9
Climate Variability and Change

OVERVIEW

If current climate projections are correct, climate change and variability over the next 10–20 years will have highly noticeable effects on society. Climate projections indicate important changes in the intensity, distribution, and frequency of severe weather; a decrease in sea ice leading to open ocean passageways in the Arctic; continued reduction of mountain glaciers; and continued trends toward record warmth (IPCC, 2001). The related effects on agriculture, water resources, human health, and ecosystems are likely to drive a public demand for climate knowledge that will require substantial changes in climate research. The magnitude and rate of the projected changes, combined with the growth in infrastructure, are expected to increase climate-related risks. Research will focus on the predictive capability over the season, decade, and century timescales that are necessary to protect life and property, promote economic vitality, enable environmental stewardship, and to help assess a broad array of policy options for decision makers. Any vision of the future of Earth observations from space must anticipate an evolving climate. As the economic impact of climate change grows, there will be both a change in research emphasis and a demand for renewed investment in climate research. Observation systems of the future must be designed with the following in mind:

  • Sustained multidecadal, global measurements of all quantities key to understanding the state of the climate and the changes taking place within it are crucial, as is adequate data management.

  • Climate change research, including the observational system, will increasingly be tied directly to understanding of the processes and interactions needed to improve predictive capabilities and resolve the probabilities associated with different outcomes.

  • Evaluation and assessment of model capability will increasingly be the focus of measurement activities; demonstrating model capability is likely to drive development and evolution of observation systems and field campaigns.

  • Higher-spatial-resolution observations, predictions, and assessments are needed to better establish the link between climate research and societal benefits.



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9 Climate Variability and Change OVERVIEW If current climate projections are correct, climate change and variability over the next 10–20 years will have highly noticeable effects on society. Climate projections indicate important changes in the intensity, distribution, and frequency of severe weather; a decrease in sea ice leading to open ocean passageways in the Arctic; continued reduction of mountain glaciers; and continued trends toward record warmth (IPCC, 2001). The related effects on agriculture, water resources, human health, and ecosystems are likely to drive a public demand for climate knowledge that will require substantial changes in climate research. The magnitude and rate of the projected changes, combined with the growth in infrastructure, are expected to increase climate-related risks. Research will focus on the predictive capability over the season, decade, and century timescales that are necessary to protect life and property, promote economic vitality, enable environmental stewardship, and to help assess a broad array of policy options for decision makers. Any vision of the future of Earth observations from space must anticipate an evolving climate. As the economic impact of climate change grows, there will be both a change in research emphasis and a demand for renewed investment in climate research. Observation systems of the future must be designed with the following in mind: Sustained multidecadal, global measurements of all quantities key to understanding the state of the climate and the changes taking place within it are crucial, as is adequate data management. Climate change research, including the observational system, will increasingly be tied directly to understanding of the processes and interactions needed to improve predictive capabilities and resolve the probabilities associated with different outcomes. Evaluation and assessment of model capability will increasingly be the focus of measurement activities; demonstrating model capability is likely to drive development and evolution of observation systems and field campaigns. Higher-spatial-resolution observations, predictions, and assessments are needed to better establish the link between climate research and societal benefits.

