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Page 44 3 Science-Driving Questions: The Polar Regions in the Context of NASA's Earth Science Enterprise Five key science questions drive NASA's Earth Science Enterprise (ESE): 1. How is the global Earth system changing? 2. What are the primary forcings of the Earth system? 3. How does the Earth system 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? The committee organized its discussion around these five questions that define the scope of the ESE program, but adapted them to focus on the cryosphere. In essence, the committee wishes to determine whether NASA-supported data sets contribute the information needed to address the cryospheric elements of the ESE. Specific, measurable biogeophysical phenomena are the focus; for example, to understand climate variability over land, researchers must be able to characterize individual elements of the Arctic water cycle such as seasonal variations in permafrost, soil moisture, evapotranspiration, precipitation minus evaporation, surface temperature, runoff, and snow cover. Each of the five ESE science questions is addressed in a section below. In each case, the committee outlines what it believes to be the primary
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Page 45 and related research questions that need to be addressed to support the ESE, and does so with sufficient specificity to allow an assessment of the supporting measurements. The scope of the science questions considered here is affected by how “cryosphere” is defined. The committee elected to begin with the definition used by the WCRP's Climate and Cryosphere Program, which defines the cryosphere as that portion of the Earth containing sea ice, snow cover, permafrost, ice sheets, and glaciers (WCRP, 2000). The committee also included critical processes that influence surface energy, freshwater fluxes in the cryosphere, and carbon and trace gas exchanges at the atmospheric interface. The committee wishes to emphasize that we did not create the basic science questions, but took the global science questions presented in the NASA ESE Science Plan and focused them on polar regions. The following science planning documents relevant to the polar regions were also used: Science Plan for the WCRP Arctic Climate System Study (WCRP, 1994) Science Plan for the WCRP Climate and the Cryosphere Programme (WCRP, 2000) Science Plan for the NSF ARCSS Human Dimensions of the Arctic System (ARCUS, 1997) The IPCC Working Group I and II contributions to the Second Assessment Report were also consulted (Houghton et al., 1996). The committee found that several cryosphere issues that we judged relevant to this study were not thoroughly addressed by the three science plans or IPCC reports. For these issues, we have formulated a few new subsidiary science questions, but these are still organized within the basic ESE framework. For each ESE question, a set of polar-specific questions, a rationale for the importance of studying the relevant phenomenon, and a short list of required measurements are given. 1 Issue No. 1: How are Polar Climate and the Biosphere System Changing? 1.1 Are changes occurring in the polar troposphere? 1.1a Is an acceleration or deceleration of the polar hydrologic cycle apparent in changes of polar precipitation rates in either hemisphere?
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Page 46 The spatial and temporal patterns of rainfall and snowfall define the fundamental character of water availability over the poles. Precipitation is the first step in a chain of events that defines the distribution and dynamics of ice sheet, glacier, and sea ice mass; soil moisture; evapotranspiration; runoff and river discharge; vegetation distribution; primary production and trace gas production. Models of contemporary climate change show increases in high latitude precipitation, shifts in the predominance of snow and rain, and possible increases in the occurrence of severe events. A contemporary and historical assessment of the spatial and temporal distribution of precipitation provides a critical benchmark for future change. One important aspect of the debate on climate change centers around the presumed intensification of the global water cycle through which excess energy trapped as a consequence of the greenhouse effect is transformed into an increase in the poleward delivery of water vapor and heat by atmospheric transport. Recent observational evidence shows an increase of storm tracks across the polar front and changes in precipitation patterns associated with the North Atlantic Oscillation (NAO). The impact of recent climate variability and ultimately greenhouse-induced circulation changes needs to be better monitored and understood. Measurement Requirements: Rainfall and snowfall over land, ocean, continental ice, and sea ice; profiles of atmospheric humidity, temperature, and winds resolved at synoptic time and space scale. 1.1b Is the radiation balance of the polar regions changing? The primary radiative energy exchange in the polar regions is the loss of energy to space, mostly from the atmosphere. This loss is regulated by clouds and water vapor: loss rate depends on the atmospheric temperature and the vertical distribution of clouds. In a changing climate, atmospheric advection of warmer and moister air into the polar regions will probably alter the vertical distribution of clouds. Tracks and frequency of storms might change. The precise interaction of the changing atmospheric and cloud properties will determine the consequent changes of the polar radiation balance. The secondary radiative exchange of energy is between the relatively warmer atmosphere and the colder sea-ice-covered
