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the extent and concentration of the sea ice surrounding Antarctica increased from the late 1970s until 2015. this increase is not reproduced by climate models, and comes despite the overall warming of the global climate and the region. At a January 2016 workshop, leading scientists gathered to discuss the potential mechanisms driving changes in Antarctic sea ice and ways to better under-stand the complex relationship between Antarctic sea ice and the broader ocean-climate system. A number of hypotheses have been posed to explain the recent expan-sion of Antarctic sea ice extent; more research is needed to make deinitive statements about the mechanisms. improvements in sea ice observations and models would improve scientists’ ability to tease apart the many local, regional, and global processes that inluence sea ice extent and thickness.
the winter sea ice surrounding Antarctica extended further between 2012 and 2014 than at any time since the late 1970s, when the satellite-based observed record began. the increase in sea ice extent was modest, but it was unexpected: climate models generally simulate a decrease in sea ice in the oceans surrounding Antarctica as climate warms. While sea ice extent decreased in 2015 and 2016, many questions remain about what controls year-to-year sea ice variability in this region.
scientists have some clues about what factors control Antarctic sea ice extent. Measurements of ocean tempera-ture show that the surface waters of the southern Ocean have warmed more slowly in response to human-caused greenhouse gas emissions than other oceans. Many local, regional, and global processes are known to inluence sea ice extent and thickness, including surface winds, snowfall, and rain rates above the ice surface, and water temperatures and salinity below.
teasing apart these factors to better understand Antarctic sea ice variability is important because sea ice plays many critical roles in the climate and Earth system. in addition to helping to control the lux of heat, gases, and momen-tum between the atmosphere and the ocean, sea ice is a vital part of Antarctic ecosystems and an important inluence on the stability of Antarctica’s glacial ice sheets (see Box 1).
THE OBSERVATIONAL RECORD OF ANTARCTIC SEA ICE VARIABILITY
the longest running and most geographically extensive of sea ice records come from satellite observations, which began in the late 1970s with the launch of satellites carrying passive microwave radiometers. these observations are almost universally drawn upon for studies of sea ice variability and change in both hemispheres. sea ice thickness
Antarctic Sea Ice Variability in the Southern
Presented by Dr. Sarah Das, map modiied from Abram et al., 2013.
observations also have been gathered from iCEsat, a laser altimeter, and from ship-based observations.
Workshop participants noted that the accuracy and precision of some sea-ice observations are affected by data, algorithm, and environmental factors. in addition, a better understanding of the mechanisms and processes driving sea ice variability and trends is limited by the lack of a homogenous record of sea ice extent and concentra-tion that extends prior to the satellite era.
In situ ocean observations are also critical to under-stand the important role of the southern Ocean in many processes that affect sea ice. Argo loats located in the southern Ocean record data on the temperature and salinity of the water, but coverage is geographically sparse—much of the southern Ocean sea ice zone has only 25 percent of target Argo loat coverage. Precipita-tion measurements for southern sea ice are also available, but these measurements are challenging because most precipitation is either blown off into the ocean or accumu-lates in ridges.
several participants expressed the view that more effort should go into extending the observational record using proxies, historical records, and data assimilation. At this time, the data that have been captured to extend the historical record indicate a larger sea ice extent prior to the satellite period. However, questions were raised as to
whether there is enough conidence in the diverse set of proxies to make such a deinitive statement. in addition, some participants highlighted the need for more vali-dation of the passive microwave observations of sea ice concentrations taken from space-based platforms.
Workshop discussions emphasized the regional vari-ability of Antarctic sea ice patterns. there are large and strongly contrasting regional trends in sea ice extent, some with magnitudes equivalent to those observed in the Arctic. in particular, there have been strongly negative trends in monthly sea ice extent in the Bellingshausen and Amundsen seas, and strongly positive trends in the Ross sea. the length of the sea ice season also has changed dramatically in some areas, becoming shorter in the Bell-ingshausen sea area and longer in the Ross sea.
For example, the southern Ocean has warmed more slowly than the global ocean in response to greenhouse gas emissions since 1950. the mechanism traditionally proposed for this slow warming is eficient heat uptake by the southern Ocean, where there is signiicant upwelling of cool, deep ocean waters that don’t have time to warm at the ocean’s surface. there are also divergent lows: a stronger northward low that transports heat away from the continent and a smaller southward low that brings heat up toward the continent. the Arctic has no compa-rable upwelling mechanism and is therefore free to warm under climate warming.
THE DRIVERS OF SEA ICE VARIABILITY
Workshop presentations and discussions highlighted several processes and mechanisms that could inluence Antarctic sea ice growth and melt. these include inter-nal variability from interactions among the atmosphere, ocean, and sea ice; and external forcings such as those caused by stratospheric ozone depletion and emissions of greenhouse gases. these processes are not necessar-ily mutually exclusive and each region of the Antarctic is sensitive to different climate anomaly patterns. At present, no single mode or mechanism could explain the recent records in total sea ice extent.
One possible driver of Antarctic sea ice variabil-ity discussed at the workshop is the feedback between atmospheric warming, hydrology, sea ice formation, and ocean circulation. For example, atmospheric warming could accelerate the melting of Antarctic ice shelves, lead-ing to increased freshwater input to the surface ocean, which in turn that reduces ocean lux from deep ocean to surface waters. this would result in cooler, fresher sea surface conditions and enhanced sea ice formation.
Another atmospheric driver of Antarctic sea ice vari-ability is the Amundsen sea low, which controls meridional winds in the Amundsen, Bellinghausen, and Ross seas. some studies indicate that changes in the Amundsen sea
Box 1. The Roles of Antarctic Sea Ice
Antarctic sea ice functions in many Earth and climate processes including:
•the sea ice-albedo feedback—in which melting sea ice can accelerate climate warming by expos-ing dark ocean areas that absorb more sunlight—is widely recognized as a factor in the ampliication of climate warming in the Arctic. Antarctic sea ice extent is approximately 20 percent greater than in the Arctic, and thus changes in its sea ice extent could result in relatively large change in albedo.
•Feedbacks between sea ice production and ocean water temperature and salinity may play a role in determining the stability of Antarctica’s massive sheets of glacial ice. thus, understanding sea ice variability may be important for anticipating the rate of ice sheet melt and sea level rise over the next few decades.
low (Asl) and increases in ocean heat content are driving decreases in sea ice cover and duration in the Bellings-hausen sea.
sea ice variability could also be related to an increase in westerly winds due to the depletion of stratospheric ozone. this has led to colder winds rushing over the waters surrounding Antarctica, creating areas of open water near the coast, known as polynyas, that promote sea ice production.
Climate models provide an important tool for interpreting Antarctic sea ice observations, many participants said, as well as for exploring the potential mechanisms inluencing the observed variability. However, model improve-ment is necessary given that models generally fail to exhibit the observed decline in Antarctic sea ice extent over the past 30–50 years.
Many of the models have a poor representation of the mean state of the southern Ocean, which is important to reproduce observed trends in Antarctic
sea ice. Furthermore, model biases affect how the models respond to forcing, although there is uncertainty on how large an impact the biases have.
if the models are improved to the point that they can reliably reproduce past sea ice conditions, then they could also be used to disentangle the roles of internal variability and human-caused drivers of changes in sea ice, as well as to project how sea ice may change in the future, several participants said.
A number of overarching themes arose from the work-shop discussions related to needs for future observations and research. Many participants said that process-based understanding is critical for improving understanding of the mechanisms of Antarctic sea ice variability. Process studies also provide validation of global coupled models. However, higher-resolution atmosphere and ocean mod-els may be necessary to more fully understand some
Box 2. Poles Apart: Important Distinctions Between the Arctic and Antarctic Oceans
the increase in Antarctic sea ice extent contrasts sharply with pronounced declines in Arctic sea ice over the same period. the Arctic average monthly sea ice index has been declining at a rate of 2.7 percent per decade for March (the month in which sea ice reaches its maximum extent in the northern hemisphere). the direction of the Arctic trend is consistent with global climate model simulations, and shrinking Arctic sea ice extent is frequently cited as an indi-cator of greenhouse gas-induced climate warming.
the Antarctic sea ice record is often presented as a conun-drum for global climate change science—but workshop participants noted that the southern and Arctic Oceans are very different dynamic systems, and so the two regions are not expected to respond to warming in the same way. Antarctica has a large overlying atmospheric variability
compared to the Arctic. Furthermore, the two regions have different geographies, heat exchange processes, and ocean circulation patterns.
For example, the southern Ocean has warmed more slowly than the global ocean in response to greenhouse gas emissions since 1950. the mechanism traditionally pro-posed for this slow warming is eficient heat uptake by the southern Ocean, where there is signiicant upwelling of cool, deep ocean waters that don’t have time to warm at the ocean’s surface. there are also divergent lows: a stron-ger northward low that transports heat away from the continent and a smaller southward low that brings heat up toward the continent. the Arctic has no comparable upwelling mechanism and is therefore free to warm under climate forcing.
Copyright 2017 by the National Academy of Sciences. All rights reserved. Division on Earth and life studies
DISCLAIMER: this Workshop Highlights was prepared by Alison Macalady and Katie Thomas as a factual summary of what occurred at the workshop. the planning committee’s role was limited to planning the workshop. the statements made are those of the rapporteurs or individual meeting participants and do not necessarily represent the views of all meet-ing participants, the plannmeet-ing committee, or the National Academies of sciences, Engineermeet-ing, and Medicine.
PLAnnIng COMMIttEE On AntARCtIC SEA ICE VARIAbILIty In thE SOuthERn CLIMAtE-OCEAn SyStEM: Julienne Stroeve (Chair), University of Colorado Boulder; David Holland, New York University, New York; Marika Holland, National Center for Atmospheric Research, Boulder, Colorado; Ted Maksym, Woods Hole Oceanographic institution, Woods Hole, Massachusetts; Marilyn Raphael, University of California, los Angeles; Susan Solomon, Massachusetts institute of technology, Cambridge; Xiaojun Yuan, Columbia University, Palisades, New York; Katie Thomas (Senior Program Oficer), Alison Macalady (Program Oficer [until August 2016]), Amanda Staudt (Director), Yasmin Romitti
(Research Associate), Rob Greenway (Program Associate), Michael Hudson (Senior Program Assistant), Shelly Freeland
(Financial Associate), National Academies of sciences, Engineering, and Medicine
SPOnSORS: this Workshop was supported by the National Aeronautics and space Administration and the National science Foundation.
For More Information . . . contact the Polar Research Board at (202) 334-3479 or visit www.nationalacademies.org/ prb. Antarctic Sea Ice Variability in the Southern Ocean-Climate System can be purchased or downloaded free from the National Academies Press, 500 Fifth street, NW, Washington, DC 20001; (800) 624-6242; www.nap.edu.
important processes, such as mixed layer depth, and the relationship between polynyas and ice formation.
Another barrier to process-based understanding is the dearth of observa-tions in the southern Ocean (e.g., under the sea ice), particularly those that are necessary to make heat and freshwater budget calculations. such calculations require estimates of sea ice thickness, esti-mates of precipitation over the southern Ocean, estimates of exchange of water between the ocean and under-ice-shelf cavities, and hydrographic measurements of temperature, salinity, and isotopes.
Models that project climate condi-tions decades and longer into the future indicate that the Antarctic sea ice will eventually respond to global warming
and decline. Observations from late 2016 and early 2017 indeed show decreases in Antarctic sea ice extent. Never-theless, many participants said a better understanding of
the mechanisms is critical to making conident statements about the future of Antarctic sea ice.