REDUCED RISK OF NORTHAMERICAN COLD EXTREMESDUE TO CONTINUED ARCTICSEA ICE LOSS
by James A. Screen, Clara Deser, and L antao Sun

Contrary to recent claims, North American cold extremes are expected to become lessfrequent as a result of continuing Arctic sea ice loss.

In early January 2014, an Arctic air outbreakbrought extreme cold to central and eastern North
America. Record low minimum temperatures forthe calendar date were set at many weather stations,
including at Chicago, Illinois (O’Hare Airport,–26.7°C/–16°F, 6 January); New York, New York(Central Park, –15.6°C/4°F, 7 January); Washington,
D.C. (Dulles Airport, –17.2°C/1°F, 7 January); and as
far south as Atlanta, Georgia (–14.4°C/6°F, 7 January)

The abstract for this article can be found in this issue, following the tableof contents.
DOI:10.1175/BAMS-D-14-00185.1

and Austin, Texas (Bergstrom Airport, –11.1°C/12°F,
7 January).1 Daily maximum snowfall records werealso broken at several stations, including Buffalo,
New York (7.6˝, 8 January), and St. Louis, Missouri(10.8˝, 5 January).
The cold temperatures and heavy snowfall caused
widespread disruption to transport and power supply, closure of work places and public services, anddamage to agricultural crops—all with significant
economic implications. Unsurprisingly, given the
disruption, the national and global media extensively reported the cold snap, including debate onwhether human-induced climate change was partly
responsible. Related to this, one particular hypothesis
garnered considerable attention: the suggestion that
rapid Arctic warming and associated sea ice loss may
be increasing the risk of cold extremes.
The media were not alone in making this link. In
the midst of the frigid conditions, the White House
released a public information video claiming that,paradoxically, cold extremes will become more likely
as a result of global warming. President Obama’s
Science Advisor, Dr. John Holdren, stated that “theIn final form 11 November 2014©2015 American Meteorological Society

1

AFFILIATIONS: Screen—College of Engineering, Mathematics andPhysical Sciences, University of Exeter, Exeter, United Kingdom;
Deser and Sun—Climate and Global Dynamics, National Center for
Atmospheric Research,* Boulder, Colorado* The National Center for Atmospheric Research is sponsored by the National Science Foundation.
CORRESPONDING AUTHOR: James A. Screen, College of Engi-

neering, Mathematics and Physical Sciences, University of Exeter,
Laver Building, 6th FL., North Park Road, Exeter EX4 4QF, United
KingdomE-mail: j.screen@exeter.ac.uk

AMERICAN METEOROLOGICAL SOCIETY

Data from the National Weather Service (www.nws.noaa.gov/climate).
SEPTEMBER 2015

| 1489kind of extreme cold being experienced by much of
the United States as we speak is a pattern that we can
expect to see with increasing frequency as global
warming continues.”
The cited explanation was that Arctic sea ice
loss specifically, or Arctic amplification (the greater
warming of the Arctic than lower latitudes) moregenerally, is increasing the likelihood of the typeof weather patterns that lead to cold extremes. The
scientific basis for this statement is derived from anumber of recent observational and modeling studies(Honda et al. 2009; Petoukhov and Semenov 2010;
Francis and Vavrus 2012; Inoue et al. 2012; Liu et al.
2012; Yang and Christensen 2012; Tang et al. 2013;
Cohen et al. 2014; Vihma 2014; Walsh 2014).
However, key aspects of some of these aforementioned studies have been questioned (Barnes 2013;
Screen and Simmonds 2013; Barnes et al. 2014; Gerber
et al. 2014; Woollings et al. 2014) and counterarguments put forward (Hassanzadeh et al. 2014; Fischer
and Knutti 2014; Screen 2014; Wallace et al. 2014).
Furthermore, these studies have largely focusedon relationships in the present-day climate. Only afew studies have considered the global impacts offuture sea ice loss (e.g., Deser et al. 2010; Peings andMagnusdottir 2014; Deser et al. 2015) and these havefocused on seasonal-mean changes. Future changes incold extremes in response to projected Arctic sea ice
loss require further study. Here we specifically focus
on North America, prompted by the events of winter2013/14 and the extensive media coverage it received.
HOW UNUSUAL WAS WINTER 2013/14?
We start with a brief overview of the winter of 2013/14,
based on gridded temperature data from the National
Centers for Environmental Prediction–National
Center for Atmospheric Research (NCEP–NCAR)
reanalysis (Kalnay et al. 1996). The period December 2013 through February 2014 was, on average,
anomalously cold over most of North America east of
the Rocky Mountains (Fig. 1a), while the Southwest
United States and northeast Canada were anomalously warm. The winter of 2013/14 was punctuatedby several cold air outbreaks, the most severe of which
occurred around 7 January 2014. Compared to thedaily average for this date in period 1980–99, thelargest anomalies on 7 January 2014 were experienced
in the eastern United States, with –20°C anomalies
stretching from Ohio to as far south as Florida (Fig.
1b). Averaged over central to eastern North America(CENA; 26°–58°N, 70°–100°W; black box in Fig. 1b),
daily mean temperatures were well below average forFig. 1. North American temperature anomalies for (a) the winter of 2013/14 and (b) 7 Jan 2014.
Anomalies are relative to the period 1980–99. (c) Daily mean temperature averaged over CENA
[black box in (b)] for 1 Nov 2013–31 Mar 2014 (black curve) and the daily 1980–99 climatology(gray line). Blue (orange) shading shows days colder (warmer) than the average for that day. (d)North American temperatures for 7 Jan 2014.