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Page 47 ocean; this exchange regulates the heat content of the polar oceans. Clouds regulate the solar heating of the surface, usually melting ice in summertime, and the heating by terrestrial radiation, primarily in wintertime. These interactions among the atmosphere, clouds, and sea ice is central to determining the sensitivity of the polar climate. Measurement Requirements: Atmospheric temperature and humidity profiles, aerosol abundances and properties, surface albedo and temperature, cloud horizontal and vertical extent and water content (also phase and particle sizes), surface radiation fluxes, and top of atmosphere radiation fluxes. 1.2 Are changes occurring in the polar ice sheets? 1.2a Is the surface elevation of the ice sheets changing? An important concern regarding the impact of climate change across the high latitudes is the potential release of the huge quantities of water stored in the cryosphere. Measurements of changes in ice sheet elevation are not of themselves diagnostic of the processes that are producing change, but the measurements are nearly directly translatable into a component of present sea level rise. Recent NASA PARCA results showing a decrease in the overall mass of the Greenland ice sheet over the last five years is the first reliable indication of the impact of the ice sheets on global sea level rise, there is a clear need to continue this time series and provide similar information for the Antarctic. Measurement Requirements: Ice sheet elevation time series. 1.2b In coastal Greenland, are present-day 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? Ice sheet mass discharge varies over both space (often locally) and time, and accumulation and melt patterns are affected by changes in atmospheric circulation; it is critical to understand these variations in order to provide a predictive understanding of the change. The societal impacts of ice-sheet-generated sea level rise will come
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Page 48 from an acceleration of these processes. Melting has been identified as a causative factor in ice mass changes in Greenland and has been suggested as a cause of ice shelf changes in Antarctica as well. Ice melt is dependent on the surface energy balance, and this makes an understanding of surface albedo and radiative forcing particularly important, especially at the ice margins. In order to determine if accelerated melting, reduced accumulation, or ice dynamics are responsible, it will be necessary to collect time series data allowing the determination of the energy available for melting, the amount of runoff, accumulation variability, and changes in ice flow. Ice core measurements of past accumulation variation are required to place modern measurements into a longer-term context. Measurement Requirements: Time series observations of ice velocity and grounding-line position, ice thickness distributions for outlet glaciers and drainage basins, surface energy balance, albedo, precipitation, and ice-melt runoff. 1.2c In West Antarctica, are the present local imbalances, caused by flow variations in large outlet glaciers and ice streams and possibly by local accumulation variations, consistent with long-term retreat or possible instability? Rapid ice discharge in large outlet glaciers and ice streams carries much of the mass lost from the large ice sheets. In West Antarctica, intensive study of ice streams discharging into the Ross Ice Shelf has revealed a complex history of variably flow rates caused by basal conditions such as soft, deforming sediments and the presence of water at high pressure. The record shows a substantial retreat of the ice over the last twelve thousand years; however, the size of the region involved in the rapid flow has made detailed understanding of present conditions difficult. Satellite data used by both NASA and NSF funded research are changing our understanding of ice flow patterns continent-wide, revealing new sites of rapid variations in ice flow and ice thickness. These types of observations will indicate the scale of ongoing change and provide the foundation for a predictive understanding of the processes involved. Measurement Requirements: Time series observations of ice velocity and grounding-line positions, the distribution and velocity of ice streams, ice thickness (topography and mass), surface energy bal-
