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Page 1 Executive Summary The high latitudes of the Arctic and Antarctic, together with some mountainous areas with glaciers and long-lasting snow, are sometimes called the cryosphere—defined as that portion of the planet where water is perennially or seasonally frozen as sea ice, snow cover, permafrost, ice sheets, and glaciers. Variations in the extent and characteristics of surface ice and snow in the high latitudes are of fundamental importance to global climate because of the amount of the sun's radiation that is reflected from these often white surfaces. Thus, the cryosphere is an important frontier for scientists seeking to understand past climate events, current weather, and climate variability. Obtaining the data necessary for such research requires the capability to observe and measure a variety of characteristics and processes exhibited by major ice sheets and large-scale patterns of snow and sea ice extent, and much of these data are gathered using satellites. As part of its efforts to better support the researchers studying the cryosphere and climate, the National Aeronautics and Space Administration (NASA)—using sophisticated satellite technology—measures a range of variables from atmospheric temperature, cloud properties, and aerosol concentration to ice sheet elevation, snow cover on land, and ocean salinity. These raw data are compiled and processed into products, or data sets, useful to scientists. These so-called “polar geophysical data sets” can then be studied and interpreted to answer questions related to atmosphere and climate, ice sheets, terrestrial systems, sea ice, ocean processes, and many other phenomena in the cryosphere. The goal of this report is
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Page 2 to provide a brief review of the strategy, scope, and quality of existing polar geophysical data sets and help NASA find ways to make these products and future polar data sets more useful to researchers, especially those working on the global change questions that lie at the heart of NASA's Earth Science Enterprise. THE POLAR REGIONS AND NASA'S EARTH SCIENCE ENTERPRISE NASA's Earth Science Enterprise (ESE) is one of four strategic enterprises being used to guide the agency's overall research direction. The ESE is dedicated to understanding the total Earth system and the effects of humans on the global environment. Within this enterprise, NASA works with inter-agency and international partners to understand patterns in climate that, ultimately, should allow the nation to predict and respond more quickly to environmental events such as floods and severe winters (NASA, 2000). ESE priorities are driven by five key science questions: 1. How is the global Earth system changing? 2. What are the primary forcings of the Earth system? 3. How does the Earth respond to natural and human-induced changes? 4. What are the consequences of change in the Earth system for human civilization? 5. How well can we predict the changes in the Earth system that will occur in the future? These science questions drive current NASA research initiatives and thus help define the focus and scope of future data set needs. In this report, the committee looks at these key science-driving questions from a cryospheric perspective and describes the most important research issues and the measurements required to address those issues. Thus, the first ESE question, “How is the global Earth system changing?,” which addresses variability and trends in the climate and biosphere system, becomes “Are changes occurring in the polar atmosphere, ice sheets, oceans, and terrestrial regime?” and various sub-questions of relevance. The second question about primary forcing agents of the climate system becomes “What are the major fluxes of CO2 and other trace gases from the polar land surfaces and oceans?” and “What are the spatial and temporal distributions and variability of aerosols in the polar atmosphere?” The third question, focused on responses to change, breaks down into a number of important questions, such as “How will the atmospheric contribution to the mass balance of ice sheets change with the effects of
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Page 3 global warming?,” “How do the polar oceans respond to and affect global ocean circulation?,” “How will the albedo-temperature feedback amplify future climate change?,” and “How will land surface hydrology and energy exchanges be influenced by the climate-induced changes to vegetation structure and distribution across the polar regions?” The fourth question, about consequences, is focused on understanding the possible effects of change on permafrost, shipping, offshore mineral extraction, commercial and subsistence fishing, storms and coastal erosion, water supplies, growing seasons, and changes in range of vegetation and animal species. Finally, the fifth question—pertaining to predictive capabilities—translates to improving capabilities for understanding climate variation in polar regions, defining and obtaining data sets needed by modelers, and improving model simulations of polar processes. The science-driving questions and the associated measurement requirements formulated by the committee are summarized in Table ES-1 and discussed in full in Chapter 3. Discussion of whether existing data-sets provide the information needed to answer the science-driving questions and the committee's conclusions and recommendation follow Table ES-1, begining on page 9.