1490 |SEPTEMBER 2015

Fig. 2. (a) Histogram of daily winter temperatures averaged over CENA during the period
1980–99. (b) As in (a), but based on the period 2000–13. Green lines are drawn at –16.8°C andcorrespond to the temperature on 7 Jan 2014. Numbers in the top left and top right of eachpanel are the mean temperature (°C) and standard deviation (°C), respectively.

large portions of the winter (Fig. 1c). The coldest dailymean temperature over CENA during the winter of
2013/14 occurred on 7 January, recording –16.8°C.
On this day, temperatures averaged below –20°C overcentral Canada and west of the Great Lakes and below –10°C over most of the United States east of theRockies (Fig. 1d).
Next, we consider how “extreme” the cold conditions were over CENA on 7 January 2014. Figure 2ashows the probability distribution function (PDF) of
daily mean temperatures, averaged over CENA, during the winter months of the late twentieth century(1980–99). The vertical green line is drawn at –16.8°C,
corresponding to the mean temperature on 7 January 2014. The 7 January event falls in the tail of the
distribution, but it is not unprecedented in the recent
past. The coldest day over this period occurred on
19 January 1994 (–20.3°C). During 1980–99, 20 days
had a daily mean CENA temperature as cold or colder
than –16.8°C, spread across six winters (Table 1). Such
events have often occurred in clusters, with multiple
days this cold in several years. Based on the 1980–99
PDF (Fig. 2a), the 7 January 2014 event has a probability of 1.1%, which equates to an average return
period of one year (since there are 90 winter days peryear). Viewed in this light, the recent event does notseem to be a rare occurrence.
At first glance it may seem odd that so many longterm station records were broken on 7 January 2014, if
an event of such severity is not uncommon. However,
the records referred to in the opening paragraph,and comparable records widely quoted in the media
AMERICAN METEOROLOGICAL SOCIETY

reporting of this event, refer to the fact that the temperature on 7 January 2014 was colder than those on
the same date in previous years, but it was not necessarily colder than on all dates in previous years. Cold
extremes occur throughout the winter and not always
on the same date. For example, days equally cold orcolder than –16.8°C over CENA since 1980 have occurred on dates from mid-December to early February(Table 1) but only once on 7 January—and that was in2014. The probability of a cold extreme occurring on aparticular date is therefore much smaller than the probability of it occurring on any date. Hence, the breakingof records for a particular date is not necessarily a goodmeasure of how extreme an event is.
A somewhat different perspective on the extremity of the 7 January event arises if a more recent
reference period is considered. Figure 2b showsan analogous PDF based on daily winter CENA
temperature during the period 2000–13. Arctic sea
ice loss has accelerated in this period (Stroeve et al.
2012), so if there were a detectable influence of sea
ice loss on cold extremes, we would expect to see itover this time period. Between 2000 and 2013, only
one day (16 January 2009) was colder than –16.8°C,
giving a probability of 0.08% (one day in 14 years).
Thus, the 7 January 2014 event could be perceived
as extreme compared to temperature minima in theearly twenty-first century, which may help explain
the media and public perception of this event being
extreme. However, clearly this event was not uncommon in a longer-term context. Only a decade or two
earlier, events of comparable magnitude occurred
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Table 1. Days equally as cold or colder than 7 Jan 2014 over CENA since 1980, based on the NCEP–
NCAR reanalysis.
Date

CENA T (°C)

Date

CENA T (°C)

Date

CENA T (°C)