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Page 49 ances, albedo, precipitation accumulation rates, melting rates (oceanic salinity), and rates (mass) of calving. 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? Simulations of future climate change indicate an increase of high-latitude precipitation and related changes in surface runoff. This will likely have a strong influence on the freshwater budget of the Arctic Ocean as direct input to the sea surface and a delayed input via snow melt from ice and land. Each of these inputs has a distinctive geographic and seasonal signature that may change in response to changes in high-latitude precipitation. The resultant freshening might influence the strength and location of deep water formation in the North Atlantic Ocean. Measurement Requirements: 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? The spatial and temporal distribution of sea ice is a fundamental property of the high-latitude oceans. Concentration and thickness of sea ice reflect the state of ocean circulation and heat fluxes. These variables may also provide an important feedback to the larger Earth system by regulating planetary heat balance and the formation of deep ocean water, producing global consequences. It is important to monitor and understand changes in the character of sea ice, since it serves as a sensitive indicator of climate change while being an important physical control on important oceanic processes. Together with its concentration and thickness, the dynamic properties of sea ice are an important characteristic of the Arctic Ocean. Improving our knowledge of contemporary sea ice flow fields will improve our understanding of how climate variations (e.g., response to NAO) and progressive climate change will influ-
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Page 50 ence ocean circulation. Monitoring sea ice concentrations and thickness, together with flow fields out of the Arctic Ocean will provide important estimates of ice mass—and hence freshwater—transport from the Arctic Ocean to the Atlantic Ocean. There are similar and equally important issues to be addressed in the Southern Ocean. Measurement Requirements: Sea ice thickness, concentration, and motion. 1.3c Are significant changes occurring in ocean productivity? The physics and nutrient chemistry of the high latitude oceans define patterns of primary productivity and open-water CO2 exchange. Ice edge productivity is a primary source of productivity in the polar oceans that is particularly sensitive to warming. Projected reductions in sea ice concentration, enhanced light availability, and increased water temperatures can increase algal productivity. At the same time a possible reduction in upwelling associated with a weakening of the ocean conveyor belt could reduce nutrient inputs from deep ocean and thereby limit production and uptake of carbon. Linkages from the Arctic land mass to the Arctic Ocean are also important because Arctic coastal ecosystems are highly dependent on the sediments and nutrients delivered from land masses. These issues are important not only to Earth system analysis but to society as well since the high-latitude oceans provide important fisheries resources that depend directly on primary productivity. Measurement Requirements: 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 and thaw changing? The seasonal freezing and thawing of the Arctic land surface is a key trigger for virtually all the major land-based hydrological, biophysical, and biogeochemical processes. Seasonal snow pack represents both potential recharge to soils and groundwater, and delayed runoff, and, at larger scales, discharge to the Arctic Ocean. The phenology of higher plants is keyed to active layer dynamics. Plant growth, evapotranspiration, and exchange of carbon com-
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Page 51 mence during the thaw period. Trace gases are also liberated by microbial processes activated by temperatures above freezing. The distribution of permafrost has recently been digitized by piecing together former paper map products having contrasting spatial resolutions and classification schemes, and without quantitative information on active layer dynamics. Modeling experiments under greenhouse warming show an important redistribution of permafrost, suggesting that a major improvement in the state of the art describing permafrost is urgently needed. Measurement Requirements: Permafrost extent, timing of freeze and thaw, vertical temperature profile, and thermokarst topography. 1.4b Is the hydrology of Arctic terrestrial regions changing? Drainage basins provide a useful organizing framework for tracking water, energy, and biogeochemical fluxes associated with the terrestrial hydrological cycle. When the Arctic is considered from the standpoint of a contributing drainage area to the Arctic Ocean, it constitutes the most land-dominated of all ocean basins with the greatest impact from freshwater discharge. Some Arctic drainage basins extend well south of 50° N, meaning that it constitutes an enormously complex landscape composed of many non-tundra, non-boreal forested ecosystems, including temperate grass and agricultural lands and deciduous forest. The dynamics of the local water cycle define the quantities and timing of the runoff, that moves water horizontally into sequentially larger channels and ultimately to the Arctic Ocean. Water is delayed or diverted with passage through floodplains and through ice dams on rivers. Seasonal ponding of water and resulting sheet flows are important in low-relief areas in many parts of the Arctic. The timing of ice-out in rivers and lakes also affects terrestrial ecosystems by affecting aquatic biology, biogeochemistry and water fluxes through drainage basins aquatic biology, biogeochemistry, and water fluxes through drainage basins. Measurement Requirements: Precipitation, temperature, surface radiation parameters (roughness, albedo), winds, humidity, permafrost state, land cover, run-off, and river discharge.