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The “family” of climate observing and forecasting products will continue to grow and will involve innovative research into societal connections with energy, agriculture, water, human health, world economies, and a host of other subjects, creating new public and private partnerships. The demand to understand the connection between climate and specific effects on natural and human systems will require a more comprehensive approach to environmental observation and modeling in order to integrate the multiple stresses that influence human and natural systems (climate, land use, and other human stressors, such as pollutants). Those six points are based on the remarkably consistent set of evaluations of climate-change and global-change research over the last 2 decades. The call for stable, accurate, long-term measurements of climate variables is nearly universal regardless of whether the reviews were focused on the adequacy of climate observations (NRC, 1999a), on strategies for Earth science from space (NRC, 1985), on integration of research and operations (NRC, 2000b, 2003b), on improving the effectiveness of climate modeling (NRC, 2001 c), on enabling societal use of information (NRC, 1999c,d, 2000a; National Assessment Synthesis Team, 2000), or on providing an overview of the future direction of global-change research (NRC, 1998a, 1999b, 2001a). Equally evident in those assessments are the lack of a suitable sustained climate-observing system and the effect of this gap in limiting progress in all aspects of climate research and applications. The most frequently cited reasons for the failure to develop a climate-observing system are the pressure to produce short-term products that are suitable for addressing severe weather, the difficulty of maintaining a commitment to monitoring slowly changing variables, the lack of clear federal stewards with a defined climate mandate, and the disconnect between operational and research needs. The difficulty of maintaining critical climate observations has recently been demonstrated by the loss of key climate-monitoring elements on the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The importance of tying observational systems more directly to the improvement of predictive capabilities and to understanding uncertainties is equally well articulated in research strategies focusing on key climate feedbacks and improved estimates of climate sensitivity (NRC, 2001a, 2003a,c) and the key components of seasonal to interannual variability (NRC, 1994, 1998a). Those strategies advocate a vigorous comparison of climate models and observations and a focus on specific observations that test how well climate simulations incorporate feedback processes and elucidate aspects of spatial and temporal variability. Greater effort is needed to resolve the interactions at the atmosphere’s boundaries (oceans, ice, and land surface and vegetation), enable an improved understanding of clouds and cloud feedbacks, and characterize the role of aerosols. The growing emphasis on regional and higher-spatial-resolution predictions, on expansion of the family of forecasting products, and on the role of multiple stresses in environmental-impact research is directly linked to the goal of realizing the full potential of climate research to benefit society. The value of climate information to society depends on knowledge of the nature and strength of the linkages between climate and human endeavors, on improved understanding of the uncertainties associated with forecasts or predictions, on the accessibility of credible information, on knowledge of societal needs, and on the ability of users to respond to information (NRC, 1999c, 2001b,d). Such research is in its infancy, but the demand for it will grow substantially. The potential societal benefits are large. Even modest improvement in seasonal to interannual predictions has the potential for important societal benefits in agriculture, energy, and management of weather-related risk (NRC, 1994, 1998a). The ability to characterize or reduce uncertainties in climate change prediction is a critical element in supporting energy and conservation policy related to global warming (NRC, 2001a). The ability to assess potential climate effects, and then to define adaptation and mitigation strategies, depends both on improving the effectiveness of climate modeling (NRC, 1998b, 2001c) and on

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FIGURE 9.1 Sea ice minimums calculated using a 3-year moving average for the period 1979–1981 (left) and 2003–2005 (right). Satellites have made continual observations of Arctic sea ice extent since 1978, recording a general decline throughout that period. Since 2002, satellite records have revealed unusually early onsets of springtime melting in the areas north of Alaska and Siberia. In addition, the 2004–2005 winter season showed a smaller recovery of sea ice extent than any previous winter in the satellite record, and the earliest onset of melt throughout the Arctic SOURCE: NASA (2005). Courtesy of NASA. implementing more comprehensive approaches to environmental study (NRC, 1999d). The first recommendation of the U.S. National Assessment of Climate Change Impacts (National Assessment Synthesis Team, 2000) calls for a more integrated approach to examining the impacts and vulnerabilities associated with multiple stresses (Figure 9.1). Several effects and vulnerabilities are particularly noteworthy. Changes in the volume of water stored on land as ice and snow are of critical importance to coastal populations and infrastructure because of the effects on sea level. Water resource management is strongly tied to climate and weather, and adaptation strategies are expensive and often require decades to implement. Climate change research has considerable potential to improve the anticipation of adverse health outcomes specifically related to heat mortality, changes in the pattern and character of vector-borne diseases, and air quality. Finally, climate change research is a major factor in improving the ability to be better stewards of natural ecosystems. This vision recognizes that the demand for knowledge of climate change and variability will intensify. The objective is to improve the ability to anticipate the future and thus increase the capability to use the knowledge to limit adverse outcomes and maximize benefits to society. Failure to obtain that knowledge carries high risks. OBSERVATIONAL NEEDS AND REQUIREMENTS The Panel on Climate Variability and Change focused on four fundamental questions in its approach to specific space-based and supporting in situ and surface-based observations required for studies of Earth’s climate: What governs Earth’s climate? What forces climate change? What feedbacks affect climate vari-