25 Dec 1980

–16.8

25 Dec 1983

–20.2

18 Jan 1994

–17.5

10 Jan 1982

–18.2

20 Jan 1984

–17.9

19 Jan 1994

–20.3

11 Jan 1982

–18.6

21 Jan 1984

–17.4

1 Feb 1996

–18.4

17 Jan 1982

–20.1

22 Dec 1989

–20.2

2 Feb 1996

–18.2

19 Dec 1983

–17.5

23 Dec 1989

–19.6

3 Feb 1996

–20.2

20 Dec 1983

–17.2

15 Jan 1994

–18.2

4 Feb 1996

–19.8

24 Dec 1983

–18.1

16 Jan 1994

–18.4

16 Jan 2009–17.2

relatively frequently. This simple comparison suggests that cold extremes are becoming less frequent,
not more frequent, consistent with previous studies(Alexander et al. 2006; Donat et al. 2013) and theanticipated response to global warming (Kharin
et al. 2007, 2013). However, such interpretation mustbe treated with caution, as the time periods considered are very short with few extremes (by definition)
upon which to calculate robust statistics.
MODELS AND SIMUL ATIONS. We now
turn our attention to quantifying future changes in
CENA cold extremes, with particular focus on the
projected changes driven by continued Arctic sea
ice loss. To do this, we analyze large ensembles ofcoupled model simulations that have been forced bygreenhouse gas (GHG) increases and that produce
reductions in Arctic sea ice (Stroeve et al. 2012),
among other impacts, and ensembles of atmosphericmodel simulations forced by solely the GHG-induced
Arctic sea ice loss with all other forcing factors held
constant.
To estimate the response to projected increases in
GHG, we utilize coupled climate model simulations
from the fifth phase of the Coupled Model Intercomparison Project (CMIP5; Taylor et al. 2012). We chose
to use the representative concentration pathway 8.5(RCP8.5), which is a high-end (“business as usual”)
scenario with a rapid rise in GHG concentrations
through the twenty-first century, for two reasons:first, to maximize the signal-to-noise ratio, and second, because observed Arctic sea ice reductions track
those simulated under RCP8.5 more closely thanthose under any of the lower-end scenarios (Stroeve
et al. 2012). We sample the projections at two time
periods, 2030–49 and 2080–99, representative of the
mid-twenty-first century (C21) and late C21, respectively. The projections are compared to the baseline
period 1980–99, representative of the late twentieth
1492 |SEPTEMBER 2015

century (C20). Data for this period come from theCMIP5 historical simulations of the models. The historical simulations have all been forced with observed
concentrations of GHG, aerosols, ozone, and naturalforcings (solar, volcanic eruptions) from 1850 to 2005.
We analyzed one ensemble member from each of the
34 models that had all necessary data available for thehistorical and RCP8.5 experiments.
To isolate the influence of sea ice, we performed
atmospheric model simulations with prescribed sea
ice concentration, sea ice thickness, and sea surface
temperature (SST). For this we used the atmosphericcomponents of the Hadley Centre Global Environmental Model, version 2 (HadGEM2; Collins et al.
2011), and the Community Climate System Model,
version 4 (CCSM4; Gent et al. 2011)—namely, theHadley Centre Global Atmospheric Model, version 2(HadGAM2), and the Community Atmosphere Model, version 4 (CAM4), respectively. The version of HadGAM2 used here has a horizontal resolution of 1.875°
longitude × 1.25° latitude and 38 vertical levels. CAM4
has a horizontal resolution of 1.25° longitude × 0.9°
latitude and 26 vertical levels. We performed threeexperiments using both models, each experiment
having repeating seasonal cycles of sea ice conditions representative of a different time period—thelate C20, the mid-C21, and the late C21 (as defined
above). These sea ice conditions were taken from theCMIP5 integrations of HadGEM2-ES and CCSM4(i.e., sea ice from HadGEM2-ES was prescribed in
HadGAM2 and sea ice from CCSM4 was prescribedin CAM4), averaged across the 20 years of the chosen
period and all available ensemble members. Specifically, we used five HadGEM2-ES historical runs, four
HadGEM2-ES RCP8.5 runs, six CCSM4 historicalruns, and six CCSM4 RCP8.5 runs. In the HadGAM2
experiments, sea ice thickness was derived empirically from the sea ice concentrations. In the CAM4
simulations, the prescribed sea ice thicknesses werebased on climatologies from the CCSM4 simulations
for each period (the late C20, the mid-C21, and thelate-C21; i.e., in the same manner as the sea ice concentrations). The treatment of SST was as follows. In
the C20 experiment, SSTs were held to the climatologyof the late C20, using the ensemble-mean SST fromthe HadGEM2-ES and CCSM4 historical simulations.
In the mid- and late C21 experiments, the SST was
also held to the climatology of the late C20, except at
grid boxes where sea ice was lost. At these locations,the climatological SST of the mid-C21 or late C21
was used, taken from the HadGEM2-ES and CCSM4
RCP8.5 ensemble means. This procedure accounts forthe local SST warming associated with reduced sea ice
cover, but it excludes remote SST changes that are notdirectly tied to the ice loss (see Screen et al. 2013; Deseret al. 2015). The three experiments were each run for
260 years. In this modeling framework, each modelyear can be considered an independent ensemble
member starting from a different atmospheric initialcondition. By running very large ensembles, we aim to
fully capture the large intrinsic atmospheric variability.
The details of each model experiment are summarizedin Tables 2 and 3.