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Page 4 TABLE ES-1 Key Questions, Research Issues, and Associated Measurement Requirements 1. HOW ARE POLAR CLIMATE AND THE BIOSPHERE CHANGING? 1.1. Are changes occurring in the polar troposhere? 1.1a: Is an acceleration of the polar hydrologic cycle apparent in changes of polar precipitation rates in either hemisphere? Measurement Requirements 1.1a: Rainfall and snowfall over land, ocean, continental ice and sea ice; profiles of atmospheric humidity, temperature, winds at synoptic time and space scale resolution. 1.1b: Is the radiation balance of the polar regions changing? Measurement Requirements 1.1b: Atmospheric temperature and humidity profiles, aerosol abundances and properties, surface albedo and temperature, cloud horizontal/vertical extent and water content (also phase and particle sizes), surface radiation fluxes, top of atmsphere radiation fluxes. 1.2 Are changes occurring in the polar ice sheets? 1.2a: Is the surface elevation of the ice sheets changing? Measurement Requirements 1.2a: Ice sheet elevation time series. 1.2b: In coastal Greenland, are presentday changes in ice sheet mass due to changes in discharge rates, changes in accumulation, and/or changes in melt rates? How do the changes in ice sheet mass compare to past changes? Measurement Requirements 1.2b: Time series of ice velocity and grounding line position, ice thickness distributions for outlet glaciers and drainage basins, surface energy balance, albedo, and ice-melt runoff. 1.2c: Are present day changes in the West Antarctic Ice Sheet due to local or global phenomena? Local warming appears to be independent of or decoupled from global trends and mass wasting of the West Antarctic Ice Sheet is believed to be occurring at an unprecedented rate. Is this related to human activities, local weather phenomena, or long-term climatic oscillations? Measurement Requirements 1.2c: Time series observations of ice velocity and grounding line positions, the distribution and velocity of ice streams, ice thickness (topography and mass), surface energy balances, precipitation accumulation rates, melting rates (oceanic salinity) and rates (mass) of calving.
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Page 5 1.3 Are changes occurring in the polar oceans? 1.3a: Are changes in high latitude precipitation and surface runoff influencing the Arctic Ocean's salinity, sea ice, and circulation structure? Measurement Requirements 1.3a: Precipitation, river runoff, snow cover, glacial runoff, ocean circulation, temperature, and salinity. 1.3b: Are changes occurring in the thickness, coverage, and circulation of sea ice? Measurement Requirements 1.3b: Sea ice thickness, concentration, and motion. 1.3c: Are significant changes occurring in ocean productivity? Measurement Requirements 1.3b: Ocean color, sea ice concentration and thickness, river discharge, and chemical fluxes. 1.4 Are changes occurring in the polar terrestrial regime? 1.4a: Is the distribution of permafrost and Arctic region freeze/thaw changing? Measurement Requirements 1.4a: Permafrost extent, timing of freeze and thaw, vertical temperature profile, and thermokarst topography. 1.4b: Is the hydrology of Arctic terrestrial regions changing? Measurement Requirements 1.4b: Precipitation, temperature, surface radiation parameters (roughness, albedo), winds, humidity, permafrost state, land cover, runoff, and river discharge. 1.4c: Are significant changes occurring in the distribution and productivity of high-latitude vegetation? Measurement Requirements 1.4c: Vegetation characteristics (leaf area index, canopy density, albedo, structural composition, vegetation class), disturbance characteristics (type, timing, severity), wetlands extent, and nitrogen deposition. 2. PRIMARY FORCINGS OF THE POLAR CLIMATE SYSTEM 2.1 What are the major fluxes of CO2 and other trace gases from the polar land surfaces and oceans? Measurement Requirements 2.1: Sea ice concentration and extent, evapotranspiration, soil moisture, surface temperature, permafrost characteristics, vegetation characteristics, disturbance characteristics, wetland extent, CO2 and CH4 fluxes, nitrogen deposition, river discharge, and chemistry.