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Page 52 1.4c Are significant changes occurring in the distribution and productivity of high-latitude vegetation? Changes in high-latitude vegetation may influence climate through effects of vegetation structure and function on surface energy balance and on trace gas concentrations in the atmosphere. High-latitude vegetation may change in response to changes in climate, climate variability, and disturbance regimes. Disturbance is an important agent of change because it can cause abrupt changes in vegetation structure and function. Major disturbance types in high latitudes include insect infestations and fire. It is important to monitor the characteristics of these disturbance regimes for information on the type, the timing, the extent, and the severity of the alteration. In the context of this study, pollution of the terrestrial landscape encompasses atmospheric deposition, which can supply nutrients that are potentially limiting to plant productivity (such as inorganic nitrogen) or chemicals that are stressful to the resident vegetation. Estimation of nitrogen deposition requires a mapping of source terms (e.g., urban and industrial areas) and their variation over time. Measurement Requirements: Vegetation characteristics (leaf area index, canopy density, albedo, structural composition, vegetation class), disturbance characteristics (type, timing, severity), wetlands extent, and nitrogen deposition. 2 Issue No. 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? The exchange of carbon and trace gases from polar ecosystems is regulated by a host of complex processes. On land the exchange of these gases is regulated by the distribution of soil wetness and oxygenation state, vegetation characteristics, disturbance characteristics, carbon substrates in soils, pH, and temperature. Productivity is also regulated by the internal cycling of nutrients and by the delivery of exogenous inorganic nitrogen from atmospheric deposition. Hence the distributions of permafrost, upland and wetland ecosystems, and atmospheric circulation patterns are important features of the Arctic system that must be defined before large-scale assessments can be made. Greenhouse warming and the concomi-
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Page 53 tant changes in the physics of the atmosphere are linked to gas exchange. Carbon balances in the oceans are regulated by the physics of the oceans and nutrient supplies from upwelling and riverborne inputs. A presumed weakening of upwelling associated with a warming Earth could reduce nutrient-dependent CO2 uptake and oceanic carbon sequestration, but could also reduce the capacity of the ocean to outgas CO2 from rising deepwater. Reduced extent of sea ice could also lengthen the growing season and expose more of the Arctic Ocean to wind-driven circulation. Changes in ice edge productivity determined by the interplay between altered upwelling and stabilization by increased meltwater constitutes an important process requiring further study. The implications for storage of CO2 by the high-latitude oceans is a critical global change question. Measurement Requirements: 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. 2.2 What are the spatial and temporal distributions and variability of aerosols in the polar atmosphere? Aerosol radiative effects are important primarily in the spring and summer, when they may influence melt onset and cloud phase changes. Changes in the characteristics of aerosols in the polar atmosphere could change how solar energy is distributed in the system. Since the ice-covered surfaces have very large albedos, most aerosols increase solar absorption in the atmosphere at the expense of the (melting) surface, but once melt begins these same aerosols reflect more sunlight than the surface and inhibit surface heating. Moreover, changes in aerosol composition could affect the seasonal change-over of clouds from ice to liquid phase, which may also alter the surface solar heating. Measurement Requirements: Aerosol concentration, size distribution, vertical distribution, and composition (index of refraction).
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Page 54 3 Issue No. 3: Responses to Forcing and Associated Feedback 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? Determining ice sheet mass balance requires knowledge of both input (i.e., precipitation) and output (i.e., sublimation, ice flow, melt discharge) fluxes. These in turn will be determined by changes in atmospheric circulation and surface energy characteristics. Global climate change is predicted to show large relative changes in these characteristics across the polar regions. Projected increases in winter precipitation in high latitudes may or may not be balanced against increases in evaporation associated with elevated air temperatures, thus making the corresponding changes in ice mass difficult to predict. Whether the ice sheets are responding primarily to recent forcings or to longer-timescale forcings (i.e., are the ice sheets in balance with the recent climate) remains an open question. Measurement Requirements: Precipitation and accumulation history, surface heat fluxes, surface temperature, inversion strength, snowpack structure, ice sheet elevation, ice sheet discharge, and run off. 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? The Arctic Ocean is a pathway through which the relatively fresh waters of the North Pacific flow toward the saltier North Atlantic. Along the way these waters pick up an additional freshwater component from river runoff and precipitation, and some of this is converted into sea ice. The outflow of these fresh waters into the North Atlantic may strongly affect the formation of North Atlantic deep water which, in part, drives the global oceanic circulation. Evidence suggests that this mechanism undergoes substantial interannual and decadal variability that may have significant effects on worldwide climate. This variability of polar ocean circulation has
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Page 61 Although the long-term effects of the disappearance of permafrost may reduce construction problems in high latitudes, the thawing of permafrost may damage buildings built on permafrost soils, may damage roads and increase road maintenance costs, and may lead to soil instability, which may increase erosion and landslides, thereby affecting man-made structures. To determine variability, all of the data sets should be in time-series. Measurement Requirements: Permafrost extent, thermokarst topography, soil temperature profile, land-surface temperature, snow characteristics, soil moisture, vegetation characteristics, disturbance characteristics, wetlands extent. 4.2 How will changes in coastal sea ice-coverage and sea-level rise affect storm surges, coastal erosion, and inundation of the coastal freshwater supply? The coastal zones in and outside the polar regions are highly sensitive to global change, both from the direct human modification of the coastal environment and from changes in sea level. Net changes in sea level have a complex geography, determined by the competing factors of thermal expansion of the global oceans and such local conditions as glacial rebound. Coastal responses to such changes are highly dynamic, as evidenced by the landward migration and net loss of deltas under sea-level rise or reduced river sediment delivery. Coastal systems are adapted to natural variability, but a key question surrounds their resiliency in the face of rapid change—in particular, contemporary sea level rise—which makes them susceptible to increased storm damage. Coastal ecosystems, which provide important support to both commercial and indigenous fisheries species, are therefore placed in jeopardy. Coastal habitation is also at risk from increased storm flooding and damage, especially if the ice-free season lengthens, and the incursion of the sea jeopardizes ground and surface freshwater supplies. On the other hand, if the predicted decrease in meridional temperature gradients were to occur in a warming climate, high latitude storminess may decrease. Measurement Requirements: Sea level height, sea ice concentration, coastal erosion rates, and coastal topography.