CONTINUED ARCTIC SEA ICE LOSS.
Ensemble-mean winter sea ice concentrations in
HadGEM2-ES during the late C20 and the mid- andlate C21 are shown in Figs. 3a–c, respectively. The
projected loss of sea ice in the mid-C21, relative to thelate C20, is fairly small (–1.5 million km2). The largest
local changes in sea ice cover are found in the Barents
Sea (cf. Figs. 3a,b). By late C21, however, HadGEM2ES simulates almost ice-free conditions in winter (Fig.
3c). Ice cover is maintained predominantly in coastal
regions and embayments. Analogous plots for CCSM4
are shown in Figs. 3d–f. CCSM4 also simulates amodest change in winter sea ice cover between thelate C20 and the mid-C21 (–1.3 million km 2). The
largest changes in sea ice cover in CCSM4, in the late
C21 relative to the late C20, are found in the Bering,
Beaufort, and Chukchi Seas (Fig. 3f). In the late C21,
CCSM4 simulates considerably more winter ice (8.6
million km2) than HadGEM2-ES (3.5 million km2).
The change in winter sea ice area between the late
C20 and the late C21 is –10.3 and –4.8 million km2, as
simulated by HadGEM2-ES and CCSM4, respectively.
Figure 3g shows the winter sea ice area changes inthese two models overlaid on the projected changes

Table 2. Details of the model simulations analyzed. Asterisk means one ensemble member permodel. The 34 CMIP5 models analyzed are listed in Table 3.
Time period
analyzed

Ensemble members

Years of simulation

Historical

1980–99

34*

680

RCP8.52030–49

34*

680

RCP8.52080–99

34*

680

Historical (A)1980–99

5100

RCP8.5 (B)

2030–49

4

80

RCP8.5 (C)2080–99

4

80

Historical (D)1980–99

3

60

RCP8.5 (E)2030–49

6

120

RCP8.5 (F)2080–99

6

120

Mean sea ice from A, SST from A

Annually
repeating

260

260

Mean sea ice from B, SST from A

Annually
repeating

260

260

Mean sea ice from C, SST from A

Annually
repeating

260

260

Mean sea ice from D, SST from D

Annually
repeating

260

260

Mean sea ice from E, SST from D

Annually
repeating

260

260

Mean sea ice from F, SST from D

Annually
repeating

260

260

Model(s)CMIP5HadGEM2-ES

CCSM4

HadGAM2

CAM4

Forcing

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Table 3. The names of the 34 CMIP5 models.
Acronym

Expansion

ACCESS1.0

Australian Community Climate and Earth-System Simulator, version 1.0

ACCESS1.3

Australian Community Climate and Earth-System Simulator, version 1.3

BCC_CSM1.1

Beijing Climate Center, Climate System Model, version 1.1

BCC_CSM1.1(m)Beijing Climate Center, Climate System Model, version 1.1 (moderate resolution)

BNU-ESM

Beijing Normal University–Earth System Model

CanESM2

Second Generation Canadian Earth System Model

CCSM4Community Climate System Model, version 4

CESM1-BGC

Community Earth System Model, version 1 (biogeochemistry)

CESM1(CAM5)

Community Earth System Model, version 1 (Community Atmosphere Model, version 5)

CMCC-CESM

Centro Euro-Mediterraneo per I Cambiamenti Climatici Carbon Cycle Earth System Model

CMCC-CM

Centro Euro-Mediterraneo per I Cambiamenti Climatici Climate Model

CMCC-CMS

Centro Euro-Mediterraneo per I Cambiamenti Climatici Stratosphere-Resolving Climate Model

CNRM-CM5

Centre National de Recherches Météorologiques Coupled Global Climate Model, version 5

CSIRO Mk3.6.0

Commonwealth Scientific and Industrial Research Organisation Mark 3.6.0

EC-EARTH

European Consortium Earth System ModelGFDL CM3

Geophysical Fluid Dynamics Laboratory Climate Model, version 3

GFDL-ESM2G

Geophysical Fluid Dynamics Laboratory Earth System Model with the Generalized Ocean Layer
Dynamics (GOLD) component

GFDL-ESM2M

Geophysical Fluid Dynamics Laboratory Earth System Model with the Modular Ocean Model (MOM),
version 4 component

GISS-E2-H

Goddard Institute for Space Studies Model E2, coupled with the Hybrid Coordinate Ocean Model(HYCOM)

GISS-E2-R

Goddard Institute for Space Studies Model E2, coupled with the Russell ocean model

HadGEM2-CC

Hadley Centre Global Environment Model, version 2–Carbon Cycle

HadGEM2-ES

Hadley Centre Global Environment Model, version 2–Earth System

INM-CM4.0

Institute of Numerical Mathematics Coupled Model, version 4.0

IPSL-CM5A-LR

L’Institut Pierre-Simon Laplace Coupled Model, version 5A, low resolution

IPSL-CM5A-MR

L’Institut Pierre-Simon Laplace Coupled Model, version 5A, medium resolution

IPSL-CM5B-LR

L’Institut Pierre-Simon Laplace Coupled Model, version 5B, low resolution

MIROC-ESM

Model for Interdisciplinary Research on Climate, Earth System Model

MIROC-ESM-CHEM

Model for Interdisciplinary Research on Climate, Earth System Model, Chemistry Coupled