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Page 6 2.2: What are the spatial/temporal distributions and variability of aerosols in the polar atmosphere? Measurement Requirements 2.2: Aerosol concentration, size distribution, vertical distribution, and composition (index of refraction). 3. RESPONSES TO FORCING AND ASSOCIATED FEEDBACKS INVOLVING THE POLAR REGIONS 3.1 How will the atmospheric contribution to the mass balance of the ice sheets (i.e., precipitation and energy fluxes) change with the effects of global warming? Measurement Requirements 3.1: Precipitation and accumulation history, surface heat fluxes, surface temperature, inversion strength, snowpack structure, ice sheet elevation, ice sheet discharge and runoff. 3.2 How do the polar oceans respond to and affect global ocean circulation? 3.2a: How sensitive are the polar oceans to changes in freshwater inputs and how does the outflow of sea ice and freshwater affect the global thermohaline circulation? Measurement Requirements 3.2a: Sea ice concentration, thickness, velocity; precipitation, evaporation, river runoff, sea surface height, ocean temperature, salinity, and circulation. 3.2b: What is the relationship between polar ocean circulation and the large-scale interrannual/decadal modes of atmospheric variability (e.g., ENSO, AO, NAO, AAO)? Measurement Requirements 3.2b: Surface pressure and temperature, vertical profiles of atmospheric winds, sea ice concentration, ocean temperature, salinity, and circulation. 3.2c: How will changes in ice sheets and polynyas affect water mass formation and circulation? Measurement Requirements 3.2c: Glacier runoff and iceberg calving, sea ice concentration and thickness, sea surface height, ocean temperature, salinity, and circulation. 3.3 How will the albedo-temperature feedback amplify future climate change? 3.3a: How is the albedo-temperature feedback affected by the physical characteristics of melting snow and ice? Measurement Requirements 3.3a: Surface albedo, sea ice concentration and thickness, melt pond fraction, snow depth on sea ice, surface temperature. 3.3b: How is the albedo-temperature feedback affected by polar clouds and aerosols? Measurement Requirements 3.3b: Atmospheric temperature and humidity profiles, aerosols, surface temperature and albedo, cloud horizontal/vertical extents and water contents (particle size and phase).
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Page 7 3.4 How do seasonal snow and ice cover interact with the modes of interdecadal variability in the Northern Hemisphere and the Southern Hemisphere? Measurement Requirements 3.4: Snow cover and albedo, sea ice concentration and albedo, global surface temperature, precipitation, pressure. 3.5 Are changes in sea ice concentration influencing the amount of water vapor in the polar atmosphere? Measurement Requirements 3.5: Surface temperature, surface evaporative flux (surface air humidity and winds), sea ice concentration, snow on sea ice. 3.6 How do atmospheric boundary layer processes under polar conditions influence the exchanges of heat and freshwater between the atmosphere and cryosphere? Measurement Requirements 3.6: Atmospheric temperature and humidity profiles, cloud and vertical layer structure, wind profiles and advective fluxes, surface turbulent fluxes, surface temperature and salinity. 3.7 What role does the cryosphere play in determining the dependence of the large-scale atmospheric circulation on the global meridional temperature gradient? Measurement Requirements 3.7: Profiles of atmospheric winds, temperature, humidity, clouds (layer structure and water content), surface and atmospheric radiative fluxes, precipitation. 3.8 How will land surface hydrology and energy exchanges be influenced by the climate-induced changes to vegetation structure and distribution across the polar regions? Measurement Requirements 3.8: Profiles of atmospheric winds, temperature, humidity, clouds (layer structure and water content), surface and atmospheric radiative fluxes, precipitation. 3.9 How will ecosystems of the high latitudes change in response to CO2 and trace gas loading of the atmosphere? Measurement Requirements 3.9: Vegetation characteristics, albedo, snow characteristics, precipitation, evapotranspiration, disturbance characteristics, permafrost characteristics, soil moisture, river runoff, rainfall, snowfall, and snow cover. 4. CONSEQUENCES OF CHANGE IN THE POLAR REGIONS 4.1 How does permafrost variability impact human infrastructure (roads, buildings) in high latitudes? Measurement Requirements 4.1: Permafrost extent, thermokarst topography, soil temperature profile, land-surface temperature, snow characteristics, soil moisture, vegetation characteristics, disturbance characteristics, wetlands extent.
