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Scientific Value of Arctic Sea Ice Imagery Derived Products 1 Sea Ice and the Global Climate System THE IMPORTANCE OF SEA ICE As an integral and interactive part of the global climate system, the presence or absence of sea ice has a number of important economic, societal, legal, and national security implications. If the current trend toward a loss of sea ice, especially during summer, continues, unimpeded shipping between the commercial centers of the Atlantic and the Pacific is bound to be an issue of enormous consequence (Stroeve et al., 2008). Such shipping will raise many questions regarding the Law of the Sea, territorial waters, and economic zones, as well as the monitoring and policing of international agreements, environmental pollution, and search and rescue (e.g. INSROP, 1999; AMSA, 2006). Another important consequence of a diminished ice cover would be the greater access to areas with potential fossil fuel deposits and other mineral resources (Bird et al., 2008). All these activities will require the development of appropriate policies and infrastructures. The diminishing amount of thick, multiyear ice poses a well-publicized threat to the survival of the polar bear (Figure 1.1; Durner et al., 2009). The survival of a dwindling population of bears in coastal sea ice areas is bound to induce additional shifts in the ecosystems, including potential threats to the basis of subsistence living of coastal native populations (Durner et al., 2005; USGS, 2007; FWS, 2008). Another issue receiving increased attention is the possibility that the recent warming and the prolonged periods of open water during the Arctic summer are already having a significant impact on the marine ecosystem of the Bering Sea (Mueter and Lutzow, 2008). The Arctic Basin receives more continental runoff and nutrients per unit area than any other, and the very large and shallow Arctic shelf waters are well mixed. This holds the possibility that, in the future, the immigration of sub-arctic species into these areas may transform them into commercially significant fisheries. The seasonal cycle of freezing and melting of sea ice imparts an annual cycle of salt injection (freezing) and dilution (melting) to the upper ocean layers. The amplitude of this cycle is small for thick, multiyear ice and much larger for seasonal ice. If the Arctic sea ice cycle were to evolve toward an ice-free summer, the increased amplitude of the upper ocean salinity variations may have profound consequences for the entire thermohaline regime of the Nordic Seas, including the properties and rate of production of North Atlantic Deep Water and the global thermohaline circulation (Wadhams, 2005).
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Scientific Value of Arctic Sea Ice Imagery Derived Products ARCTIC SEA ICE IN INTERACTIVE CLIMATE MODELS To prepare for managing the transition to an Arctic that may be nearly ice free during summer, it is critical to have accurate projections of Arctic environmental changes over the next several decades. Forecasts of regional sea-ice conditions on seasonal timescales can help different stakeholders prepare for and adapt to the impacts of climate change and minimize environmental risks associated with development. While the Intergovernmental Panel on Climate Change (IPCC) models provide meaningful projections of future global temperature and precipitation, projections of Arctic sea ice cover range widely, from almost no change to the end of the 21st century, to a disappearance of the ice cover at the end of summer 20 years from now. Clouds (Figure 1.2, top figure) exert the strongest forcing on the surface heat balance, which controls the freezing and melting of ice (Kay et al., 2008; Curry et al., 2006). In view of several other uncertainties concerning sea ice physics, it is not surprising that the models show a huge variance in their predictions of the Arctic ice cover during the rest of our century. FIGURE 1.1 Projected changes in the spatial distribution and integrated annual area of optimal polar bear habitat for 2050. Map shows the cumulative number of months per decade where optimal polar bear habitat was either lost (red) or gained (blue). The green circles indicate the approximate locations of the Medea Fiducial Sites. SOURCE: Durner et al., 2009.