MIROC5Model for Interdisciplinary Research on Climate, version 5

MPI-ESM-LR

Max Planck Institute Earth System Model, low resolution

MPI-ESM-MR

Max Planck Institute Earth System Model, medium resolution

MRI-CGCM3

Meteorological Research Institute Coupled Atmosphere–Ocean General Circulation Model, version 3

MRI-ESM1

Meteorological Research Institute Earth System Model, version 1

NorESM1-M

Norwegian Earth System Model, version 1 (intermediate resolution)in all the CMIP5 models. In the late C20 and the midC21, both models have a winter sea ice cover close to
the CMIP5 ensemble mean. In the late C21, the twomodels diverge from the CMIP5 mean. HadGEM2-ES
simulates considerably less winter ice than the CMIP5
mean, whereas CCSM4 simulates more winter ice thanthe CMIP5 mean. Both models, however, lie within the1494 |SEPTEMBER 2015

10%–90% range of the CMIP5 model spread. Thus,
we consider the simulations by these two models
to capture some of the uncertainty in future sea ice
cover, but neither of the models are obvious outliers.
In terms of winter sea ice volume, CCSM4 lies near to
the CMIP5 mean in all three time periods (Fig. 3h).
HadGEM2-ES has a winter sea ice volume close tothe CMIP5 mean in the late
C20 and the late C21, but it
has a larger volume in the
mid-C21 (primarily due tothicker ice). We note thatthe sea ice thicknesses used
to calculate these values arethose derived empiricallyfrom the sea ice concentration (and prescribed to
HadGAM2; see above) andnot those simulated in the
HadGEM2-ES RCP8.5 experiment.
WARMER AND LESSVARIABLE. Figure 4ashows PDFs of winter daily
CENA temperature from34 CMIP5 models. The
three histograms show
distributions based on thelate C20 (gray bars), themid-C21 (blue), and thelate C21 (red). Each histogram is based upon 61,200
daily values (34 models ×
20 years × 90 days). As agroup, the CMIP5 models
project a shift toward theright and a narrowing ofFig. 3. Ensemble-mean winter sea ice concentrations from HadGEM2-ES during
the PDF, the former implythe period (a) 1980–99, (b) 2030–49, and (c) 2080–99. (d)–(f) As in (a)–(c), but
for CCSM4. (g) Evolution of winter sea ice area in the CMIP5 historical anding mean warming and the
RCP8.5 experiments, 1980–2099. Blue curve denotes the multimodel mean,latter less variability. The
dense hatching denotes the 10%–90% range of the model spread, and lightmean warming measures
hatching denotes the full model spread. Black and green lines show the values
2.6°C by the mid-C21 andprescribed in the sea ice–forced HadGAM2 and CAM4 simulations, respectively.
6.5°C by the late C21, both(h) As in (g), but for sea ice volume.
relative to the late C20. The
standard deviation decreases by –0.3°C by the mid-C21 and by –0.7°C by the late declines of –0.3° and –1.1°C in the mid- and late C21,
C21, again relative to the late C20. All these changes respectively. The CCSM4 simulations show a mean
are statistically significant (95% confidence).
warming of 2.6°C by the mid-C21 and 5.7°C by the
Figures 4b,c show analogous PDFs from simu- late C21 and a standard deviation decrease of –0.24°C
lations of the HadGEM2-ES and CCSM4 models, by the mid-C21 and –0.58°C by the late C21. Again,
respectively. We present the simulations from Had- all quoted changes are statistically significant (95%
GEM2-ES and CCSM4 here to enable direct compari- confidence).
sons with the sea ice–forced runs that were conducted
The results from the sea ice–forced experimentswith the atmospheric components of these coupled are shown in Figs. 4d,e for HadGAM2 and CAM4,
models. Both models show broadly the same response respectively. As under GHG forcing, the sea ice–forced
as the CMIP5 ensemble—namely, mean warming simulations show mean warming and a decrease in
and a decrease in variability. The HadGEM2-ES variability, but to a lesser degree than in the GHGsimulations show warming of 3.5° and 8.7°C in the forced experiments. The sea ice–forced changes in
mid- and the late C21, respectively, and variability mean temperature and standard deviation are relatively
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Fig. 4. (a) Histograms of simulated daily winter temperature averaged over CENA taken from 34
coupled climate models for the periods 1980–99 (gray bars), 2030–49 (blue), and 2088–99 (red).
Numbers in the top left and top right of each panel are the mean temperature (°C) and standard
deviation (°C), respectively. Vertical green line denotes the value of the model’s 1.1 percentile,
the simulated analog of the 7 Jan 2014 event in observations. (b)–(e) As in (a), but for coupledsimulations with (b) HadGEM2-ES and (c) CCSM4 and for sea ice–forced simulations with (d)HadGAM2 and (e) CAM4. (f) Probability of daily temperature as cold or colder than the 1.1
percentile in the period 1980–99 in each model ensemble and period.