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Page 8 4.2 What changes will occur in water supplies from snow and snow-fed rivers as the climate changes? Measurement Requirements 4.2: Precipitation, snow cover, ground water, river runoff. 4.3 How will shipping, offshore mineral extraction, commercial fishing, and subsistence fishing/hunting be affected by changes in coastal sea ice characteristics? Measurement Requirements 4.3: Sea ice concentration, ice thickness distribution, surface winds, ocean productivity. 4.4 How will changes in coastal sea ice coverage and sea level rise impact storm surges, coastal erosion and inundation of the coastal fresh water supply? Measurement Requirements 4.4: Sea level height, sea ice concentration, coastal erosion rates, coastal topography. 4.5 How will primary terrestrial productivity, vegetation, and higher organisms be impacted by changes in the Arctic's physical environment? Measurement Requirements 4.5: Permafrost characteristics, vegetation characteristics, rainfall, snowfall, snow characteristics, disturbance characteristics, wetlands extent, soil moisture, land surface temperature, photosynthetically active radiation, evapotranspiration, vegetation characteristics, sea ice extent and thickness, sea surface temperature, salinity, and color, UV radiation. 5. PREDICTING CHANGES IN THE POLAR CLIMATE SYSTEM AND THEIR GLOBAL IMPACTS 5.1 To what extent can transient climate variations in the polar regions be understood and predicted? Measurement Requirements 5.1: Relevant models and supporting data as described in Chapter 3. 5.2 For the purposes of data assimilation by atmosphere and ocean/ice models, including numerical weather prediction models, is there a need for more and/or new observations from the polar regions? Measurement Requirements 5.2: Relevant models and supporting data as described in Chapter 3. 5.3 What specific improvements to formulations of polar processes (e.g., sea ice, land surface energy exchanges) are necessary for the accurate simulation and prediction of climate and climate change? Measurement Requirements 5.3: Relevant models and supporting data as described in Chapter 3.
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Page 9 DO EXISTING DATA SETS PROVIDE WHAT IS NEEDED TO ANSWER THE SCIENCE-DRIVING QUESTIONS? The science-driving questions in Chapter 3 can be analyzed to lead to a clear set of research priorities. That is, by defining the measurements needed to study these questions, one can then evaluate whether NASA's existing polar geophysical data sets adequately meet these needs or whether additional measurements are needed. NASA and other organizations already support and collect data for many variables that are relevant to the polar science-driving questions, but the temporal and spatial coverage can be improved. The committee's analysis identified the following priority measurement needs: atmospheric profiles (including vertical profiles of atmospheric temperature, humidity, and wind) cloud properties aerosol properties surface temperature surface heat fluxes surface albedo precipitation permafrost land surface characteristics evapotranspiration soil moisture terrestrial CO2 and CH4 flux river runoff ice sheet elevation ice sheet dynamics snow cover sea ice concentration sea ice thickness sea ice velocity ocean surface temperature and salinity sea surface height ocean productivity and CO2 flux wildlife habitat and migration NASA makes significant contributions to the development, evaluation, and availability of many data sets of importance in addressing the science-driving questions. For instance, it has done well in supporting demonstration projects and field campaigns that have helped identify the potential applications of remote-sensing technologies to development of data sets addressing variables relevant to the terrestrial land surface (e.g., land surface characteristics, terrestrial CO2 and CH4 flux, freeze/thaw dynamics, snow cover, evapotranspiration and soil moisture). The agency also makes a valuable contribution in terms of ground-based monitoring for carbon fluxes. NASA has also supported analysis of satellite data collected by other agencies, for instance to determine atmospheric temperature and humidity profiles enhanced for polar conditions, surface temperature and visible
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Page 10 albedo, cloud properties, and surface radiative fluxes. NASA's work on mass change for the Greenland ice sheet, its contributions toward understanding ice mass balance in Antarctica, its contributions to understanding surface energy balance and local meteorology, and its high resolution mapping of ocean color also important for understanding climate changes and its potential societal impacts. Many other examples are discussed in Chapter 4. CONCLUSIONS AND RECOMMENDATIONS Based on its deliberations, the committee developed conclusions and recommendations in three categories: key data gaps and the highest priorities for measurements in the context of the polar variations of NASA's Earth Science Enterprise science-driving questions; the general NASA strategy for enhancing and providing relevant data sets to the polar science community; and issues relating to the effectiveness of the Distributed Active Archive Centers (DAACs) and their data distribution activities. Data Gaps and Measurement Needs The committee identified 10 high-priority measurements related to the cryosphere (i.e., surface characteristics) and the fluxes that determine cryospheric characteristics. Thus, the committee sees them as obstacles to progress on the fundamental science issues in the ESE. These high priority measurement needs are: polar precipitation, surface albedo, freshwater discharge from terrestrial regions, surface temperature, surface turbulent fluxes, permafrost extent, ocean surface salinity, ice sheet mass flux, land surface characteristics, and sea ice thickness. For all variables, the need is for data sets sufficient to determine the spatial and temporal variations. For those variables deemed suitable for
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Page 11 monitoring, temporal continuity across instruments must be a key consideration. For polar precipitation the key need is to measure the space-time distribution of precipitation rates for moisture budget determinations. For surface albedo the key need is to capture the progression of melt and freeze-up over spatial and temporal scales adequate for surface energy budget evaluations and for testing and validation of models. Meeting this objective also requires surface temperature measurements with similar time and space resolution. Related to freshwater discharge from terrestrial regions, there is need for data on fluxes of freshwater to polar oceans from ice sheets and glaciers, as well as surface and sub-surface runoff from non-glaciated land area. With regard to surface temperature, the need for routine measurements under cloudy skies calls for multisensor analysis approaches that combine microwave, infrared, and radar measurements. For surface turbulent fluxes, the key need is to measure sensible and latent heat fluxes over all polar surfaces. For permafrost, the key need is for measurements of the extent of permafrost, the depth of permafrost, vertical temperature profiles, and the timing of thaw and freeze-up of the active layer. Key needs with regard to polar ocean surface salinity are high- to moderate-resolution measurements of coastal and ice edge environments and improved accuracy in near-freezing temperatures. For ice sheet mass flux, the key need is for the continuation and interpretation of measurements to understand the variability introduced by accumulation and ablation, both temporally and spatially. Needed measurements of land surface characteristics include leaf area index, wetlands extent and water storage, canopy density, structural composition of vegetation, and disturbance characteristics at appropriate time and space scales. Finally, for sea ice thickness the key need is for development of remote-sensing techniques (beyond those currently available for mapping ice age and type). NASA's General Strategy on Polar Geophysical Data Sets To improve NASA's general strategy for providing useful polar geophysical data sets the committee strongly believes that measurements should not be made in isolation, and that there is a need for across-sensor and across-variable evaluations to be an integral part of remote sensing activities. These themes are common to several of the following recommendations. In several respects, polar data sets have been isolated from global data sets in the overall NASA data set framework. Examples are the
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Page 12 global satellite-derived atmospheric termperature data sets and global sea surface temperature data sets versus the TOVS vertical profiles and the POLES surface air temperatures. We recommend that NASA make efforts to integrate polar data sets and global data sets in the architecture of the NASA data distribution system. The committee was struck by the multiplicity of data sets available for some polar variables. Cloud products, sea ice coverage, and sea ice motion are examples. The committee recommends that NASA support systematic comparisons of different versions of the same product, including calibration and validation activities, the establishment of error bars, and the merger or consolidation of data sets depicting the same variables. This activity should include the determination of needs for (1) ground-based validation sites in addition to those planned for EOS, and (2) additional field campaigns directed at validation of remotesensing products. In general, satellite sensors and products have not been optimized for polar regions. Examples include those used for precipitation estimation, determination of cloud properties, and sea surface salinity. The committee recommends that NASA give higher priority to sensor optimization for polar applications. For some of the variables listed in the preceding sub-section, alternative measurement approaches may be more appropriate than reliance on satellite products. Alternative and complementary approaches could include aircraft (manned and unmanned), automated underwater vehicles, and other ground-based measurements. The committee recommends that NASA explore such alternatives from a cost and benefit perspective in the planning and design of its polar programs. There is a need for enhanced interaction and feedback between modelers and data providers. This interaction can occur to some extent through data assimilation activities; however, there is often a disconnect between modelers' needs for forcing and validation data sets and the products derived from remote sensing. The committee recommends that NASA strengthen the connection between modeling and its remote sensing strategy for the polar regions. The committee recommends that NASA support research to determine optimum uses of polar remote sensing measurements in re-analyses so these measurements are better integrated into a global context, thereby enhancing their value to ESE and reducing their insularity. For