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ability and change? How is the climate changing? The coming decade will see a challenge to predict better how Earth will respond to changes in atmospheric composition and other forcings. Observations must document the forces acting on the climate system (including solar and volcanic activity, greenhouse gases and aerosols, and changes in land surface and albedo); the characteristics of internal variability that can obscure forced changes and that may evolve in response to climate change; the feedback processes that involve the atmosphere, land, and ocean; biogeochemical cycles and the hydrologic cycle; and climate change itself. Stripped to fundamentals, the climate is first affected by the long-term balance between sunlight absorbed and infrared radiation emitted by Earth. Thus, key elements to observe are incident sunlight, absorbed sunlight, and emitted infrared radiation. Achieving an understanding of how the system works requires the characterization of the influences affecting the absorbed sunlight and the emitted radiation. The influences include the composition of the atmosphere (such as greenhouse gases and aerosols), the state of the surface (whether snow- or ice-covered and whether vegetated or desert), and the effects of the various atmospheric components and the surface state on radiation loss to space. In addition, physical and chemical processes in the system feed back to affect the composition of the atmosphere and the surface state, such as the processes that affect water vapor and clouds. Other processes and conditions—such as the extent of permafrost, subsurface concentrations of phytoplankton, and the ocean’s thermohaline circulation—are hidden from direct space view. Inferences must be drawn not only from records of space-based observations but also from in situ and remotely sensed observations from surface-based, balloon, and suborbital platforms. In its consideration of the specific observations to be made and the challenges and opportunities presented by the changes anticipated in the coming decade, the Climate Change and Variability Panel adopted the list of essential climate variables in the 2003 Global Climate Observing System report (GCOS, 2003). The panel then assessed the current observing capabilities and those planned for the coming decade, mostly those from NPOESS. Table 9.A.1 in the attachment at the end of the chapter lists the status of space observations, and in some cases supporting surface-based observations, of critical climate variables. Although the table provides a valuable perspective, its limitations should also be recognized: (1) in some cases, it lists variables that can be obtained through several techniques, but not all techniques are listed; (2) it is limited to satellite observations that are in low Earth orbit, although a number of the objectives listed can also be achieved through retrievals with multispectral imagery and sounder data from platforms in geostationary and other orbits; and (3) few space-based observations can be taken as physical measurements in their own right, and interpretations are often revised as more comparisons are made between inferences based on space-based observations and alternative measures of the physical variables. The evolution of knowledge will require the oversight of scientists and continuous evaluation by the climate research community as space-based observations are transformed into the high-quality long-term records that will be invaluable for climate studies and societal benefit. The stratosphere plays a unique role in climate forcing and responds in unique ways to global warming, greenhouse gases, solar ultraviolet, and volcanic aerosols (Figure 9.2). In many cases, observed changes are challenging to explain (e.g., Santer et al., 2003; Eyring et al., 2005). As with other variables critical for climate change, consideration of changes in the stratosphere requires long-term climate data records. Current Status and Needed Improvements The following description of current observations and needed improvements is based on three basic requirements: (1) multidecadal records of primary climate variables, (2) observations dedicated to inferring

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FIGURE 9.2 Marine stratocumulus over the Arabian Sea imbedded in a plume of haze from the Asian subcontinent. The picture was taken from the NCAR C-130 during an Indian Ocean Experiment (INDOEX) research flight. Haze affects the number and sizes of cloud droplets and ice crystals and thereby alters the amount of sunlight that clouds reflect. The effect of haze on clouds is referred to as the aerosol indirect radiative forcing of climate and is among the largest uncertainties that hampers assessments of climate change due to humans. Photo courtesy of Antony Clarke, University of Hawaii. key processes that affect climate variability and change, and (3) opportunities for scientific exploration, innovation, and discovery. Multidecadal Records Increased scientific understanding and improved analysis depend heavily on the length and quality of the observational record—a key need is for space-based and surface-based observations that span many decades. Current Status of Multidecadal Records The current scientific strategy for generating and sustaining long-term records by using space-based and ground-based instrumentation of finite lifetimes is to achieve overlap of successive generations of observing systems. Global records of a few decades have been constructed from space-based observations of such variables as sea-surface temperature, sea ice, atmospheric layer temperatures, Earth’s energy budget, and cloud properties. Overlapping observations are crucial for identifying and reducing calibration uncertainties in current instruments that would otherwise exceed the geophysical changes of interest.

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The plans summarized in Table 9.A.1 show a heavy reliance on NPOESS for observations of key climate variables during the coming decades. The June 2006 descoping of the NPOESS program fails to satisfy the basic needs of climate science for several reasons. First, instruments essential for climate science have been deleted from the NPOESS program. The following cancellations are of great significance to the climate sciences: CMIS (conical-scanning microwave imager sensor). As currently proposed, the first NPOESS platform (C-1) will not embark a microwave imager-sounder. A solicitation for a “replacement” microwave imager-sounder is proposed for C-2 and beyond. APS (aerosol polarimetry sensor). Aerosol observations for the medium term will rely on the Glory research mission, with a launch anticipated for about 2008. TSIS (total solar irradiance sensor). OMPS-Limb (ozone mapping and profiler suite). With the deletion of the OMPS limb sounder, no monitoring of ozone profile below the ozone peak (where most ozone depletion occurs) is planned during the NPOESS/MetOp period. OMPS-Nadir is still included in the PM orbit. ERBS (Earth radiation-budget sensor). The CERES instrument will fly on C-1, with no plan provided after C-1. ALT (radar altimeter). No clear replacement plan is available other than proposed reliance on a future Navy mission for altimetry. There remains the option of deploying further DMSPs (one already built and in storage) in the middle-AM instead of PM orbit. CrIS/ATMS and OMPS-Nadir will be flown only on the PM orbits, although VIIRS will be flown on all NPOESS platforms. The NPOESS bus will have the capability (power and physical space) for the sensors listed above, and the Integrated Program Office (IPO) will plan for and fund integration of the sensors on the spacecraft in the NPOESS program, but only if the instruments are provided from outside NPOESS (by some other agency or partner). The impact of the proposed changes is significant. As originally proposed, NPOESS lacked the capabilities of the EOS-era satellite systems: VIIRS was missing important water vapor and temperature sounding channels, the capability of CMIS to provide useful passive winds was uncertain, OMPS had much lower horizontal resolution than Aura’s ozone-monitoring instrument (OMI) and lacked limb sounding, and ALT’s orbit made removal of tides a challenge. Compared with the EOS provision of climate data in the current decade, the original NPOESS plan provided a weak set of observations for understanding climate change in 2010 and beyond. The 2006 descoped NPOESS plan will provide a still weaker set of climate observations with substantial gaps in key variables. Second, the 2006 proposed dates for NPOESS launches indicate delays of several years in data provision: The NPOESS Preparatory Project (NPP) proposed launch date is delayed from 2006 to 2009. The proposed launch dates for NPOESS are 2013 for C-1 (in PM orbit), 2016 for C-2 (in AM orbit), 2020 for C-3 (in PM orbit), and 2022 for C-4 (in AM orbit). There will be no NPOESS platforms in middle-AM orbit; the middle-AM orbit will be covered by MetOp. The delays of the NPP and NPOESS will be felt immediately if overlap with the EOS Aqua and Aura platforms is lost. For example, the cancellation of CMIS and the delay of any microwave imager until 2016

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would create a gap in the record of sea ice concentration and extent, which extends from 1978 to the present. Sea ice is one of the best-documented and most rapidly changing elements in the climate system. In the Arctic, the lowest sea ice extent on record occurred in 2005, and September sea ice extent has been declining by about 8.5 percent per decade. This critical climate record requires continuation. In addition to sea-ice mapping, passive microwave sensors are used to map the onset and extent of melt on the Greenland ice sheet, a key to assessing climate change and the contributions of ice-sheet melt to rising sea level. In view of the fundamental role played by accurate long-term Earth radiation budget measurements and in view of the growing gap expected between the CERES observations on Aqua and the ERBS observations now scheduled for C-1 (proposed date, 2013), there will be a major gap in some of the most fundamental measurements of the climate system. Finally, regardless of the descoping, the NPOESS program lacks essential features of a well-designed climate-observing system: NPOESS lacks a transparent program for monitoring sensor calibration and performance and for verifying the products of analysis algorithms. Moreover, it lacks the direct involvement of scientists who have heretofore played a fundamental role in developing climate-quality records from space-borne observations. NOAA has initiated plans for scientific-data stewardship (NRC, 2004), but the plans are in their infancy, and NOAA’s commitment to ensuring high-quality climate records remains untested and inadequately funded (NRC, 2005c). NPOESS does not ensure the overlap that is required to preserve climate data records (CDRs). Instead, the NPOESS system is designed for launch on failure of a few key sensors. Failure of NPOESS instruments required for CDRs will probably result in gaps of many months, which will make it difficult to connect long-term climate records and future measurements. The NPOESS commitment to radiometric calibration is unclear, particularly for the VIIRS visible and near-infrared channels used to determine surface albedo, ocean color, cloud properties, and aerosol properties. VIIRS may be flown as the NOAA AVHRRs were flown, with only preflight calibrations, leaving the in-orbit calibrations of those channels to drift. Furthermore, in its current configuration, VIIRS lacks the channels now on MODIS in the 6.3-µm band of water vapor used to detect clouds in polar regions and in the 4.3- and 15-µm bands of CO2 used to obtain cloud heights, particularly heights of relatively thin cirrus. NPOESS only partly addresses the needed measurements of the stratosphere and upper troposphere. The primary variables of the stratosphere—temperature, ozone abundance, and some aerosol properties—will not be provided by NPOESS, because of the loss of OMPS-Limb, APS, and CrIS/ATMS. Other elements are poorly addressed by NPOESS plans, notably measurements of upper-troposphere and stratosphere water vapor, aerosols, and the abundance of ozone-depleting compounds. This decadal survey was intended to create a vision of the future of Earth observations from space. However, the panel believes that reliance on the operational NPOESS system as a foundation for climate observations in a decadal vision of the climate sciences has failed as a strategy. Needed Improvements and Products for Multidecadal Records The collection and maintenance of the long-term records so crucial for understanding of the climate system presents a number of challenges. Clear deficiencies in instrumentation and data analysis are evident in current plans, specifically in the transition to NPOESS. The needed improvements are in two categories: (1) actions required to address the loss of NPOESS measurements viewed as critical for climate research and (2) actions required to improve current and future observation strategies based on the lessons learned

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from space-based climate-data acquisition and use in past decades (such as MSU, ISCCP, the Global Aerosol Climatology Project). NASA and NOAA should develop an immediate plan to address the loss of continuity of critical climate measurements. The most important losses for climate include the microwave imager, Earth radiation-budget measurements, total solar irradiance, and stratospheric measurement capability. Considerable care is required to ensure successful stop-gaps and long-term plans. Several options should be considered: Every effort should be made to provide instruments from outside NPOESS (if Congress fails to act to restore NPOESS instruments) to take advantage of the plan to fund integration of these instruments into the NPOESS platforms. Every option should be considered (for example, substitute copies of existing instruments such as MODIS, AMSR-E, and SSM/I for the appropriate lost NPOESS measurements). Every effort should be made to extend the life of Terra, Aqua, Aura, SORCE, and Glory to ensure the longest possible data records and to minimize or eliminate critical data gaps. Much greater effort should be applied to improve current and future observation strategies based on lessons learned from past missions: Improved instrument calibration is required for long-term climate records. Because of the uncertain future of instrument calibration within NPOESS and the likelihood of important data gaps, the development of a space-borne calibration observatory to address accurate radiometry and reference frequencies is essential. For many variables, such as aerosol and cloud properties and water vapor concentrations, it is crucial to avoid orbital drift, which causes a substantial shift of several hours in the local time of the observations. The NPOESS satellites are designed to maintain their Sun-synchronous orbits, and this requirement should not be relaxed. Mission failures and delays can introduce gaps that compromise the detection and understanding of spatial-temporal variability in the climate system. Consequently, until such an understanding of the climate system is achieved and techniques for ensuring radiometric and timing accuracies have been shown to succeed, sequential observations of key climate variables should be overlapped for periods long enough to ensure useful comparisons. Reprocessing of critical data sets is required. Reprocessing of data allows the incorporation of gains in knowledge, the correction of errors in preflight and in-flight calibrations, inclusion of changes in instrument function, and the correction of errors in earlier processing algorithms. Validation of geophysical products inferred from satellite remote sensing is essential. In developing CDRs, validation should be an almost continuous component, providing an independent check on the performance of space-based sensors and processing algorithms. The Climate Variability and Change Panel believes that the current strategy, of ensuring overlap between measurements should be continued as recommended by GCOS (2003), CCSP (2003), and others (Ohring et al., 2005). For example, the different total-solar-irradiance instruments are tied to radiometric standards but produce measurements that depart from each other by amounts exceeding the claimed uncertainties. The panel recommends that substantial overlap be continued until reliance on absolute measurement standards has been shown to be successful. However, the long-term success of climate measurements cannot always depend on redundancy and therefore requires new approaches, with future instruments designed and built to maintain in-flight calibration to absolute radiometric standards. Temperature and humidity

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profiles derived from GPS occultations are likely to gain favor in long-term studies, so measurements of delay need to be tied to high-accuracy frequency standards as are now possible with the ultrastable oscillator flown on GRACE (Trenberth et al., 2006; Bengtsson et al., 2003). Ultimately, reliance on radiance and time measurements that are tied to absolute references will allow the climate record to tolerate gaps for some measurements, but the space-time variability of the climate during the gap in observations must be understood. Until such understanding is achieved and the reliance on calibrated radiances and accurate delays is demonstrated through comparisons of measurements by different instruments on different platforms, the need for overlap remains. Many of the calls for improvement in the satellite climate record—such as the need for radiometric calibration, launching to preserve continuity as opposed to waiting for instrument failure, and the need to validate space-based inferred products—are themes that run through many previous reports (CCSP, 2003; NRC, 2004; Ohring et al., 2005). Less common are calls that address the culture and infrastructure required to provide the kind of societal benefits that are possible from these satellite observations (see sections “Innovation and Discovery” and “Implications of the Requirements for Developing Climate Data from Satellite Observations” below in this chapter). Focused Process Studies Process studies focus on understanding the climate feedback process and are critical to improving climate models. They are generally intensive, short-duration, repeated campaigns with ground-based, airborne, satellite, and modeling components. They usually require frequent, diurnally resolved measurements and a wide variety of simultaneous products—a need typically at odds with the accuracy and stability essential for achieving reliable long-term records. Current Status of Process Studies Many climate system processes and many causes of climate variability and change are not fully understood or adequately validated with observations. The large range in climate model estimates of the change in the global surface temperature in response to a doubling of CO2 illustrates how choices in treating these processes—which vary greatly from model to model—can have sizable consequences. Reliable climate simulations require improved treatment of the processes known to be inadequate (NRC, 2003c, 2005b): clouds, aerosols, and convective systems; biosphere-atmosphere interactions; coupling of sea ice, ocean circulation, and icemelt; ice-sheet dynamics; the fluxes of heat, momentum, water, and trace species across the interfaces of ocean-atmosphere, land-atmosphere, ice-atmosphere, boundary layer and free troposphere, troposphere-stratosphere, and ice-ocean; and internal variability, such as the ENSO. Needed Improvements and Products for Process Studies There should be a more deliberate effort to focus resources on the most critical weaknesses in predictive models, specifically, the six topics listed above. Networks of surface sites should be distributed to sample the widest possible process over the globe and designed to provide long-term observations of clouds, aerosols, and their effects on surface radiative fluxes; fluxes of sensible heat, evaporation, and evapotranspiration; and concentrations of key trace species and their surface-atmosphere exchange rates. Such observations have proved invaluable in the validation of space-based inferences of aerosol and cloud properties and trace-gas concentrations.1 Properly incorporated in the scheme of climate-data stewardship, the surface-based observations will produce local climatologies that not only enhance the utility of 1 A good example of the kind of coordinated efforts that can be developed is CEOP (Coordinated Enhanced Observing Period) under the WCRP GEWEX program at http://www.gewex.org/ceop.htm.