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Page 62 4.3 What changes will occur in water supplies from snow and snow fed rivers as the climate changes? The seasonal storages of snow are important to energy exchanges with the overlying atmosphere and in many parts of the Arctic constitute the major source of water discharged through river channels to the Arctic Ocean. Although many models of future climate change project an increase of precipitation at high latitudes, several global-warming scenarios indicate a shorter season of snowfall, diminished snowpack, and a movement of snowlines northward. Such changes would have marked impacts on soil moisture recharge and the distribution of streamflow, and hence are important to both terrestrial and aquatic ecosystems. Measurement Requirements: Precipitation, snow cover, ground water, and river runoff. 4.4 How will shipping, offshore mineral extraction, commercial fishing, and subsistence fishing and hunting be impacted by changes in coastal sea ice characteristics? Changing patterns of sea ice formation, transport, and disappearance have very clear consequences on the ability of society to exploit resources in the polar regions. Longer ice-free periods mean greater opportunity for shipping and mineral extraction. All other factors being equal, a reduction of sea ice can increase coastal primary production by reducing light limitation, with possible increase in fisheries production. Changes in the physics of the high-latitude coastal oceans can, however, create unforeseen changes in ecosystem dynamics and species migrations, with unknown consequences. Changes in the timing and extent of sea ice is also of obvious importance to traditional Arctic fishing and hunting on floating ice. Measurement Requirements: Sea ice concentration, ice thickness distribution, surface winds, and ocean productivity. 4.5 How will primary terrestrial productivity, vegetation, and higher organisms be affected by changes in the Arctic's physical environment? Patterns of terrestrial primary productivity in the Arctic are keyed closely to the availability of energy and water and hence are sensi-
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Page 63 tive to projected changes associated with global warming. As discussed earlier, changes in growing season and water availability will foster a dynamic redistribution of vegetation and the species dependent on those habitats. Because water and carbon balances differ greatly between contrasting vegetation communities, potentially great differences in net primary production and ecosystem sequestration of carbon might occur. Coastal and oceanic production is regulated by available light, temperature, salinity, and the stability of the photic layer. With potential changes in sea ice distribution, precipitation rates, and potentially higher discharge from melting ice mass, higher air temperatures, and changes in net primary production may also occur. Reductions in sea ice mean a longer period of primary production in newly opened waters, but may result in the disappearance of ice-dependent higher organisms. The influence of ozone depletion and arctic contaminants on primary producers (and hence higher trophic levels) in high-latitude land and ocean systems requires further attention. Measurement Requirements: Permafrost characteristics, vegetation characteristics, rainfall, snowfall, snow characteristics, disturbance characteristics, wet lands extent, soil moisture, land surface temperature, photosynthetically active radiation, evapotranspiration, vegetation characteristics, sea ice extent and thickness, ultra-violet radiation, and sea surface temperature, salinity, and color. 4.6 How will changes in growing season and primary production influence agriculture and forestry in high-latitude regions, including disturbed regimes (e.g., fire, insects)? The economic side of net primary production is represented by forestry and agriculture in service to society. Boreal forest ecosystems are expected to be highly sensitive to Arctic climate change, and large reductions in area are predicted even as these ecosystems expand into tundra zones. Threats to the boreal forests also arise from potential increases in fire frequency and severity and insect infestations associated with global warming. Because of the severe climate, there are relatively few crops grown in the polar regions. Herding of indigenous animals, fisheries, and hunting are important to native peoples, and these activities may be sensitive to green-
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Page 64 house warming in the Arctic through changes in net primary productivity. Measurement Requirements: Permafrost characteristics, vegetation characteristics, precipitation, snowpack characteristics, disturbance characteristics (e.g., fire frequency, insect population), wetlands extent, soil moisture, land surface temperature, photosynthetically active radiation, and evapotranspiration. 5 Issue No. 5: Predicting changes in the polar climate system and their global effects (Note: The required measurements and tools for all the subsidiary questions for the questions in sections 5.1-5.3 are addressed following section 5.3.) 