Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications

270 Pages·2008·28.33 MB·English

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Geophysical Monograph 180
Arctic Sea Ice Decline:
Observations, Projections,
Mechanisms, and Implications
Eric T. DeWeaver
Cecilia M. Bitz
L.-Bruno Tremblay
Editors
American Geophysical Union
Washington, DC
Published under the aegis of the AGU Books Board
Kenneth R. Minschwaner, Chair; Gray E. Bebout, Joseph E. Borovsky, Kenneth H. Brink, Ralf R. Haese, Robert B. Jackson,
W. Berry Lyons, Thomas Nicholson, Andrew Nyblade, Nancy N. Rabalais, A. Surjalal Sharma, Darrell Strobel, Chunzai Wang, and
Paul David Williams, members.
Library of Congress Cataloging-in-Publication Data
Arctic sea ice decline : observations, projections, mechanisms, and implications / Eric T. DeWeaver, Cecilia M. Bitz,
L.-Bruno Tremblay, editors.
p. cm. — (Geophysical monograph, ISSN 0065-8448 ; 180)
ISBN 978-0-87590-445-0
1. Sea ice—Arctic regions. 2. Climatic changes—Environmental aspects—Arctic Regions. 3. Environmental impact analysis—
Arctic regions. 4. Arctic regions—Climate. I. DeWeaver, Eric T., 1964- II. Bitz, Cecilia M. III. Tremblay, L.-Bruno.
GB2595.A735 2008
551.34¢3091632—dc22
2008045367
ISBN: 978-0-87590-445-0
ISSN: 0065-8448
Cover Photo: Courtesy of Don Perovich (CRELL) during the 2005 Arctic Basin Transect HORTRAX field campaign.
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CONTENTS
Preface
Eric T. DeWeaver, Cecilia M. Bitz, and L.-Bruno Tremblay ..................................................................................vii
Arctic Sea Ice Decline: Introduction
Eric T. DeWeaver ...................................................................................................................................................1
Section I: Arctic Sea Ice in the Instrumented and Paleo–Proxy Records
Recent Trends in Arctic Sea Ice and the Evolving Role of Atmospheric Circulation Forcing, 1979–2007
Clara Deser and Haiyan Teng ................................................................................................................................7
Reconstructing Sea Ice Conditions in the Arctic and Sub-Arctic Prior to Human Observations
Anne de Vernal, Claude Hillaire-Marcel, Sandrine Solignac, Taoufik Radi, and André Rochon ............................27
Section II: Factors in Sea Ice Sensitivity
Arctic Cloud Properties and Radiative Forcing From Observations and Their Role in Sea Ice Decline
Predicted by the NCAR CCSM3 Model During the 21st Century
Irina V. Gorodetskaya and L.-Bruno Tremblay ......................................................................................................47
Some Aspects of Uncertainty in Predicting Sea Ice Thinning
Cecilia M. Bitz ......................................................................................................................................................63
Sensitivity of Arctic Sea Ice Thickness to Intermodel Variations in the Surface Energy Budget
Eric T. DeWeaver, Elizabeth C. Hunke, and Marika M. Holland ...........................................................................77
The Atmospheric Response to Realistic Reduced Summer Arctic Sea Ice Anomalies
Uma S. Bhatt, Michael A. Alexander, Clara Deser, John E. Walsh, Jack S. Miller,
Michael S. Timlin, James Scott, and Robert A. Tomas ...........................................................................................91
Section III: Rapid Loss Versus Abrupt Transition
Sea Ice–Albedo Feedback and Nonlinear Arctic Climate Change
Michael Winton .................................................................................................................................................111
The Role of Natural Versus Forced Change in Future Rapid Summer Arctic Ice Loss
Marika M. Holland, Cecilia M. Bitz, L.-Bruno Tremblay, and David A. Bailey ....................................................133
Multiple Equilibria and Abrupt Transitions in Arctic Summer Sea Ice Extent
William J. Merryfield, Marika M. Holland, and Adam H. Monahan ....................................................................151
What Is the Trajectory of Arctic Sea Ice?
Harry L. Stern, Ronald W. Lindsay, Cecilia M. Bitz, and Paul Hezel ...................................................................175
Analysis of Arctic Sea Ice Anomalies in a Coupled Model Control Simulation
Richard I. Cullather and L.-Bruno Tremblay ........................................................................................................187
Section IV: The Threat to Polar Bears From Sea Ice Decline
A Bayesian Network Modeling Approach to Forecasting the 21st Century Worldwide Status of Polar Bears
Steven C. Amstrup, Bruce G. Marcot, and David C. Douglas ..............................................................................213
Arctic Sea Ice Decline: Introduction
Eric T. DeWeaver
Center for Climate Research, University of Wisconsin-Madison, Madison, Wisconsin, USA
. THE GREAT DECLINE OF 2007 minimum was unusually thin and thus vulnerable to melting
away. The July and August extent were somewhat higher
By any measure, the loss of Arctic sea ice cover in Sep- than in 2007, but the daily loss rate accelerated in early Au-
tember 2007 was spectacular. The National Snow and Ice gust after storms broke apart thin ice in the Beaufort and
Data Center (NSIDC) called it a loss “the size of Alaska and Chukchi seas. Southerly winds following the storms further
Texas combined,” in comparison to the 979–2000 Septem- promoted opening by pushing the ice away from the eastern
ber mean. Record-breaking minima in sea ice extent are not Siberian coast (information from the “Arctic sea ice news
unexpected, given the declining trend of the past 30 years and analysis” Web pages for  August through 4 Septem-
and its recent acceleration [e.g., Meier et al., 2007; Deser ber 2008 at http://www.nsidc.org).
and Teng, this volume]. But the 2007 minimum was remark- With approxmiately 2 weeks left in the 2008 melt season,
able even compared to the decline, a full four standard devi- Arctic sea ice extent is now very close to the 2007 mini-
ations below the trend line (H. Stern, quoted by Schweiger et mum. While the lack of recovery is discouraging, the 2008
al. [2008]). Kerr [2007] reported an Alaska-sized loss com- loss could have been worse. In May the NSIDC suggested,
pared to the previous record low in 2005, which was itself based on the prevalence of thin first year ice cover in the
an Alaska-sized retreat from the value at the beginning of the Arctic, that the North Pole could become ice free in 2008, a
satellite era in 979. Deser and Teng point out that the loss prediction more commonly made for the middle of the cen-
between September 2006 and September 2007 is as large as tury. Three researchers contributing to the May Sea Ice Out-
the entire September extent loss from 979 to 2006. look (produced by the interagency Study of Environmental
Following the 2007 melt season there was some cause for Arctic Change (SEARCH)), anticipated a return toward the
optimism that 2008 could see a partial recovery. Writing at long-term trend of summer sea ice loss, six argued that 2008
the end of the melt season, Comiso et al. [2008, paragraph 5] September extent should be close to 2007, and five expected
noted that the ice was “rebounding with a rapid early autumn losses exceeding those in 2007.
growth.” Following a cold winter, the April 2008 maximum
ice extent reported by NSIDC was relatively high by recent
2. RESEARCH ON THE CAUSE OF THE LOSS
standards, although still below the long-term mean. But
while the temperatures were cooperating, the winds were
Research on the causes of the 2007 loss is already well un-
not. In early February I. Rigor noted that buoys embedded
derway. Surface wind anomalies are generally identified as
in multiyear ice flows were “streaming out” of the Arctic,
the proximal cause [Nghiem et al., 2007; Stroeve et al., 2008;
flushed through Fram Strait along with their ice floes by cir-
Deser and Teng, this volume; Overland et al., 2008; Zhang
cumpolar wind anomalies [Kizzia, 2008]. Also, as discussed
et al., 2008], as the transpolar winds dubbed the “Polar Ex-
by Maslanik et al. [2007], ice cover following the 2007
press” by Nghiem et al. pushed ice away from the Alaskan
and eastern Siberian coastlines and out of the Arctic. Kay
Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and
et al. [2008] claim an additional role for enhanced summer
Implications
Geophysical Monograph Series 80 melting as high pressure and sunny skies persisted over the
Copyright 2008 by the American Geophysical Union. western Arctic Ocean. Their claim is disputed by Schweiger
0.029/80GM02 et al. [2008], who note that the sunny skies are not well col-

2 ARCTIC SEA ICE DECLINE
located with the largest sea ice loss in the Chukchi and East 3. PAST THE TIPPING POINT?
Siberian seas. On the other hand, a strong role for insola-
tion as a positive feedback is not in dispute. Perovich et al. Has the Arctic sea ice passed a tipping point? This is per-
[2008] find a 500% increase in January to September solar haps the most consistently asked question in news accounts
heat input to the Beaufort Sea compared to the 979—2005 about the 2007 and 2008 losses. Understandably, respond-
climatology. They further determine that the large increase ents to the question have not voiced much hope for reversal:
is due to the large area of low-albedo ocean surface exposed “It’s hard to see how the system may come back (I. Rigor,
by the dramatic sea ice retreat. Accompanying the increased quoted by Kizzia [2008])”; “I’m much more open to the
heat uptake, they report a sixfold increase in bottom melt idea that we might have passed a point where it’s becoming
measured by an ice mass buoy in the Beaufort Sea. Their essentially irreversible” (J. M. Wallace, quoted by Revkin
results thus document the classical sea ice–albedo feedback, [2007]); “It’s tipping now. We’re seeing it happen now” (M.
presumably initiated by wind-driven opening of the ice pack. Serreze quoted by Borenstein and Joling [2008]). No doubt,
The modeling study of Zhang et al. [2008] concludes that the tipping point terminology aptly captures the precipitous
70% of the 2007 loss anomaly was due to amplified melting loss of 2007 and lack of recovery in 2008. But questions re-
while 30% resulted from ice motion. main as to how literally the tipping language should be taken.
In these studies, the meteorology of 2007 is generally In a formal sense, tipping refers to a sudden and irreversible
given less prominence than the vulnerability of the 2007 sea transition between two stable states of a system (e.g., right
ice cover. Maslanik et al. and Nghiem et al. document the side up versus overturned), occurring as the system crosses
long-term change from multiyear sea ice to younger floes some threshold value of a key parameter (e.g., angle to the
which are thinner and more prone to breakup and melting. local vertical). The scientific challenge would then be to find
Overland et al. and Kay et al. also question the novelty of and characterize the stable states and threshold values of the
the 2007 meteorological conditions. They relate the offshore Arctic sea ice system.
winds and sunny skies of 2007 to a surface high over the The idea of an unstable transition between ice-covered
western Arctic Ocean, a rare but not unprecedented occur- and ice-free Arctic states has a long history (see references
rence. Four years with comparable high pressure can be seen of Winton and Merryfield et al. [this volume]), and such be-
in the 50-year record shown by Overland et al. (their Figure havior does occur in simple energy balance models with dif-
11), while Kay et al. find four additional years with sunnier fusive heat transfer (the “small ice cap instability” of North
skies than 2007. The older, thicker ice in these earlier years [984]). However, unstable transitions are somewhat elusive
was not dramatically affected by the adverse meteorology. in global climate models, as Winton [this volume] shows.
The contribution of greenhouse warming in producing The rapid loss events in simulations of the Community
sharp, single-year declines is not easily quantified, since Climate System Model (CCSM) shown by Holland et al.
warming favors these events indirectly as it helps precondi- [2006] are commonly compared to the recent Arctic losses,
tion the ice to a thinner state (e.g., Overland et al.). However, yet threshold values for sea ice cover and thickness were
Stroeve et al. [2007] point out the consistency of the 2007 not found for the CCSM events. Instead, the authors argue
event with the periods of rapid loss found by Holland et al. that rapid loss can occur through the superposition of natural
[2006] in global warming simulations. Stroeve et al. note in variability and a steady downward trend (further analysis of
particular the similarity between the March 2007 thickness these simulations is given by Holland et al. [this volume],
estimates of Maslanik et al. and the mean Arctic thickness in Merrifield et al. [this volume], Stern et al. [this volume],
simulations analyzed by Holland et al. The analysis of Hol- and Gorodetskaya and Tremblay [this volume]. The lack
land et al. is expressed in terms of a three-part conceptual of identifiable thresholds in CCSM is significant, since the
framework in which ice is first preconditioned for rapid loss yearly sea ice losses during CCSM rapid declines are larger
by decades of thinning, after which loss is “triggered” by nat- than the 2007 loss observed by Holland et al., despite the
ural variability and then amplified by the sea ice–albedo feed- absence of easily identifiable tipping points.
back. The “preconditioning, trigger, feedback” framework The primary motivation for claims of a tipping point
was developed by Lindsay and Zhang [2005] to account comes from the destabilizing effect of the sea ice–albedo
for the observed sea ice decline from 988 to 2003, and the feedback. No doubt this is a strong feedback, but there is
same framework was invoked in the Zhang et al. study of the some subtlety in assessing its strength. Gorodetskaya and
2007 event. Thus, while the 2007 loss was unprecedented, Tremblay point out that the effect of sea ice removal is miti-
descriptions of it are quite consistent with descriptions of gated by the cloudiness of the Arctic in summer, and note
the longer-term Arctic losses of the recent past and the rapid that the presence of sea ice reduces the top-of-atmosphere
declines found in simulations of future Arctic change. albedo by only 10 to 20%, despite the large albedo contrast
DEWEAvER 3
between ice and open water. This finding is consistent with was comissioned to help the USFWS decide whether to list
Winton’s [2006] conclusion that the sea ice albedo feedback the polar bear as a threatened species under the Endangered
is not dominant as a cause of polar amplification in climate Species Act (ESA). Coincidentally, the results of this were
models. presented to the USFWS in September 2007, as the Arctic
Moreover, the stability of the Arctic sea ice cover depends sea ice cover approached its record low.
on the sign of the net feedback, with instability occurring It is clear even upon superficial consideration that sea ice
when the positive sea ice–albedo feedback overwhelms decline is bad for polar bears, given their dependence on
the negative feedbacks which stabilize sea ice cover under sea ice as a platform for hunting and other activities (see
colder conditions. Bitz [this volume] performed CCSM ex- references of Amstrup et al. [this volume]). However, the
periments in which Arctic Ocean surface albedo is held fixed threat to polar bears from sea ice decline cannot be rigor-
even when sea ice cover is reduced by greenhouse gas in- ously assessed without an understanding, based on obser-
creases, so that sea ice–albedo feedback is effectively disa- vational field biology, of the sea ice needs of polar bears.
bled. The sea ice decline which occurs in the absence of sea Durner et al. [2008] quantified the habitat value of sea ice
ice–albedo feedback is not dramatically different from the using observations of radio-collared polar bears over 2
sea ice decline in the control run. An explanation for this re- decades. The characteristics that make sea ice desirable
sult is given by Winton [this volume], who performed model as polar bear habitat could be identified and quantified
experiments in which sea ice cover was artificially removed. based on this data. In particular, polar bears were found
In these experiments increases in solar absorption due to in- to prefer sea ice over the shallow, productive waters of the
creased open water area are offset by increases in turbulent continental shelf. The decline of pan-Arctic sea ice extent
heat flux from the ocean because of the removal of the insu- matters less than the retreat of sea ice from the shelf areas,
lating ice cover. The implication of these results is that the as the habitat value of ice remaining over the deep Arctic
net feedback due to opening can still be negative, despite the basin is low.
strong positive sea ice–albedo feedback. Further support for Durner et al.’s resource selection functions (RSFs) quan-
this conclusion (at least in climate models) comes from Cul- tify the value of sea ice as polar bear habitat, expressed
lather and Tremblay’s [this volume] analysis of naturally as the frequency of occupation by polar bears, in terms of
occurring sea ice loss anomalies in a long CCSM control simple parameters including distance to shore, ocean depth,
run with 990 levels of greenhouse gases. Despite the sea and sea ice concentration. The RSF methodology can be
ice–albedo feedback, sea ice cover rebounded within  to 3 applied with equal ease to sea ice decline in observations
years of each anomaly. and climate model projections. Thus, they enable research-
ers to provide guidance to policy makers in terms of the
4. CLIMATE IMPACTS: POLAR BEAR policy-relevant impact, in this case the loss of polar bear
LISTING DECISION habitat, rather than generic statements regarding the over-
all sea ice decline. Further use of field data combined with
Of course, the implications of rapid sea ice loss go well model projections in the USGS reports comes from Hunter
beyond academic interest in climate stability. Policy makers et al. [2007] who used data from a capture-release study to
are particularly challenged by Arctic sea decline, since they estimate declines in polar bear population as a function of
must plan for future sea ice conditions which are without reductions in sea ice availability.
precedent in the instrumented record. Faced with the lack Projections of future sea ice loss and its impacts will in-
of observed analogs, policy makers can seek guidance from evitably be accompanied by substantial uncertainty, given
global climate model (GCM) simulations of anthropogenic the evident sensitivity of the Arctic climate system. As dis-
greenhouse warming. Such guidance can be quite valuable cussed by Amstrup et al. [this volume], the USGS research
provided that two essential issues are addressed: first, the accounted for model uncertainty by using a subset of 0
policy-relevant climate impacts of the simulated sea ice de- climate models which satisfy a selection criterion based on
cline must be determined and, second, the uncertainty inher- present day sea ice simulation quality. Projections from this
ent in GCM projections of sea ice loss must be adequately subset show a range of September sea ice loss from 30 % to
assessed and incorporated. An important case in point is the complete loss by mid century (sources of uncertainty in sea
research conducted by the U.S. Geological Survey (USGS) ice projections are discussed by Bitz and DeWeaver et al.
to advise the U.S. Fish and Wildlife Service (USFWS) on [this volume]). The uncertainty represented by the range of
the impact of sea ice decline on polar bears. The research, model simulations was propagated through the USGS analy-
which was presented in nine USGS administrative reports sis by applying techniques like the RSF calculation to the
(online at www.usgs.gov/newsroom/special/polar_bears), whole subset, so that ensemble spread in sea ice simulations
4 ARCTIC SEA ICE DECLINE
leads to ensemble spread in polar bear outcomes. However, Comiso, J. C., C. L. Parkinson, R. Gersten, and L. Stock (2008),
Amstrup et al. note that these projections may be overly op- Accelerated decline in the Arctic sea ice cover, Geophys. Res.
timistic, given Stroeve et al.’s [2007] finding that real-world Lett., 35, L0703, doi:0.029/2007GL03972.
Deser, C., and H. Teng (2008), Recent trends in Arctic sea ice and
Arctic sea ice has declined at almost twice the rate found
the evolving role of atmospheric circulation forcing, 979–2007,
in model simulations of the recent past. The USGS efforts
this volume.
culminated in Amstrup et al.’s synthesis report, which uses
de vernal, A., C. Hillaire-Marcel, S. Solignac, T. Radi, and A. Ro-
a Bayesian framework to assess the probability of decline
chon (2008), Reconstructing sea-ice conditions in the Arctic and
in polar bear population based on consideration of sea ice
subarctic prior to human observations, this volume.
decline and other factors. Despite the uncertainties of the re- DeWeaver, E. T., E. C. Hunke, and M. M. Holland (2008), Sensi-
search, none of the outcomes were favorable for polar bears; tivity of Arctic sea ice thickness to intermodel variations in the
in effect, they run the gamut from bad to extremely bad. surface energy budget, this volume.
In May 2008 the polar bear was listed as a threatened spe- Durner, G. M., et al. (2008), Predicting 2st century polar bear
cies under ESA, after considerable delay. It is clear from habitat distribution from global climate models, Ecol. Monogr.,
the final rule [U.S. Fish and Wildlife Service, 2008] that in press.
Gorodetskaya, I. v., and L.-B. Tremblay (2008), Arctic cloud prop-
the policy makers understood and considered the scientific
erties and radiative forcing from observations and their role in
guidance. Consideration of the science is also evident from
sea ice decline predicted by the NCAR CCSM3 model during the
the announcement of the decision (www.doi.gov/secretary/
2st century, this volume.
speeches/08405_speech.html), which included a prominent
Holland, M. M., C. M. Bitz, and B. Tremblay (2006), Future abrupt
display of Stroeve et al.’s [200X] work on observed and sim-
reductions in the summer Arctic sea ice, Geophys. Res. Lett., 33,
ulated sea ice trends, and maps of Arctic sea ice showing L23503, doi:0.029/2006GL028024.
the change in coverage by old (at least 5 years) and new Holland, M. M., C. M. Bitz, B. Tremblay, and D. A. Bailey (2008),
(less than 5 years old) ice, apparently from the drift model The role of natural versus forced change in future rapid summer
of Rigor and Wallace [2004]. But while the effort to pro- Arctic ice loss, this volume.
vide scientific input for the listing decision was successful Kay, J. E., T. L’Ecuyer, A. Gettelman, G. Stephens, and C. O’Dell
in some sense, it remains to be seen if the listing will have (2008), The contribution of cloud and radiation anomalies to the
2007 Arctic sea ice extent minimum, Geophys. Res. Lett., 35,
any direct effect on the status of the polar bear (see analysis
L08503, doi:0.029/2008GL03345.
of Revkin [2008]).
Kerr, R. A. (2007), Is battered sea ice down for the count?, Science,
318, 33–34.
5. CONCLUSION
Kizzia, T. (2008), Polar ice pack loss may break 2007 record, An-
chorage Daily News, 2 Feb.
The events of 2007 and 2008 highlight the need for im- Lindsay, R. W., and J. Zhang (2005), The thinning of Arctic sea
proved understanding of sea ice sensitivity and the impacts ice, 988–2003: Have we passed a tipping point?, J. Clim., 18,
of sea ice decline. Perhaps, if we are fortunate, our under- 4879–4894.
standing of the Arctic sea ice and climate system can evolve Maslanik J. A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally, D. Yi,
fast enough to keep pace with the changes occurring there. and W. Emery (2007), A younger, thinner Arctic ice cover: In-
creased potential for rapid, extensive sea-ice loss, Geophys. Res.
Acknowledgments. The author’s research is supported by the Of- Lett., 34, L2450, doi:0.029/2007GL032043.
fice of Science (BER), U.S. Department of Energy, grant DE-FG02- Meier, W. N., J. Stroeve, and F. Fetterer (2007), Wither Arctic sea
03ER63604. I thank Cecilia Bitz, Steven Amstrup and members of ice? A clear signal of decline, regionally, seasonally, and extend-
the NCAR Polar Climate Working Group for helpful conversations. ing beyond the satellite record, Ann. Glaciol., 46, 428–434.
I am indebted to the chapter authors for contributing their best work Merryfield, W. J., M. M. Holland, and A. H. Monahan (2008), Mul-
to this monograph. tiple equilibria and abrupt transitions in Arctic summer sea ice
extent, this volume.
Nghiem, S. v., I. G. Rigor, D. K. Perovich, P. Clemente-Colón,
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