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Uncertainty & confidence in tree‐ring records at
mul5‐decadal to centennial 5mescales
Tim Osborn
Clima5c Research Unit, School of Environmental Sciences, UEA
with contribu5ons from
Keith Briffa and Tom Melvin
TwiGer: @TimOsbornClim 3 September 2015
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Tree‐ring measurements as climate proxies
Problems include: • Biological proxy – Mul5ple controlling processes (T, light, moisture, nutrients etc.) – Age effects
Advantages include: • Precise, absolute da5ng – Possible because mul5ple samples with annual resolu5on & strong common signal allows cross‐da5ng
• Mul5ple samples – Stronger climate signal and allows uncertainty es5mates
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Realis5c error es5mates for proxy data & climate reconstruc5ons
Interes6ng challenge, combining process understanding with sta6s6cal analysis – Great poten5al for sta5s5cal analysis for tree‐rings because we have large samples with precise, absolute da5ng
Two components (this talk focusses on first one): • Confidence in the proxy record itself • Confidence in the clima5c interpreta5on & reconstruc5on
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Es5ma5ng confidence in tree‐ring records ..1
Centennial–millennial ambi6on but approaches oDen only measure annual–decadal behaviour
– (1) Expressed Popula5on Signal (EPS) – where n = sample size (number of trees sampled) – and rbar = mean inter‐series correla5on – But rbar determined mostly by annual–decadal common signal
– (2) Calibra5on and verifica5on against climate observa5ons – But result determined mostly by annual–decadal correspondence because limited length of instrumental record
Esper et al. (2004) Climate reconstruc5ons: low‐frequency ambi5on and high‐frequency ra5fica5on. EOS 85, 113‐120 Wigley et al. (1984) On the average value of correlated 5me series… J Clim Appl Meteor 23, 201‐213
EPS ! nr1+ n"1( )r
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Es5ma5ng confidence in tree‐ring records ..2
Errors tend to diminish from annual to decadal 6mescales, but may then increase again out to longer 6mescales – Different mean growth rates, differences between trees’ life cycles, uncertainty in the expected growth–age curve
Centennial–millennial confidence can only be assessed by considering agreement between independent long records
Briffa et al. (2013) Reassessing the evidence for tree‐growth and inferred temperature change… QSR 72, 83‐107
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Yamalia case study: Yamal & Polar Urals Living & sub‐fossil Larix sibirica trees
Polar Urals: TRW & MXD
Yamal: TRW, but bigger sample
Fig. 1, Briffa et al. (2013) QSR Yamal: 473 sub‐fossil, 160 living
Polar Urals: 61 sub‐fossil (+18 for MXD), 62 living
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Yamal chronology: Full sample mean &
standard error
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Fig. 9, Briffa et al. (2013) QSR Red/Black: two versions of the overall mean chronology (thin lines where < 6 trees sampled)
Cyan: +/‐2 SE of the mean for 50‐year smoothed data. Grey: sample counts
1000 1400 1800 2000
400 BC 0 400 CE 800
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Yamal chronology: Calibrated JJ temperature reconstruc5on with error
components
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Fig. 11, Briffa et al. (2013) QSR Red/Black: two versions of the overall mean chronology (thin lines where < 6 trees sampled)
Cyan: +/‐2 SE of the mean for 50‐year smoothed data. Grey: sample counts
1000 1400 1800 2000
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Yamal chronology: Compare different sites
Only for recent centuries where mul6ple sites with living trees are available
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Fig. 2, Briffa et al. (2013) QSR Mean chronology for each site and SUB for sub‐fossil (thin lines where < 4 trees sampled)
1600 1800 2000
1600 1800 2000
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Fig. 2, Briffa et al. (2013) QSR Mean chronology for each site and SUB for sub‐fossil (thin lines where < 4 trees sampled)
BoGom panel smoothed with a 20‐yr spline
1600 1800 2000
1600 1800 2000
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Yamal chronology: Compare different growth
rate samples Two independent chronologies by alloca6ng half samples to each group based on their mean growth rate aDer removing age effects & common (climate) signals
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Ring age (yr) 50 100 200 300 400
0 500 1000 1500 2000 Fig. 3, Briffa et al. (2013) QSR ‐ Mean chronology for each growth‐rate class (thin lines where < 8 trees sampled)
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Yamal chronology: Compare with Polar Urals
Two Polar Urals records (TRW & MXD) but both have much smaller sample sizes &
span only 1 millennium MXD sensi6ve to a longer summer season
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Fig. 3, Briffa et al. (2013) QSR Mean chronology for each growth‐rate class (thin lines where < 8 trees sampled)
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Fig. 3, Briffa et al. (2013) QSR Mean chronology for each growth‐rate class (thin lines where < 8 trees sampled)
r( Yamal , Polar Urals) = 0.82 (< 15 yr) 0.78 (15–100 yr) 0.69 (> 100 yr)
r( Yamal , Polar MXD) = 0.55 (< 15 yr) 0.72 (15–100 yr) 0.52 (> 100 yr)
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Closing remarks
Confidence in tree‐ring records on century‐to‐millennium 6mescales:
– Don’t assume they are reliable. Evaluate them – Don’t assume errors are random. Try to iden5fy systema5c biases – Larger samples are needed to establish confidence at the longest 5mescales
– Clear advantage of tree‐rings: mul5ple, precisely dated samples
Scope to extend this by collabora6on with sta6s6cians – using hierarchical sta5s5cal models of the data genera5ng process – extension to spa5o‐temporal models (this example is one site, but a widespread network exists across parts of the NH)
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Spare slides relevant for some ques5ons
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Qilian Mountains case study: NE Tibetan Plateau
Fig. 1, Yang et al. (2014) PNAS 1203 sub‐fossil + living
Qilian Juniper, very long lived, slow growing, TRW only
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Qilian chronology: Compare different
sites Living and archaeological samples
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Fig. SB2, Yang et al. (2014) PNAS
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Fig. SB2, Yang et al. (2014) PNAS
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Qilian chronology: Compare different growth
rate samples Four independent chronologies by alloca6ng quarter of samples to each group based on their mean growth rate aDer removing age
effects & common (climate) signals
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Fig. SB9, Yang et al. (2014) PNAS
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Fig. SB10, Yang et al. (2014) PNAS
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Sampling sites near the edge of a
species’ ecological range to obtain a single dominant
growth control Near the la6tudinal or eleva6onal tree line
(can also reduce influence of stand dynamics where this coincides with a more open canopy forest)
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Magnified view
Pine tree
Processed wood core
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Natural archives of past climate?
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Trees cannot grow above/beyond their tree-line In very cold regions, tree growth is limited by temperature
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Polar Urals root samples Difference in mean ring width between bole (trunk) & root collar samples can
result in biased tree‐ring record
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Example of sampling bole (trunk) at “breast height” Tom Melvin, Northern Norway, 1 June 1999
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Example of root collar with large diameter than the bole Remnant Scots Pine, near Torneträsk, Sweden
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Polar Urals TRW record: original used bole samples, “update” included root collar samples
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Iden5fying previous periods with elevated tree mortality
End‐aligned spans of tree samples, but note that sub‐fossil segments may not be complete
(outer rings not preserved, pith not sampled)
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Khadytla River One of the living-tree
sites in the Yamal region
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-6000 -4000 -2000 0 2000Year BC/AD
0
100
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300
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500
600
700
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Num
ber o
f sta
cked
sam
ples
Example for Tornetrask, northern Sweden