5.1 To what extent can transient climate variations in the polar regions be understood and predicted? Recent cyclic events such as the Arctic Oscillation, Antarctic Oscillation, and North Atlantic Oscillation have been linked to important changes in atmospheric and ocean circulation patterns. These in turn have lead to changes in the delivery of precipitation and hence runoff over the land mass. It will be important to study these quasi-oscillatory climate phenomena to explore whether they are persistent features or more or less transient events associated with the internal dynamics of the climate system. Analysis of contrasting conditions represented in paleo-oceanographic records could lend important insight to this question. Promising recent work in analysis of ENSO signals for making predictions of hurricane number and severity, for example, might be expanded to predict Arctic cyclic phenomena as well. 5.2 Is there a need for more and/or new observations from the polar regions to support atmosphere and ocean and ice models, including numerical weather prediction models? In addition to providing important boundary and initial conditions to diagnostic models of the atmosphere, oceans, and sea ice, routine monitoring of geophysical variables has been used extensively to support operational products, including weather predictions. Sur-
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Page 65 face wind, temperature, humidity and pressure data from rawinsonde observational networks are routinely assimilated with land-based meteorological station data sets into National Center for Environmental Prediction and European Center for Medium-range Weather Forecasting operational forecasts. The assimilation products are imprecise. Comparison of long-term atmospheric convergence fields for moisture versus river runoff, for example, suggests that the rawinsonde network is inadequate over much of the Arctic. The capacity of satellite-based atmospheric sounders to augment the fixed network of rawinsondes, and routine assimilation of the numerous geophysical variables mentioned above, need a systematic assessment. 5.3 What 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? Our ability to forecast changes in the polar climate system and their ultimate connection to the larger global climate system requires a capability to interpret observational data sets and use them to guide the development of process-based models. Much of this discussion has centered around the collection of data sets to help monitor and improve our process-based understanding of the high latitude region. Current model formulations are rapidly increasing their level of sophistication and are being called upon to treat coupled land-atmosphere-ocean exchanges. Fully linked models are becoming available and are being used to analyze past, contemporary, and future conditions. Retrospective analysis, including paleoanalogues to future greenhouse warming conditions, is an important avenue of investigation. Treatment of transient climate conditions is also now recognized as important as we seek to understand changes in the character of climate response to warming (e.g., potentially higher intensity of precipitation events) and to assess its reaction to quasi-periodic events in a prognostic manner. In addition, a clear strategy is needed for gathering and distributing to the research community specific polar data sets to serve as model forcings and validation. Required Measurements and Tools for Addressing Questions in Section 5 The prediction-oriented questions posed in Section 5 require a broad set of modeling tools. Some of these will be exercised in isolation to better
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Page 66 understand particular polar sub-system processes; however, as the science associated with coupled Earth systems models develops, it is likely that these simulations will be linked and applied in an increasingly integrated way. Four major classes of models will be needed: numerical weather prediction models, sea ice and ocean models, ice sheet models, and land system models for hydrology and terrestrial ecosystem dynamics. These are described below together with their key data needs. These models first require data sets for initialization of their computed fields or specification of boundary conditions. For example, ice/ocean models require salinity distributions as an initial condition. Validation data, including the many variables listed in the foregoing discussion, are also required; such key data sets are repeated below for emphasis. Many of these (for instance, polar cloud properties or surface heat flux data) have virtually no polar coverage for model validation. A good example of the need for such data sets is demonstrated by satellite- and aircraft-based measurements, which are providing new constraints on ice sheet models, including regional ice flow measurements and the age-depth distribution from internal layers (isochrons) traced from well-dated ice core sites. By their very nature, simulations and models provide a large degree of synthesis and thus require input data on a comprehensive list of specific variables, generally the same variables described earlier in this chapter. Listed below are the four major model types and the types of measurements required to support either assimilation and initiation or validation. (A) Numerical Weather Prediction Models Assimilation and Initialization: Atmospheric profiles of temperature, humidity, winds, surface temperature, winds, and pressure, precipitation, sea ice concentration. Validation (additional parameters): Cloud properties, top-of-atmosphere radiation fluxes, surface heat flux components, diabatic heating. (B) Sea Ice and Ocean Models Assimilation and Initialization: Surface radiation fluxes; surface air temperature; humidity; winds and pressure; precipitation; ice concentration, thickness, and velocity; ocean temperature; salinity; and currents. Validation (additional parameters): Ice and ocean interfacial fluxes, surface temperature and albedo.
