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Understanding Climate Change Feedbacks (2003)
Board on Atmospheric Sciences and Climate (BASC)

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3 SEA-ICE FEEDBACKS SUMMARY ice anct snow in high latitudes, and in particular sea ice, contribute importantly to climate sensitivity through ice-albedo feedback, but the magnitude of this feedback remains uncertain. Ice-albedo feedback in polar regions is coupled strongly to polar cloud processes and ocean heat transport. Better monitoring of polar ice distributions and associated atmospheric and oceanic properties is needed. Systematic global observations of sea-ice thickness are needed, but a system to make these measurements is unavailable. Improvements are needed in the parameterization of sea-ice growth, associated heat and freshwater fluxes, the variable surface albedo, and polar clouds. Parameterizations of snow and ice processes in climate models and their effect on climate sensitivity need to be tested against observations using an appropriate set of metrics. Further development and distribution of satellite and in situ datasets describing variations of polar ice and polar clouds should be a priority. Various positive feedbacks and other important linkages between the atmosphere and Earth's surface occur through sea-ice processes, which themselves are subject to conditions in the ocean's surface layer. At high latitudes when the ocean surface temperature drops to about -1.8°C, sea ice forms on the ocean surface. Ice has a strong impact on climate because the associated feedbacks are positive and large. The presence of sea ice both insulates air-sea heat exchange and increases the surface albedo, thereby affecting climate through a reduction in oceanic sensible and latent heat loss to the atmosphere, and reducing the amount of absorbed incoming solar radiation, respectively. Albedo effects are also linked with cloud radiation 4 '1

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42 UNDERSTANDING CLIME TE CHANGE FEEDBACKS balances in ice-covered regions. Ice insulation has a weaker direct effect on climate since sensitivity to ocean surface temperatures is low in high latitudes, but indirectly the impact could be large since ice extent partially depends on underlying ocean conditions. In addition, sea ice represents a source of freshwater that through advection from one location to another can affect the properties of deep and intermediate water formation in the ocean. Sea ice is a highly responsive component of the global climate system due to its high albedo and its participation in the hydrologic cycle. The IPCC TAR identified the coupling between sea ice and atmosphere and between sea ice and ocean to be of great importance in defining the sensitivity of the global system. Important sensitivities include the feedback between surface albedo and ice extent and properties; the ice-insulating effect; and the relationship between the North Atlantic thermohaline circulation and sea-ice export through Pram Strait. Of these the albedo feedback is the greatest influence. A perturbation to the surface energy balance of the sea ice results in a perturbation to ice area, surface temperature, melt pond and lead fraction, snow depth, ice thickness and other sea-ice characteristics. A positive (warming) perturbation will lead to an increase in the amount of solar radiation that is absorbed by the planet. Thus, increases in the temperature cause increases in the amount of solar radiation absorbed by the surface, leading to further increases in temperature. This association of temperature, ice cover and characteristics, and albedo is called ice-albedo feedback. Ice albedo feedback is a positive feedback process in that it amplifies the temperature response to climate forcing. However, until our physical understanding of the component processes is improved, the interdependence among these processes remains unquantified. The magnitude and even the sign of some of the other polar feedback processes are also associated with significant uncertainties. Much of this uncertainty is related to cloud radiation feedbacks and how polar cloud characteristics will be altered in a changing climate. Because of the impact of clouds on the surface radiation flux and thus the state of the sea-ice surface, the cloud radiation feedback processes in the polar regions are inextricably linked with sea ice and snow feedback processes. Our best estimate at present is that all of the individual cloud, snow, and sea-ice feedbacks in the polar regions are positive, with the exception of the aerosol- dehydration feedback. It remains a major task in climate modeling to explain the relative stability of the polar climate in the presence of these positive feedbacks. Possibilities include unexpected negative cloud feedbacks, or negative feedbacks between the sea ice and ocean.

