Category Archives: Blog

Blog 10: Tree rings and ENSO

By Ignacio Jara

Since the 1980’s a huge amount of time and effort has been put to build a climate-sensitive tree-ring chronology for New Zealand. A noteworthy achievement of this scientific effort was the publication in 2006 of two independent tree-ring chronologies, a 2,300-year record from Silver Pine (Lagarostrobos colensoi) and a 3,700-year chronology from Kauri (Agathis australis) (1). Since 2006, the kauri chronology has been refined and extended up to 4,500 years, becoming one of the longest tree-ring records from the Southern Hemisphere (2). To generate such a long series, Kauri wood samples were collected from more than 60 sites across its whole distribution range in northern New Zealand; including modern trees, archaeological sites and sub-fossil trees preserved in swamp areas (that is a lot of annual tree rings to count!).

Kauri

Figure 1: The present-day distribution of Kauri forest in northern New Zealand has been severely reduced by centuries of human forestry and farming. In the picture a remnant Kauri forest patch. Photo courtesy of Andamare.com

But perhaps what makes this chronology most scientifically relevant is that the dominant driver behind the present-day Kauri radial growth is El Nino Southern Oscillation (ENSO (3)). Wider tree-rings tend to occur during the relatively dry/cold El Nino years, while narrower rings are observed during the warmer/wetter La Nina years. This ENSO-kauri correlation seems to be strong throughout the entire distribution range of kauri forest, which facilitates a robust ENSO signal even if samples from multiple areas are integrated.

The potential of using the kauri chronology to decipher the waxing and waning of ENSO at timescales longer that the period instrumental record has been fully exploit in a recent publication in the journal Nature Climate Change (3). Focussing in the last 700 years of the chronology, the period with the highest statistical quality, the published ENSO reconstruction clearly depicts the twentieth century as the most “ENSO active” period in the context of the last 400 years. However, a high ENSO activity period is observed between 1,300 and 1,450 AD.

700 yr Kauri

Figure 2: The 700-years kauri-based ENSO reconstruction (in blue) in conjunction with other ENSO reconstruction from the Tropical Pacific. Image taken from Fowler et al 2012 (reference #2).

By comparing this tree-ring reconstruction with other ENSO proxies from the core of ENSO activity in the tropical Pacific, the authors further investigate the climate connection between the tropics and New Zealand (Figure 2). Relatively low variation in the kauri-based record compared with the tropical ENSO reconstructions between 1300-1750 AD suggests a reduced ENSO-New Zealand connection during the so-called Little Ice Age period. Interestingly, the authors point out that a northward shift of the Southern Westerly Winds and the subtropical front – a miniature version of a climate shift previously identified during the last Glaciation- could have resulted in a northward movement of the ENSO-south Pacific connection, pulling it away from northern New Zealand and therefore explaining the differences between these reconstructions observed during the Little Ice Age. Unfortunately, a confident ENSO reconstruction using the whole extend of the Kauri chronology is still impossible as some segments of the 4,500-year tree-ring record do not have enough samples to provide a statistically significant interpretation (2).

ENSO is one of the most influential modes of climate variation in the world. Abrupt annual anomalies in its behaviour can have a large impact on ecosystems and societies around the Pacific. For instance, in western South America -where ENSO was first identified- El Nino years are associated with very wet and warm conditions, as well as with a reduction in the upwelling of nutrient rich waters along the Pacific coast, with devastating effects on the local ecosystem and the local fishing industry. While in Queensland, Australia – the opposite side of the Pacific- El Nino is associated with drier conditions, which are linked to droughts, fires and mass coral bleaching on the Great Barrier Reef. The biggest question then is how ENSO behaviour might be affected by future global warming. Will the ENSO mode of variation be enhanced, will it increase in frequency, or will be unaffected, or reduced? Over the last few decades (pretty much the whole extension of the ENSO instrumental record), there has been an increase in ENSO activity associated with the warming trend observed during that same period. The continuation of this trend in the future is uncertain since coupled atmospheric-ocean models suggest contrasting results. This highlights the importance of developing high-resolution paleoclimate records able to investigate past associations between temperature changes and ENSO activity during the Holocene and beyond.

References

1.            E. R. Cook, B. M. Buckley, J. G. Palmer, P. Fenwick, M. J. Peterson, G. Boswijk, A. Fowler, Millennia-long tree-ring records from Tasmania and New Zealand: a basis for modelling climate variability and forcing, past, present and future. Journal of Quaternary Science 21, 689-699 (2006)10.1002/jqs.1071).

2.            G. Boswijk, A. M. Fowler, J. G. Palmer, P. Fenwick, A. Hogg, A. Lorrey, J. Wunder, The late Holocene kauri chronology: assessing the potential of a 4500-year record for palaeoclimate reconstruction. Quaternary Science Reviews 90, 128-142 (2014); published online Epub4/15/ (http://dx.doi.org/10.1016/j.quascirev.2014.02.022).

3.            A. M. Fowler, G. Boswijk, A. M. Lorrey, J. Gergis, M. Pirie, S. P. J. McCloskey, J. G. Palmer, J. Wunder, Multi-centennial tree-ring record of ENSO-related activity in New Zealand. Nature Clim. Change 2, 172-176 (2012); published online Epub03//print (http://www.nature.com/nclimate/journal/v2/n3/abs/nclimate1374.html#supplementary-information).

Blog 9: Rapid weathering and erosion of the NZ Southern Alps

By Ignacio Jara and Helen Bostock

The debate about the role the Southern Alps (New Zealand) uplifting to offset global climate change has experienced a recent renewal with the publication of a couple of new articles. According to the Uplifting Theory, over geological time the tectonic erection of large mountain systems has been associated with lower global temperatures due to the enhanced consumption of atmospheric CO2 during the chemical weathering of terrestrial (silicate) material from the uplifting landscapes.

This year, Larsen et al (2014) reported new catchment erosion and soil production rates from the Southern Alps. Using Beryllium 10 (10Be) -a radioactive isotope produced by cosmic rays bombarding exposed mineral surfaces- the team found the highest measured rates of erosion and soil production (1). These chemical weathering rates (of about 2.5 mm/yr) are an order of magnitude higher than previously measured and higher than the suggested kinetically controlled limit. Moreover, the authors point out that soil weathering increases with erosion rate. The study also challenges the current dogma that high erosional environments are inefficient for soil production due to low residence time of key minerals. Even though denudation rates –the speed at which the landscape surface is eroding- exceed soil production in every catchment studied, the landscape is able to maintain a continuous soil mantle as a result of the dense vegetation cover (the main biotic soil producer in New Zealand due to the lack of native burrowing mammals). This seems to be a key point, as root expansion is an efficient mechanism for converting rock into soil, either by the physical breaking of bedrock or by increasing the concentration of organic acid and sub-surface CO2.

Thus, contrary to previous assumptions, this study demonstrates that rapidly uplifting mountains with dense vegetation cover are active weathering systems. The authors thus suggest that –

“These high weathering rates support the view that mountains play a key role in global-scale chemical weathering and thus have potentially important implications for the global carbon cycle”

Since weathering can only drive global climate trends if the erosion of silicate rocks removes enough CO2 from the atmosphere, the next step is to evaluate the amount of CO2 consumption resulting from the uplift of the Southern Alps.

This was the focus of another recent study in the Southern Alps using Calcium isotopes and Ca/Na ratios from rivers (2). They also show that weathering rates were an order of magnitude higher than the global mean and that the region has some of the highest chemical weathering rates in the world. However, they found that the majority of the weathering in non-glacial catchments was from carbonate rather than silicate rocks (a reaction that does not use atmospheric CO2), and thus the consumption of CO2 was no higher than the global average. Higher silicate weathering was only found in rivers downstream of glaciated regions. When these increased silicate weathering rates are extrapolated to all glaciated montane regions around the world, however, they only account for <1% of the global silicate weathering and global atmospheric CO2 consumption. Therefore they conclude that silicate weathering in uplifting mountain ranges like the Southern Alps has little effect on the global carbon cycle and long-term climate change.