small between the late C20 and the mid-C21, but they
emerge more clearly by the late C21, consistent withthe magnitude of the sea ice loss (recall Fig. 3g). HadGAM2 simulates warming of 0.3°C by the mid-C21
and 2.0°C by the late C21, and CAM4 exhibits warming of 0.7° and 2.2°C, respectively (all statisticallysignificant). These CENA temperature changes canbe divided by the changes in winter sea ice area toyield sensitivity terms. In HadGAM2 this sensitivity
is 0.2°C (million km)–2 (multiplied by –1 to yield avalue for sea ice area loss) between the late C20 andthe mid-C21 and the same value between the midand late C21. In CAM4 the corresponding values are
0.5 and 0.4°C (million km)–2. Thus, in both models
there is an approximately linear relationship between
winter sea ice area loss and CENA warming; however,CAM4 has a higher sensitivity than HadGAM2. Bothmodels simulate a statistically significant decrease in
the standard deviation of CENA temperature by the late
C21 in response to sea ice loss, –1.0°C in HadGAM2
and –0.8°C in CAM4, which represents a 27% and 18%
decrease relative to the late C20, respectively. Althoughmuch smaller in magnitude, a statistically significant
decrease in variability in response to sea ice loss is1496 |SEPTEMBER 2015

evident by the mid-C21 in both models. None of the
C21 sea ice–forced experiments show evidence of cooling or increased variability relative to the C20: in otherwords, there is no evidence for increased cold extremes.
So, why does Arctic sea ice loss make CENA
temperature warmer and less variable? Arctic sea ice
loss drives local warming via changes in the surface
heat fluxes (Deser et al. 2010; Screen and Simmonds
2010a,b; Screen et al. 2013). This warming signal isspread to lower latitudes primarily due to temperature advection by transient eddies (Deser et al. 2010).
Temperature advection can also help explain the variability decrease. Cold winter days in the midlatitudes
tend to coincide with northerly wind (from the Arctic)
and warm winter days with southerly wind (from thesubtropics). Arctic warming, induced by sea ice loss,
leads to warmer northerly wind but little change intemperature of southerly wind (Screen 2014). As aresult, cold days warm faster than warm days, leading
to a decrease in daily temperature variability (Screen2014). Figure 4 clearly shows that the cold (left hand)tail of the CENA temperature PDF warms more (i.e.,
shifts farther to the right) than does the warm (righthand) tail, supporting this simple mechanism.

Contrasting the GHG-forced and sea ice–forced
simulations, sea ice loss in HadGAM2 explains 9% of themean warming and 52% of the decreased standard deviation seen in HadGEM2-ES between the late C20 andthe mid-C21, and sea ice loss in CAM4 accounts for 25%of the mean warming and 58% of the decreased standard
deviation seen in CCSM4 in the CENA region (Table 4).
By the late C21, sea ice loss accounts for 24% of the mean
warming and 87% of the decreased standard deviationin HadGEM2-ES and 38% of the mean warming and141% of the decreased standard deviation in CCSM4.
The latter percentage, being larger than 100%, implies
that other processes (not directly related to Arctic sea
ice loss) in the GHG-forced experiment are responsible
for an increase in variability that partially offsets the sea
ice–driven variability decrease. Evidently Arctic sea ice
loss is the key driver of the projected decrease in variability by the late C21, supporting similar conclusions
for the midlatitudes as a whole (Screen 2014).
REDUCED RISK OF COLD EXTREMES. For
each experiment, we identified a threshold CENA temperature that occurs with 1.1% (one day per year) frequency during the late C20. This represents the model
analog to 7 January 2014. We note that because themodels are generally biased cold relative to the reanalysis, the chosen threshold temperatures are lower than
–16.8°C (–18.7°, –20.5°, –18.6°, –17.1°, and –18.5°C
for the CMIP5, HadGEM2-ES, CCSM4, HadGAM2,
and CAM4 simulations, respectively). Figure 4f shows
how the probability of CENA temperature equal to or
below this threshold changes in the future in response
to increased GHG and to Arctic sea ice loss. In the
CMIP5 models, the probability reduces to 0.21% (oneday in five years) by the mid-C21. By the late C21, the
CENA temperature never equals or falls below thethreshold. In the HadGEM2-ES coupled simulations,the probability reduces to 0.014% (one day in 80 years)
by the mid-C21 and, again, reduces to zero by the late