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SEA-ICE FEEDBACKS 43 Outlined below are some of the potentially most important polar feedbacks. These feedbacks should not be viewed as mutually independent, but rather as interconnected components of a complex system. OVERVIEW OF SEA-ICE FEEDBACKS Ice-Albedo Feedback Warming of high latitudes can decrease the areal extent of sea ice, especially in the summer, leading to a decrease in surface albedo and an increase in the absorption of solar radiation at Earth's surface, which would favor further warming. In model studies the magnitude of the positive ice- albedo feedback has been seen to increase by the inclusion of melt ponds, and to diminish by the inclusion of ice thickness distribution and ridging. Ice Insulating Feedback Warming of high latitudes decreases the areal extent of sea ice, especially in the summer, providing an enhancement to the warming through removing the insulating effect of sea ice on air-sea heat exchange (Manatee and Stouffer, 1980~. Meridional Overturning Circulation and SST-Sea-Ice Feedback While the actual future path of the Atlantic meridional overturning circulation (MOC) is not known, it is possible that in the short term the ocean could act as a negative feedback to high-latitude warming (Bryan, et al. 1988; Gent, 2001~. The role of deep ocean heat in the Antarctic subpolar gyres (delivered by the MOC) plays a critical role in regulating the thickness of the insulating Antarctic sea-ice cover (Martinson, 1990~. Consequently, one may assume that any change in the MOC may result in a change in this deep ocean heat content and thus the sea-ice thickness. The latter will impact the length of the sea-ice season, insulating effectiveness, freshwater transport by sea-ice drift, and deep and intermediate water formation (feeding back into the MOC directly). It is difficult to predict the nature of the sign of the net feedback, since we need a better understanding of how changes in the MOC may impact the properties of the subpolar deepwaters.

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44 UNDERSTANDING CLIMA TE CHANGE FEEDBACKS The net change will depend upon the balance of a variety of detailed local air-sea-ice exchange processes, and this is difficult to estimate in typical low-resolution climate models. Ice Cloud Feedback Processes Because of the impact of clouds on the surface radiation flux and thus the state of the sea-ice surface, cloud radiation feedback processes in the Arctic are inextricably linked with albedo feedback processes. A perturbation in the surface radiation balance of the snow or ice, which could be produced by input of greenhouse gases and aerosols, results in a change in snow or ice characteristics (i.e., ice thickness and areal distribution, surface temperature, and surface albedo). These changes in surface characteristics, particularly the surface temperature and fraction of open water, will modify fluxes of radiation and surface sensible and latent heat, which will modify the atmospheric temperature, humidity, and dynamics. Modifications to the atmospheric thermodynamic and dynamic structure will modify cloud properties (e.g., cloud fraction, cloud optical depth), which will in turn modify the radiative fluxes. DEVELOPING A SCIENTIFIC STRATEGY The polar climate community is poised to make rapid progress in these areas. In particular the United States is uniquely positioned to improve our understanding of these feedbacks, because many of the relevant satellite datasets are being developed in the United States, and some of the relevant modeling activity is concentrated here as well. Many of the assembled datasets are already in place, or field campaigns are planned that will address deficiencies in in situ data requirements. The one caveat to this assessment is the paucity of ice thickness data over large space and time scales. There remain serious technological difficulties in making extensive observations of this type. It may require further development of upward-looking sonar (ULS) technology before reliable collection will be possible. Two of the other potential impediments to progress are insufficient data- processing and archival facilities and inadequate funding for creating detailed climate-quality satellite datasets (e.g., NRC, 2000b) over longer time periods. Another point worth noting is that most current funding addresses specific science questions. We view this favorably. However,