Further work is clearly needed to better assess the weathering rates in tectonically active regions and then assess the role of mountain building and erosion in the global carbon cycle. Interestingly, enhancing weathering rates has recently been assessed as a potential geo-engineering strategy to offset increasing anthropogenic CO2 (3).

  1. Larsen et al., 2014. Rapid Soil Production and Weathering in the Southern Alps, New Zealand. Science 343, 637-640, doi:10.1126/science.1244908.
  2. Moore et al., 2013. Tracking the relationship between mountain uplift, silicate weathering, and long-term CO2 consumption with Ca isotopes: Southern Alps, New Zealand. Chemical Geology, 341, 110-127, doi:10.1016/j.chemgeo.2013.01.005.
  3. Hartmann, et al., 2013. Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification. Reviews of geophysics, 51, 113-149.

Blog 8: Global volcanism linked to late-Quaternary deglaciations

By Ignacio A. Jara

It has been widely documented that volcanic aerosols can alter the radiative balance of the atmosphere, producing measurable temperature depressions following large explosive eruptions. Perhaps the most renowned of these cases is the eruption of the mount Tambora in Indonesia, which in 1815 caused a decline of 0.5°C in the Northern Hemisphere in what was known as “the year without a summer” (1).

But, what if this causal relationship is inverted and climate change affects the number of volcanic eruptions?

In 1979 Rampino et al., first used proxy data to argue that periods of climate cooling were associated with catastrophic volcanic events such as the latest Taupo eruption in New Zealand or the mega eruption of the Toba volcano in Indonesia (~70,000 cal yrs) (2). The authors boldly proposed that the changes in ice extent and sea level during glaciations resulted in sufficiently large variations in the Earth’s crustal stress to alter volcanic activity over longer time scales.

Since this pioneering study, a considerable number of publications have added local evidence supporting long-term climate variations as a driver for volcanic activity. However, solid evidence pointing to a definitive link between glacial cycles and volcanism at global scales as remained elusive.

An interesting article published in 2012 adds new insights into this theory. They analysed the timing of more than 400 tephra layers identified in marine sediment records around the Pacific “Ring of Fire” over the last 1 Myr (3). The spectral analysis of tephras deposited off South and Central America, Japan, the Philippines and the Southern Pacific islands reveals that periods of increased volcanic eruptions have a recurrence of 41 kyr, the exact periodicity of the Earth’s obliquity. Moreover, phase analysis indicates that peaks in volcanic eruptions lag behind minimum ice volume and maximum sea level by about 4 kyr.

Since obliquity has been recognized as one of the orbital pacemakers of the Pleistocene ice ages, these results indicate a direct link between orbital cycles, glacial/interglacial climate and global volcanism. The authors further suggest that this link could be mediated by surface pressure variations resulted from ice-ocean mass redistribution during periods of abrupt climate change. In this regard, volcanic eruptions along the edge of continental plates are expected to occur at greater frequency during periods of deglaciation, when ice retreat caused crustal pressure to decrease, lowering the compression of the rock overlying the magma chambers.

Undoubtedly these results are preliminary and much more work is required to better understand this phenomena. Even though the duration of the present-day warming trend is several orders of magnitude quicker than climate variations presented here, the interplay between climate change, ice extent and volcanism might be relevant in high-latitude regions with ice caps and active volcanic zones. As the atmosphere heats up at an unprecedented rate, the removal of large ice loads may contribute to the reactivation of volcanic complexes or the emergence of unknown volcanic systems.

References

1.            K. R. Briffa, P. D. Jones, F. H. Schweingruber, T. J. Osborn, (1998) Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years. Nature 393, 450-455.
2.            M. R. Rampino, S. Self, R. W. Fairbridge, (1979) Can rapid climatic change cause volcanic eruptions? Science 206, 826-829.
3.            S. Kutterolf, M. Jegen, J. X. Mitrovica, T. Kwasnitschka, A. Freundt, P. J. Huybers, (2012) A detection of Milankovitch frequencies in global volcanic activity. Geology, (doi: 10.1130/g33419.1).