C21. The probability reduces to 0.093% (one day in
12 years) and zero in the CCSM4 simulations by the
mid-C21 and the late C21, respectively. Thus, by the
mid-C21, increased GHG reduce the odds of an event
as severe as 7 January 2014 by a factor of 5 based on
the CMIP5 models as a group, a factor of 80 based on
HadGEM2-ES, and a factor of 12 based on CCSM4.
In response to projected sea ice loss, the probability of CENA temperature below the threshold
temperature reduces to 0.7% (one day in 1.6 years)
in HadGAM2 and to 0.8% (one day in 1.4 years) in
CAM4 by the mid-C21. Thus, projected Arctic sea
ice loss alone reduces the odds of such an event by
one-quarter to one-third in the mid-C21 compared
to late C20. By late C21, the probability falls to zero in
HadGAM2 and 0.06% (one day in 19 years) in CAM4.
The sea ice–forced simulations presented here
were not coupled to an ocean model and, thus, ocean
feedbacks are not represented. Deser et al. (2015) examined the climate response to projected Arctic sea ice
in coupled and uncoupled versions of CCSM4. Theseauthors show that the coupled response to Arctic sea ice
loss resembles a weaker version of the full response to
GHG in CCSM4. Since North American warming anddecreased temperature variance are robust characteristics of the full coupled response to GHG, we speculatethat ocean feedbacks may further reduce the risk of cold
extremes or, at least, are unlikely to increase the risk.
HEMISPHERIC PERSPECTIVE. A wider geographical perspective on the simulated response to sea
ice loss is provided in Fig. 5, which shows maps of mean
temperature and standard deviation change between
the late C20 and the mid-C21. Both models show
warming over the high-latitude continents (Figs. 5a,c),
accompanied by a decrease in standard deviation (Figs.
5b,d). Both the warming and variability decrease are
more widespread in CAM4 than in HadGAM2. By thelate C21, both models depict larger warming over the

Table 4. Changes in winter daily mean CENA temperature and variance simulated in response to
GHG increases and Arctic sea ice decreases. Numbers in parentheses denote the percentage ofsimulated change under GHG forcing that can be explained solely by projected Arctic sea ice loss.
All changes are statistically significant at the 95% confidence level.
Temperature change
relative to 1980–99 (°C)

Std dev change relative to
1980–99 (°C)

Model

PeriodGHG

Sea iceGHG

Sea ice

HadGEM2-ES/
HadGAM2

2030–49

3.49

0.32 (9%)–0.31–0.16 (52%)

2080–99

8.67

2.08 (24%)–1.11–0.97 (87%)

CCSM4/CAM4

2030–492.62

0.65 (25%)–0.24–0.14 (58%)

2080–99

5.74

2.17 (38%)–0.58–0.82 (141%)

AMERICAN METEOROLOGICAL SOCIETYSEPTEMBER 2015

| 1497

Fig. 5. Differences in (a) mean winter near-surface air temperature and (b) standard deviation of daily winter nearsurface air temperature between the periods 2030–49 and 1980–99 in the HadGAM2 sea ice–forced experiments.
(c),(d) As in (a),(b), but for the CAM4 sea ice–forced experiments. Colored shading is shown only at grid points
where the difference is statistically significant at the 95% confidence level.

high-latitude continents and an extension of the warming signal into parts of the midlatitudes (Fig. 6). In particular, warming is simulated over much of the eastern
United States. Also by the late C21, there are largermagnitude and more widespread simulated reductions
in standard deviation, with significant decreases overmost of North America, Russia, and northern Europe
by the late C21. In contrast, both models depict weakcooling and patches of increased standard deviation
1498 |SEPTEMBER 2015

over eastern Asia (China, Mongolia). This cooling isrelated to a simulated strengthening of the Siberianhigh (not shown), consistent with the model results ofMori et al. (2014). It is noteworthy that the two models
depict a robust spatial pattern of mean temperature andvariability change in response to Arctic sea ice loss.
These changes in mean temperature and variability
due to Arctic sea ice loss would be expected to translate
into altered frequencies of cold extremes. To show thisFig. 6. As in Fig. 5, but for differences between the period 2080–99 and 1980–99. Note the different color scales.

explicitly, Fig. 7 presents the spatial pattern of the sea
ice–forced change in the probability of cold extremes.
Here, the temperature threshold is calculated as the
1.1 percentile of the late C20 distribution at each grid
point. For clarity in Fig. 7, we simplify these probabilitychanges into broad categories that emphasize the sign
and relative magnitude of the sea ice–forced change
from the late C20. Focusing first on the changes by the
mid-C21, both models depict reduced probabilities ofcold extremes over the high latitudes (Figs. 7a,c). The
midlatitude responses are dissimilar in the two models.
For example, HadGAM2 shows reduced probability
AMERICAN METEOROLOGICAL SOCIETY

over the majority of North America, whereas CAM4
depicts comparably large reductions over northernand eastern North America but modest increases oversouthern and western parts of the continent. The late
C21 changes are in very good agreement between themodels, however (Figs. 7b,d). This suggests that thediscrepancies in the mid-C21 responses arise due to thesmall signal-to-noise ratio and not model differences in
the forced response. By the late C21 both models simulate large probability reductions over North America,
Europe, and Russia. In parts of northern Canada andnortheast Asia, the probability reduces to zero, andSEPTEMBER 2015

| 1499

Fig. 7. (a) Probability of extreme cold (winter daily temperature as cold or colder than the 1.1 percentile during
the period 1980–99) in the sea ice–forced HadGAM2 simulation for the period 2030–49. Colored shading categories
are based on the relative change in probability compared to the period 1980–99. (b) As in (a), but for the period
2080–99 relative to 1980–99. (c),(d) As in (a),(b), but for the sea ice–forced CAM4 simulations.