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Page 67 (C) Ice Sheet Models Assimilation and Initialization: Ice sheet elevation and thickness, basal conditions accumulation, ice temperature distribution, melt estimates (surface radiation flux components, surface temperature, albedo, surface air temperature, humidity and winds). Validation (additional parameters): Ice sheet velocity, runoff or discharge, internal layer distribution. (D) Land System Models for Hydrology and Terrestrial Ecosystem Dynamics Assimilation and Initialization: Surface elevation and topography, precipitation, surface radiation flux components, surface temperature and albedo, surface air temperature, humidity and winds, vegetation characteristics, river and lake morphology, soil moisture, permafrost extent and temperature profiles, snow cover. Validation (additional parameters): Surface temperature and albedo, river runoff. SUMMARY This chapter began with a broad set of scientific questions that reflect our incomplete understanding of the roles that polar systems play in the Earth system and in global change. For clarity the committee further divided the discussion into a set of more disciplinary issues, specifically around the issues of ice sheets, sea ice, atmospheric dynamics, land surface hydrology, and ecosystems. The committee, of course, fully recognizes the importance of integrative and cross-disciplinary study. The first major science-driving question and its subsidiary research issues were devoted to the detection of coherent signals of change, either natural or anthropogenic. The committee found that there are significant difficulties in establishing the contemporary spatial and temporal distribution of change and in quantifying such basic biogeophysical variables as precipitation, ice sheet mass, sea ice concentration, land-surface hydrological variables, permafrost, and vegetation distributions. The committee also concluded that our understanding and capability to detect variability and progressive changes to polar systems remains inadequate. The high-latitude oceans and land mass exert an important influence on the Earth system, and in the second part of this chapter the focus was on how fluxes of CO2 and trace gases in polar regions affect global atmo-
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Page 68 spheric composition and how aerosols in high latitudes affect atmospheric energy budgets. The third part of the chapter addressed feedback of polar systems in response to global change and thus included a complex array of processes that influence ice sheet atmosphere, sea ice and ocean-atmosphere, land and vegetation atmosphere, land-ocean linkages, and couplings between the polar and non-polar atmosphere. The complexity of these issues means that numerous biogeophysical variables must be observed simultaneously before the questions can adequately be addressed. The fourth part of this chapter treated the consequences of change in the polar regions, with an emphasis on human systems. Many traditional and industrial activities are located in polar regions and several of these are intimately tied to the state of the natural system and thus are likely to experience the most direct impact. Many of the geophysical variables monitored for Earth System studies also can be used in the assessment of societal impacts. The final portion of the chapter dealt with the ability to forecast polar change and its effects on the global system. This is fundamentally a modeling and synthesis exercise that will rely on knowledge from observational programs as well as process-based studies of individual components of the polar system. Four classes of models are crucial for progressing in polar system science: ice sheet models, sea and ice ocean models, operational weather prediction models, and terrestrial hydrology and ecosystem dynamics models. The complexities of feedbacks within and among these realms requires a major synthesis effort. The linkage of these models through an integrated initiative would constitute a major step forward in our understanding of the polar regions and their broader role within the Earth system (as was depicted in Chapter 1, Figure 1-1). Reliable and coherent geophysical data sets are critical to the calibration and validation of such models and are therefore essential to continued scientific progress. Table 3-1 summarizes the measurements required to address the science questions described in this chapter. We have attempted to construct a matrix that relates the specific measurement variables to the specific science questions. In the variable/parameter column, all variables listed in sections 3.1-3.5 (in association with the science questions) are listed. Columns 1-5 relate directly to the science questions 1-5, as discussed in sections 3.1-3.5. The specific numbers in columns 1-5 correspond to specific science questions in section 3. For example, a 9 in column 3 corresponds to question 3.3.9. In column 5, the letters correspond to the parenthetical letters assigned to specific models where they were described in the text. That is, (A) in column 5 corresponds to numerical weather prediction models; (B) indicates sea ice and ocean models; (C) indicates ice sheet
