the hemispheres given unknown forcing and the dramatically different character of the evidence. However, if the NH was warm and there is no convincing evidence for a cool SH then, I guess, global warmth is a logical conclusion. 

W.v.B.: I want to thank you for being my interview partner and wish you all the best for the future.

copyright W.v.B.