over large swaths of North America and northern
Asia, the probability is more than halved. Both models
show slightly increased probabilities over central andsouthern Asia, although the exact regions differ, relatedcooling is induced by a strengthened Siberian high.
So far we have only considered the changing probability of extremely cold days. There has been recent
speculation that Arctic warming and sea ice loss may
increase the frequency of longer-duration cold extremes
1500 |SEPTEMBER 2015as a result of more persistent weather patterns over
North America (Francis and Vavrus 2012). Motivatedby this, we have also examined the changing probabilityof 5-day and 9-day cold extremes (Fig. 8). The simulated changes in the frequency of these longer-duration
extremes closely match those shown previously fordaily extremes (as do the patterns of standard deviation
change; not shown). Therefore, our simulations do notsupport the hypothesis of more frequent cold spells over

Fig. 8. (a) Probability of extreme cold (winter 5-day mean temperature as cold or colder than the 1.1 percentile
during the period 1980–99) in the sea ice–forced HadGAM2 simulation for the period 2080–99. Colored shading
categories are based on the relative change in probability compared to the period 1980–99. (b) As in (a), but for
9-day means. (c),(d) As in (a),(b), but for the sea ice–forced CAM4 simulations.

central and eastern North America in response to sea ice
loss. They do suggest that, in isolation, Arctic sea ice loss
favors increased cold spells over central Asia, consistent
with Mori et al. (2014). However, it should be noted thatthe net effect of GHG increases is to reduce the chanceof central Asian cold extremes (Mori et al. 2014).
CONCLUDING REMARKS. We have used large
ensembles of model simulations to explore how the risk
AMERICAN METEOROLOGICAL SOCIETY

of North American daily cold extremes is anticipated tochange in the future in response to increases in GHG
and the component of that response solely due to Arctic
sea ice loss. Specifically, we have examined the changing probability of daily cold extremes as (un)common
as the 7 January 2014 event. Projected increases in
GHG will decrease the likelihood of North American
cold extremes in the future. Days as cold or colder
than 7 January 2014 are still projected to occur in the
SEPTEMBER 2015

| 1501

mid-C21 (2030–49), albeit less frequently than in thelate C20 (1980–99). However, such events will cease tooccur by the late C21 (2080–99), assuming GHG emissions continue unabated. Continued Arctic sea ice loss
is a major driver of decreased—not increased—North
America cold extremes. Projected Arctic sea ice loss
alone reduces the odds of such an event by one-quarterto one-third in the mid-C21 compared to late C20 andto zero (or near zero) by the late C21. Both projected
mean warming and a decrease in temperature variability contribute to the decrease in daily cold extremes.
Recent claims that Arctic sea ice loss may increase
the risk of midlatitude cold extremes are primarily
based on hypothesized increases in the latitudinal
extent of north–south excursions of the jet stream.
The simple reasoning is that a more meandering jet
stream will increase the frequency of cold Arctic air
migrating southward and thus lead to more frequent
cold extremes in the midlatitudes. However, this logic
ignores two important factors, even putting aside theconsiderable uncertainty in future changes in thejet stream (Barnes and Polvani 2013) and associatedfeatures of the atmospheric circulation (Masato et al.
2013). The first factor ignored is that the midlatitudes
are warming. This means it takes a larger-magnitude
cold anomaly to cause a cold extreme than in a cooler
climate. The second factor ignored is that disproportionally large warming of the high latitudes comparedto the midlatitudes reduces the average temperature
gradient between these two regions. This means that
if an Arctic air mass is displaced southward into the
midlatitudes, then the resulting temperature anomaly
is smaller than is the case for a larger north–southtemperature gradient. These two factors translate into
a reduced chance of cold extremes. Our results suggest these thermodynamically induced changes areof first-order importance in determining the futurerisk of cold extremes and that dynamically inducedchanges play a secondary role (such as changes inthe behavior of the jet stream). As a result, we should
expect fewer—and not more—cold extremes over the
coming decades in the midlatitudes including North
America.
ACKNOWLEDGMENTS. Michael Alexander and threeanonymous reviewers are thanked for their time and feedback. We thank Robert A. Tomas for conducting some of
the CAM4 simulations. The HadGAM2 simulations were
performed on the ARCHER U.K. National Supercomputing
Service. James Screen is supported by U.K. Natural Environment Research Council Grant NE/J019585/1. Lantao
Sun is supported by a grant from the U.S. National Science
Foundation’s Office of Polar Programs.

1502 |SEPTEMBER 2015

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