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Page 69 TABLE 3-1 Measurements Required to Address the ESE Science Questions a ESE POLAR SCIENCE QUESTION 1 2 3 4 5 VARIABLE OR PARAMETER Monitoring External Forcing Feedback Impact Prediction Top of atmosphere radiation flux 1 7 A Atmospheric profiles -Temperature 1 3,6,7 A -Humidity 1 3,6,7 A -Winds 1 2,7 A Cloud properties -Cloud cover 1 3,6,7 A -Ice/liquid content 1 3,6,7 A -Particle eff. radius 1 3,6,7 Aerosol properties -Concentration 1 2 3 -Size distribution 1 2 3 -Refractive index 1 2 3 Surface temperature 1,2,4 1,2,3,4,5,6 5 A,B,C,D Surface albedo 1 3,4 B,C,D Surface pressure Surface heat flux -Radiation flux 1,2,4 1,7 B,C,D -Sfc air temp 2,4 1,6 B,C,D -Sfc winds 2,4 1,5,6 4 B,C,D -Sfc air humidity 2,4 1,5,6 B,C,D Precipitation -Rainfall 1,2,3,4 7,8,9 5 A,B,D -Snowfall 1,2,3,4 1,4,7,8,9 5 A,B,C,D CO2 Flux 3 1 CH4 Flux 3 1 Sfc UV radiation 5 Permafrost -Extent 4 8,9 1,5,6 D -Freeze/thaw dates 4 8,9 1,5,6 -Temperature profile 4 8,9 1,5,6 D -Thermokarst topog 4 8,9 1,5,6 Land snow characteristics -Cover 4,8,9 3,5 D -Depth 8,9 3,5 -Water Equivalent 8,9 3,5 continued
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Page 70 TABLE 3-1 continued ESE POLAR SCIENCE QUESTION 1 2 3 4 5 VARIABLE OR PARAMETER Monitoring External Forcing Feedback Impact Prediction Vegetation characteristics -Leaf area index 4 1 8,9 1,5 D -Canopy density 4 1 8,9 1,5 D -Albedo 4 1 8,9 1 D -Structural composition 4 1 8,9 1,5 D -Land-cover type 4 1 8,9 1 D Evapotranspiration 8,9 1,5 Photosynthetically active Radiation (PAR) 5 Coastal morphology 2 Surface topography D River, lake morphology D Wetlands extent 4 1,5 Sfc elevation, topog Soil moisture 9 1,5 4 N deposition & contaminants 4 Disturbance characteristics (land) -Type 4 8,9 1,5 -Timing 4 8,9 1,5 -Extent 4 8,9 1,5 -Severity 4 8,9 1,5 River runoff 3 1 9 3 Ice sheet -Elevation 2 1 C -Thickness 2 C -Velocity 2 C -Discharge 2 1,2 C -Runoff 2 1,2 C Sea ice -Concentration 3 1 2,3,4,5 2,4,5 A,B -Thickness 3 2,3 4,5 B -Velocity 3 B -Snow cover 3,5 B -Melt pond cover 3 Ocean -Temperature, salinity 3 2,6 -Currents 3 2 -Surface height 2 -Color (productivity) 3 4,5 -CO2 flux continued
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Page 71 TABLE 3-1 continued NOTE: eff = effective; sfc = surface; TOA = top of atmosphere. a This table is a matrix that relates the specific measurement variables to the specific science questions. In the variable/parameter column, all variables listed in sections 3.1-3.5 (in association with the science questions) are listed. Columns 1-5 relate directly to the science questions 1-5, as discussed in sections 3.1-3.5. The specific numbers in columns 1-5 correspond to specific science questions in section 3. For example, a 9 in column #3 corresponds to question 3.3.9. In column 5, the letters correspond to the parenthetical letters assigned to specific models where they were described in the text. For example, A in column 5 corresponds to numerical weather prediction models. Examination of the number of times that a specific variable is cited under the various science questions provides a crude measure of the priority of need for measuring that variable. models; and (D) indicates land system models for hydrology and terrestrial dynamics. Examination of the number of times that a specific variable is cited under the various science questions provides a crude measure of the priority of need for measuring that variable. For instance, column one addresses the polar variation of ESE science question #1, “How are polar climate and the biosphere changing?” (which is fundamentally a monitoring question). It identifies atmospheric profiles (temperature, humidity, and winds) and a variety of other measurements as being most relevant to this science question. Chapter 4 looks further at the most frequently cited variables and parameters in this table, such as atmospheric profiles (including temperature, humidity, and winds), cloud properties (including cover, ice/liquid content, and particle radius), etc. The committee recognizes that this is a subjective judgement but believes the logic is sound and provides some indication of the kinds of measurements most needed to answer the ESE questions.
Representative terms from entire chapter: