Auxiliary material for
Unprecedented recent summer warmth in Arctic Canada
Gifford H. Miller1,2, Scott J. Lehman1, Kurt A. Refsnider1,2,4, John R. Southon3 and
Yafang Zhong5.
Introduction
These auxiliary files describe our calculations to estimate maximum ice cap
thickness as a function of the area of the flat-lying ground they occupy, tables with
all of the radiocarbon dates and their metadata (Table S1), and summaries of the
CMIP5 climate model results for 6 ka and 1850 AD simulations (Table S2), our
calculations to evaluate whether there is a strong elevation dependency in CMIP5
climate models between 6 ka and pre-industrial simulations (Fig. S1), and the
Maps of ~1960 AD ELA and Glacierization Level used to calculate the change in
ELA between mid-20th Century and present.
Maximum size of a small ice cap
The relationship between ice-cap thickness and ice-cap area for an idealized
circular ice cap on a horizontal plane is given by Nye, J. F. (1952). "The mechanics
of glacier flow."J. Glaciology 2: 82-93. Assuming a basal shear stress of 1 bar, the
equation collapses to h = (23x)0.5, where h is the ice thickness at distance x from
the ice edge, both expressed in meters. For a circular mountain summit of 200 m
radius, the maximum ice thickness is calculated to be 68 m.
Supplementary Table and Figure Captions
Table S1 List of all radiocarbon-dated Holocene samples with relevant metadata.
Table S2
The Coupled Model Intercomparsion Project, phase five (CMIP5;
(http://cmip-pcmdi.llnl.gov/cmip5/) coordinates a set of climate model experiments
to address outstanding scientific questions that are relevant to future international
assessments of climate science, e.g., the IPCC’s AR5. One focus of the model
experiments is long-term simulations for understanding historical and
paleoclimates using atmosphere-ocean global climate models (AOGCMs). For our
data-model comparison, we use outputs from a pre-industrial control (1850 AD),
and mid-Holocene (6 ka) experiments. Nine model groups have made data outputs
available for these experiments. There are at least 450 years of data (after initial
spin-up) for the pre-industrial control runs, and at least 100 years for the midHolocene runs. Some of the nine model groups also performed additional
ensemble simulations of pre-industrial and/or mid-Holocene runs. We use all years
and ensemble members to get mean temperatures of pre-industrial and midHolocene. As the proxy data reflect summertime conditions over land, we
calculate June-July averaged surface air temperatures over land for each CMIP
sumulation. Listed in the table are areal averages over the globe, Northern
Hemisphere (NH; 0-90°N), Arctic (60-90°N), and Atlantic sector of the Arctic
(90°W-30°E, 60-90°N) for each of the nine models and for CMIP5 model averages.
Table S3 6ka and Pre-Industrial temperature differences in 200 m elevation bins
between 0 and 2600 m asl for the North Atlantic Arctic for the 9 CMIP5 models
used in Table 2. Figure S2 is derived from these data. The rip files for Tables S2
and S3 are given in the table below.
rip code
BCCCSM1.1
CCSM4
CNRMCM5
CSIROMK3-6-0
FGOALSg2
GISSE2-R
MIROCESM
MidHolocene
Preindustrial
r1i1p1
r1i1p1
r2i1p1
r1i1p1
r2i1p1
r3i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p141
r1i1p2
r1i1p3
r1i1p1
MRICGCM3
r1i1p1
MPIESMP
r1i1p1
r1i1p1
r1i1p1
r1i1p1
r1i1p1
Figure S1 Maps of ~1960 ELA and Glacierization Level (Andrews, J.T., Miller, G.
H. (1972). “Quaternary history of northern Cumberland Peninsula, Baffin Island, N.
W. T., Canada: Part IV: Maps of the present glaciation limits and lowest equilibrium
line altitude for north and south Baffin Island” Arctic Antarctic and Alpine Research
4, 45-59) based on 1:250k topographic maps derived from vertical aerial
photography obtained between 1957 and 1961. All of our dated collection sites are
superimposed In their correct geographic coordinates, and identified by the
elevation of their collection site. For the small ice caps we collect from, the ELA is
nearly the same elevation as the regional snowline, and the difference between
collection elevation and ~1960 ELA (Fig. 2A) is a measure of the change in
snowline since the vegetation was last snow-covered.
Figure S.2 The average temperature change over land areas in the North Atlantic
Arctic (see Table S3) between 6 ka and pre-industrial time for the 9 CMIP5 models
listed in Table S2, binned in 200-m elevation increments. The slope of a linear
trend through the data suggests a very slight bias toward increasing change in
temperature at greater elevations. However, the average increase (0.035 °C km -1)
is far to small to explain the large model data differences identified in the text.