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the record derived from satellites but also provide valuable information for society on local trends. Satellite observations provide global perspective and facilitate the incorporation of in situ and surface-based observations to develop regional-scale trends. Innovation and Discovery Specific, focused investigations may emerge as urgent priorities because of new knowledge or unexpected events, such as abrupt climate change. For example, a major volcanic or ENSO event, an unusual hurricane season, chronic atmospheric pollution plumes, or new insight into a poorly understood process, such as convection, may catalyze research interest or public attention and lead to substantial societal benefit. Current Status of Opportunities for Innovation and Discovery The panel notes that the NASA Earth System Science Pathfinder missions have provided important opportunities for space-based technical innovation and innovative scientific exploration. Three ESSP missions have flown, and two expected to launch in 2008. The timing of future opportunities is highly uncertain. Current budget restrictions have nearly eliminated this source of flexibility in the science that provides opportunities for the community to make critical measurements and to test new technologies. The loss of flexibility and innovation is highly significant. In addition, budget restrictions have led to the cancellation of DSCOVR, a completed satellite that would have provided innovative Earth and space observations from the L1 orbit but now sits in storage. Needed Improvements and Products for Innovation and Discovery Climate science needs to have the capability and flexibility to respond promptly and creatively to emerging climate-change issues with the best technology. The panel recognizes that focused investigations can be successful only in the context of a broad understanding of the climate system, which in turn is made possible by the long-term climate data records described above. Alternative views of Earth through new orbital vantage points, new instrumental and retrieval techniques, and new scientific hypotheses may all advance climate science in unpredictable ways. Some of the greatest challenges in improving the long-term record and in advancing the ability to predict change to benefit society (e.g., issues of calibration, cloud-climate feedbacks, convective processes, and better understanding of surface fluxes) will require greater opportunities for innovation. New knowledge may also result from investigations that were not directed at climate variability and change. The necessary drive to design observing systems that address known deficiencies in knowledge should not be allowed to preclude opportunities for ongoing curiosity-driven discoveries whose tremendous contributions can continue to revolutionize the Earth sciences. Requirements for Developing Climate Data from Satellite Observations Involvement of the Climate Science Community The success of NASA’s Earth Science Enterprise in developing records of climate variables that have been validated over long periods is unprecedented. It was achieved through the involvement of many scientists who represented a wide array of interests in the climate community. The level of involvement should be continued regardless of the source of observations (such as NPOESS).

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Accuracy and Time-Space Scales To secure long-duration climate records, observations must have relative accuracy (precision) sufficient to detect the changes being sought. Ultimately, the acquisition of long-term climate records will require traceability to absolute calibration standards. In principle, once knowledge of the climate system is sufficient, accurate calibration standards may allow relaxation of the requirement that observations with independent instruments be substantially overlapped, at least for some climate variables. Clearly, the records must be able to characterize seasonal and internal variations on appropriate spatial scales so that the relatively small secular changes can be reliably extracted. Validation of Satellite-Derived Climate Data Products Validation of climate observations—for example, through comparisons with observations from balloons, aircraft, and ground-based instruments—is crucial to ensure the quality of data sets. For example, the panel notes that the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) sites and the federation of AERONET sites have been heavily relied on to characterize cloud and aerosol properties and temperature and moisture profiles used in the validation of satellite-derived products. The operational weather network is also important. The existing networks should be maintained. The networks should also be expanded in geographic extent and the types of measurements made. Top-of-the-atmosphere radiative forcing is often considered interchangeable with climate forcing, but new insight calls for equal attention to the surface energy budget (NRC, 2005b). The panel calls for the development of surface-based networks focused on climate observations and the development of the associated climate records as set forth in climate-stewardship principles. Use of Climate Records in Climate Model Development Because simulations with climate models provide useful climate information, future observational systems need to recognize impending and ongoing model changes and improvements that will require validation and observational inputs. For example, global climate models are expanding to include higher altitudes (top of the atmosphere and above), delineate more surface features (e.g., vegetation on land), utilize higher spatial and vertical resolution, and add more detailed calculations for various processes now incorporated only through rough approximations (simple parameterizations). Quantitative data sets will be needed not only for model validation but also for assimilation (e.g., cloud assimilation). Because climate records have played and continue to play a fundamental role in climate model validation and development, the need for an Earth radiation-budget continuation mission is reiterated. There is also a pressing need for measurements of the vertical distribution of water vapor, cloud-ice and liquid-water path, and convective processes. Large-Volume, Accessible Archives of Long-Term Climate Observations Climate science requirements have substantial implications for data management, distribution, access, reprocessing, scientific oversight, and value-added analyses that are all part of comprehensive data stewardship. Those activities are crucial to provide the data sets that prove useful for a wide array of climate science investigations, are easy for scientists in diverse disciplines to access, and facilitate the generation of accessible climate products for societal needs. The panel envisions a virtual observatory that provides access to multiple data records and facilitates analysis of disparate observations and integration with model

