Past and Future Temperature Evolution off West Svalbard Arctic Ocean
To arrive at appropriate starting conditions for modeling the next three centuries, we
preconditioned the model of the present-day gas hydrate system, using a combination
of measured and estimated records of temperature for the past two millennia.
Temperature series were constructed for a period of 2005 yr at 350, 400, 420, 450,
500, 600, 700 and 800 m water depth (mwd) (Figure S1a) on a transect across the
margin (Figure 1). To model the period 1975-2005, we used near-seabed temperature
means at the above water depths, given by May-to-October CTD measurements in our
study area, from Westbrook et al. [2009, Figure 3b] and the temperature anomalies,
temperature deviations from the mean at that period, at 400 mwd from Thatcher et al.
[2013]. The temperature anomalies for the other water depths were calculated by
scaling the variation in temperature measured at 400 mwd in proportion to the ratio
between the standard deviation from the mean at each depth and the standard
deviation from the mean at 400 mwd. We took the variation of standard deviation of
temperature with depth from the profiles of Polyakov et al. [2003, Figure 2] and,
assuming that the temperature variability with depth at the profile location, 82.85°N
25.79°E, is the same as that in our study site, scaled these standard deviations to be
appropriate to our study transect using the ratio between the standard deviation of the
temperature anomalies at 400 mwd at the study transect and 400 mwd at Polyakov et
al.’s [2003] profile. For the period 1950-1975, we used mean values for the period
1975-2005 from Westbrook et al. [2009], and the anomalies from Ocean Station Mike
at 50 mwd [Holliday et al., 2008]. These anomalies were scaled to match the standard
deviation of the anomalies for the period 1975-2005 for each water depth. For the
period 1900-1950, we used the mean temperature and 15-year-running mean
anomalies at 150-300 mwd for region 4 from Polyakov [2004]. For each water depth,
these anomalies were scaled to match the standard deviation of the anomalies for the
period 1950-1975, and the temperatures were offset to make the temperature at 1950
the same as the mean for the period 1950-2005. For the period 1-1900, we used 50year-running summer temperature means at 50 mwd, obtained from planktic
foraminifera proxy data in a borehole nearby our study area and using the SINMAX
modern analog technique [Spielhagen et al., 2011], and scaled them to our location
and water depths by applying a linear regression between the near-surface
temperatures at the site of Spielhagen et al. [2011] and the temperatures at the water
depths along our transect given by the ocean/sea-ice model NEMO [Madec, 2012] at
1/12° resolution (ORCA12). The temperatures for this period were offset to make the
temperature at 1900 the same as the mean for the period 1900-1950.
The temperatures for the period 2005-2300 were mean annual seabed temperatures
given by climate models HadGEM2 [Collins et al., 2011a, 2011b] and CCSM4 [Gent
et al., 2011] under climate-forcing scenarios Representative Concentration Pathways
(RCPs) 8.5 and 2.6 [Moss et al., 2010], except for that from CCSM4 model RCP 8.5
which goes from 2005-2250 (Figures 2a and 2b). These temperatures were
interpolated to our study location and offset (Table S1) to make the temperature at
2005 the same as the mean temperature for the period 1975-2005 given by CTD
measurements in our study area.
1900
4
1940
1980
2005
2
Temperature [°C]
4
3
0
Water Depth [m]
350
400
450
RWP
500
DACP
600
700
IP
800
MCA
LIA
2
1
Methane Flow [mol!
y-1m-2]
0
(a)
-1
100
1
1940
1900
100
1E-5
1980
2005
1
1E-10
0.01
1E-20
1
(b)
200
400
600
800
1000
Time [yr]
1200
1400
1600
1800
2005
Figure S1. (a) Past temperature constructed for our study transect (Figure 1) for the
period 1-2005 yr and for 800, 700, 600, 500, 450, 400, 350 meters water depth (mwd).
This period is subdivided into the Roman Warm Period (RWP, until ~600 CE), the
Dark Ages Cold Period (DACP, ~600 to ~900 CE), the Medieval Climate Anomaly
(MCA, ~900 to ~1500 CE), the Little Ice Age (LIA, ~1500 to ~1900 CE), and the
Industrial Period (IP) [Spielhagen et al., 2011]. Temperature data were obtained from
Spielhagen et al., [2011] for the period 1-1900, from Polyakov [2004] for 1900-1950,
from Holliday et al. [2008] for 950-1975, and from Westbrook et al. [2009] and
Thatcher et al. [2013] for 1975-2005. (b) Methane flow at the seabed. The insets in
(a-b) show details for the period 1900-2005. Note that at 350 mwd (red line), the
increase in methane outflow at ~1967 yr is due to free methane gas coming from the
deeper part of the model contributing to the methane outflow in the upper part.
Table S1: Temperature bias at 2005 yr given by the climate models with respect to
the mean seabed temperature for the period 1975-2005 given by CTD measurements
in our study area [Westbrook et al., 2009]. Positive temperature offsets indicate higher
temperatures predicted by the climate models.
Climate Model
HadGEM2
CCSM4
HadGEM2
CCSM4
RCP
8.5
8.5
2.6
2.6
350
3.32
-0.26
3.09
-0.35
Offsets applied [°C] for each water depth [m]
400
420
450
500
600
700
3.54
3.55
3.56
3.64
3.56
3.46
-0.06 0.00
0.09
0.35
0.82
1.49
3.31
3.29
3.26
3.31
3.25
3.08
0.13
0.06
0.05
0.32
0.82
1.55
800
3.94
2.15
3.49
2.26
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