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SEA-ICE FEEDBACKS 45 insufficient funding has been available for developing the necessary new monitoring technologies, large datasets, and comprehensive models. Specific strategies for characterizing and reducing the uncertainty in polar feedbacks need to consider not just cloud processes and sea-ice processes but linkages between them and the relationships between these processes and interannual variability. Observations The most comprehensive source of sea-ice data of large space and time scales is satellite-derived data, which includes sea-ice concentration, snow extent and ice motion from passive microwave data, sea-ice concentration from MODIS and leads from the Advanced Very High Resolution Radiometer (AVHRR). Some ice thickness data may become available from Icesat (Zwally et al. 2002~. Less extensive data from, for example, the Arctic and Antarctic Drifting Buoy programs and ice draft from ULS add in situ data. In addition, an unprecedented Arctic sea-ice dataset is being assembled under the auspices of the U.S. Surface Heat Budget of the Arctic Ocean (SHEBA) project (Uttar et al., 2002~. High-quality surface data is available at the SHEBA ice camp in the Beaufort and Chukchi seas; aircraft observations were made during a four-month period in a region over the Beaufort and Chukchi seas, and several satellite remote-sensing groups are focusing on the SHEBA field season of October 1997 to October 1998. Several remotely sensed datasets will be made available on a basin-scale for this project. These data should be fully utilized to advance understanding and improve model parameterizations. Although not directly related to ice, an understanding and correct simulation of the cloud radiation feedback in polar regions requires observations of (1) cloud fractional coverage and vertical distribution as the vertical temperature and humidity profiles change, and (2) changes in cloud water content, phase, and particle size as atmospheric temperature and composition changes. The largest uncertainty in assessing the cloud-climate feedback mechanism is the change in cloud cover in response to a change in atmospheric temperature. Even the sign of the cloud-climate feedback over the Arctic is unknown. Cloud radiation feedbacks and the required observations are also discussed in Chapter 3. Because of the different thermodynamic and radiative environment in the polar regions, conclusions drawn for the globe regarding these feedback processes may be inappropriate over the Arctic and Antarctic. Detailed

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46 UNDERSTANDING CLIMATE CHANGE FEEDBACKS satellite datasets must be extended to include Antarctic products at the same level of detail as for the Arctic. In addition, the record length of detailed satellite datasets must be extended to account for interannual variability and characterization of ice thickness must be extended in time and space. To advance understanding and thereby possibly reduce uncertainty about important cryospheric feedback processes, the committee recommends that detailed satellite datasets must be extended to include Antarctic products at the same level of detail as for the Arctic. In additions detailed satellite datasets must be extended in time and space to account for interannual variability and characterization of ice thickness. The ability of climate models to simulate the observed annual cycle of sea-ice extent, thickness, and concentration should be carefully tested. In addition, the interannual variations of these quantities in free-running climate models should be compared against observations. Modeling The state of the art in sea-ice modeling is fairly advanced relative to what is currently being used in most state-of-the-art climate models. Most if not all major coupled climate models have crude representations of sea-ice physics. Some models still use a purely thermodynamic treatment of sea ice and others often only incorporate crude representations of sea-ice dynamics (e.g., cavitating fluid, free drift). Climate models show strong sensitivity to sea-ice representations (Holland et al., 2001; Liu et al., 2003~. It is difficult to assess the importance of sea-ice-climate feedbacks without coupling sea- ice models to prognostic ocean and atmosphere models. It can be argued that on large scales, the use of current, state-of-the-art parameterizations of ice dynamics and correct atmospheric dynamical forcing will lead to reasonable simulations of ice extent, if the ice thermodynamics is well represented. Local thermodynamic processes (even over multiyear ice) and exchange with the atmosphere influence surface type and hence albedo. The detailed exchanges of heat and freshwater with the atmosphere and ocean are processes that disciplinary modelers (ice, ocean and atmosphere) often neglect. These interracial processes are crucial to an understanding of sea-ice feedbacks. Taken together the uncertainties outlined in Chapter 3 on water vapor and cloud feedbacks highlight several areas of priority where substantial and rapid scientific advances can be made in the areas of process parameterization and model development, especially in light of improved and expanded datasets. 1

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SEA-ICE FEEDBACKS 47 In the area of sea-ice feedbacks our general modeling recommendations are the following. Initiatives should be developed to improve the parameterization of new sea-ice growth and its associated heat and freshwater fluxes, snow over sea ice (especially the surface temperature) and surface albedo that responds to surface ice characteristics, including melt ponds. In addition, parameterizations are urgently needed for the unique properties of Arctic and Antarctic clouds. We also recommend that major U.S. modeling groups incorporate and rigorously test more sophisticated treatments of sea ice and related parameterizations in coupled models.

Representative terms from entire chapter:

feedback processes