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Page 13 the broadest impact, this activity should entrain, in the earliest stages, the two leading global re-analysis centers, NOAA's National Centers for Environmental Prediction and European Center for Medium-range Weather Forecasts. Consistency within and among data sets is a necessary prerequisite for detecting, with a known degree of certainty, the effects of climate change on high-latitude systems. Such an effort would also enhance the interaction of modelers and data providers and constitute an important framework activity for developing and testing new sensor designs. The committee recommends that NASA give high priority to fostering the development of spatially and temporally coherent, internally consistent polar geophysical data sets. Finally, the continued use of satellite-derived data sets in intercomparison and synthesis activities, such as those described above, argues for the long-term archiving of satellite data sets within the NASA data distribution system. The committee considers the DAACs to be the natural vehicles for long-term archival. Improving the Effectiveness of Distributed Active Archive Centers As a result of its deliberations—including consideration of the 1998 National Research Council report, Review of NASA's Distributed Active Archive Centers, conversations with DAAC managers and representatives of associated user groups, and a survey of the science community—the committee learned much about the effectiveness of the polar-oriented DAACs. Unlike the recommendations in the two preceding sections, the following recommendations pertain specifically to the polar DAACs. Redundancies: The committee recommends that NASA support quantitative evaluations of possibly redundant data sets to help provide a rational basis for decisions about discontinuing certain data sets. The results of coordinated evaluations should be used by the DAACs to minimize redundancies in their holdings and their distribution responsibilities. Outreach: The survey responses showed a surprising lack of awareness of the availability of holdings of the DAACs. The committee recommends that the DAACs increase the efforts to disseminate and publicize their holdings, particularly among investigators who are not involved in large NASA programs, as well as provide overview documentation of the broad spectrum of data sets available at a site.
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Page 14 Links: With regard to the distribution of data products by the DAACs, the committee recommends that the DAACs install more extensive web links to other global products that contain polar data, with brief descriptions of the holdings in the sites. These “pointers” should include available information about data sources, quality, and limitations specific to the polar regions. This information can be provided through literature references or through DAAC-initiated assessments. Improvements in Tools: The committee recommends that data set providers better document their individual holdings as well as provide overview documentation of the broad spectrum of data sets available at their site. The addition of browse products to data sets would help users unfamiliar with the data to judge the utility of the data set for their uses. To help overcome obstacles faced when trying to combine data sets from different sensors, data set providers should increase availability of user-friendly software tools to help with tasks such as converting from one format to another and among standard grid formats. Attention should also be given to the issue of changing technology so that archived data will remain readable with future technologies. Feedback: The committee recommends that data set providers provide additional opportunity for community feedback via the creation of web bulletin boards where users may comment on their experience using data at that site. This will encourage a more coherent user community where, for example, problems may be solved between users without direct intervention by the data site provider. P-I Web Sites: The committee recommends creation of an archive of principal investigator-generated websites containing relevant data sets or information about these data sets. This type of archive will complement the data distribution activities of the DAACs and will enhance their utility as information resources for the polar community. Alaska SAR Facility: While ASF received expressions of both praise and frustration from our survey respondents, the use of ASF products has been limited by data product availability, by costs, by ease of access, and by access and distribution restrictions. There has been some recent improvement in access to SAR products. Nevertheless, we share the concern of the 1998 DAAC review and reiterate the need to facilitate access to and utilization of ASF products. In particular, the committee recommends that ASF become more proactive in the assembly of pan-Arctic data sets.
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