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Scientific Value of Arctic Sea Ice Imagery Derived Products FIGURE 1.2 Selected results from 16 models used in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). Top figure: monthly contemporary mean cloud amount north of 66 degrees N from 16 climate models from IPCC-AR4. The black line shows the mean monthly cloud amount observed at the Russian drifting stations in the Arctic Basin (Lindsay, 1998). Bottom figure: Projected retreat of the summer ice extent from the same models. SOURCE: Reproduced with permission from Ian Eisenman, using data from the Program for Climate Model Diagnostics and Intercomparison at Lawrence Livermore National Laboratory. The discrepancies in the total cloud amount alone (especially in winter) can make a major contribution to the vast range of the predicted ice cover shown in Figure 1.2. We cannot offer a discussion of the various parameterizations used in the different models and their effects on the outcome, but it seems clear that the vast range of results signifies a profound problem with model physics. Analyses of ice models with constant seasonal forcing suggest that the same applies to the ice models. In part, the models are hampered by the fact that many of the physical processes are poorly understood due to a lack of observations at appropriate times and scales. Maintaining manned drifting stations on the ice and conducting observational flights is difficult and expensive. The spring melt and fall freeze-up periods are particularly
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Scientific Value of Arctic Sea Ice Imagery Derived Products challenging due to the rapidly changing environmental conditions and the weak platform offered by thin ice. The essential ingredients of an ice model are shown in Figure 1.3. The external forcing consists of wind, ocean currents, short- and long-wave radiation, and turbulent fluxes in the atmospheric and oceanic boundary layer. A flow law describes the relationship of external stress and strain rate, and a thickness re-distribution function (psi) relates the ice strain to the formation of leads and pressure ridges (Thorndike et al., 1975). The external forcing functions are the subject of interactive atmosphere/ocean models, and the prediction of their effect on the ice is only as realistic as they are. The thermodynamic processes are dominated by the radiation balance, i.e. by the amount of incoming radiation and by the reflectivity (albedo) of the surface. In summer, the surface albedo is strongly affected by the area of ice covered by low-albedo melt ponds, which in turn is affected by the surface topography, i.e. the thickness distribution. Currently available data on this relationship are sparse, to say the least. Hence the albedo assigned to the different thickness categories is a powerful tuning parameter in the models. The amount of cloud cover exerts a strong control on the downwelling radiation that reaches the surface of the ice. In the absence of sunshine, the total downwelling radiation is composed only of the longwave component, whose magnitude is determined unilaterally by the presence or absence of cloud cover. In the presence of sunshine, the total downwelling radiation is the sum of longwave and shortwave components. Because cloud amount decreases downwelling shortwave but increases downwelling longwave radiation, the relative impact of clouds on the total downwelling radiation that is observed by the ice may be greater in winter than summer. The extremely high inter-model variance of cloudiness, particularly during the winter months (Figure 1.2, top panel) may well make a major contribution to the widely differing model results seen in the bottom panel of Figure 1.2. FIGURE 1.3 Schematic representation of sea ice in a climate model. SOURCE: Thorndike et al., 1975; Reprinted by permission of American Geophysical Union.
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Scientific Value of Arctic Sea Ice Imagery Derived Products A half-century-long record shows a trend downward in sea ice extent. This trend is much steeper for the summer months and, until recent years, relatively insignificant for winter months (Meier et al., 2007; Figure 1.4). From the viewpoint of greenhouse gas (GHG) forcing, the opposite might be expected: during the warm season, variations in atmospheric water vapor dominate GHG forcing and are likely to overwhelm the effect of the other greenhouse gases. During the cold season, when the atmosphere is drier, the effect of the non-vapor GHG should dominate. The stronger decline in the summer sea ice extent compared to the winter extent indicates that an increasing amount of multiyear ice either melts or is exported through Fram Strait, which is compensated to some extent by an increase in first-year (and thus thinner) sea ice (Figure 1.4). Kwok (2009, unpublished) reports that the export of multiyear ice has not increased during the past 5 years, hence the loss of multiyear ice must be ascribed to in situ melting (Figure 1.5). According to energy balance climatology, established mostly during the second half of the 20th century, multiyear ice is too thick to melt completely in one summer. The preceding remarks are intended to illustrate that many unresolved problems exist regarding the processes occurring in sea ice. The Medea data set and future collections, described in the following chapter, will deepen our understanding of sea ice and help us cope with the consequences of a changing Arctic. FIGURE 1.4 Three-monthly and annual means of the surface area of Arctic sea ice since the inception of observations by satellite. SOURCE: The Cryosphere Today, http://arctic.atmos.uiuc.edu/cryosphere/.
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Scientific Value of Arctic Sea Ice Imagery Derived Products FIGURE 1.5 Decline in Arctic Ocean Multiyear Sea Ice Coverage. SOURCE: Figure courtesy of Ron Kwok, JPL.