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Observations are needed to characterize the convective event (e.g., to enable the derivatives of the large-scale convergence of heat and water vapor, cloud base and top, and precipitation) and to measure the redistribution of trace species, including water isotopes, to derive the net convective transports. For example, missions might be designed to accumulate the statistics of convection, building up the patterns of trace species before and after convective transport with the magnitude and type of convective transport. Measurements taken during the airborne CRYSTAL-FACE experiments are able to follow specific events, measure the altitude of convective outflow along with the abundance of boundary-layer tracers, and obtain needed measurements. A mission focused on convection cannot now be assembled from known instruments (or directly from RFI responses). However, the components might include a limb-scanning instrument with high vertical resolution (1 km) and sensitivity to trace species and water isotopes; a lidar or similar measurement of boundary-layer pollutants, such as CO or aerosols; cloud measurements and imager; and a precipitation measure. Given the diurnal cycles in convection and rainfall, the observations cannot be made usefully from a Sun-synchronous orbit. The most important convective transports occur in the tropics or midlatitudes and could be served by a diurnally shifting low-inclination low-Earth-orbit mission like TRMM. Furthermore, these measurements would be greatly enhanced by a cloud-aerosol mission (such as Climate Mission 1). The auxiliary use of a GEO pollution or storm mapper might help to fill in the full cycle of convection. As noted earlier, some intensive in situ campaigns are required to more fully understand the convective cycle. OTHER SPECIAL ISSUES The spaced-based measurements of the Earth system that will be collected in the next two decades will provide scientists with a unique opportunity to gauge climate trends in terms of both the mean state of the system and its variability, including the probability of extreme events (Climate Missions 1–4). The possibility of investigating processes (focus areas Alpha and Beta) that can improve modeling efforts has the potential to advance climate forecasts substantially and to produce models that will be ever more useful for regional impact studies. However, providing the type of information necessary for detection of climate variability and change requires coordination of instruments, missions, and analysis programs. The realization of the program also depends on interagency collaboration and international cooperation. The transition of science-driven missions to operational missions presents challenges related to the integrity of the scientific data. The problems of data continuity, relative and absolute calibration of the measurement sequence, open access to and availability of data, standardization of processing, and distribution standards must all be considered. Interagency Issues A number of institutional challenges must be addressed to achieve the full potential of the climate missions outlined above. It is necessary to identify clearly the respective roles of NASA, NOAA, NSF, DOE, DOD, and other agencies in advancing sensor technology, system calibration and validation, and data archiving and management. NOAA’s plans for data calibration after the NPOESS Preparatory Project (NPP) fall short of those required for climate studies because of budgetary constraints and institutional culture. Other issues related to transparency of processing methods are of concern. Given the national commitment to NPOESS and current problems with it as a suitable and cost-effective platform for climate studies, NOAA, NASA, and other agencies with climate interests should actively participate in a plan to ensure adequate long-term, high-quality data sets on climate. A number of recommendations related to those issues can be found

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in previous NRC reports (e.g., NRC, 2000a,b,c). For NOAA to realize its mandate as the federal agency charged with collecting and managing space-based observations of climate it must have funding to acquire the infrastructure and workforce and must embrace a culture in which climate has high priority. That will require a plan whereby research and operations responsibilities are integrated to balance technical innovation, data quality and stability, and flexibility to meet emerging science questions and concerns. International Partnerships International partnering on instrument development, satellite operations, data exchange, and data analysis spreads the cost burden, mitigates risks of gaps in particular data streams, encourages technical innovation by broadening the engineering expertise base, and increases the number of science users. NASA and its international partners have enjoyed those benefits through numerous programs—including joint ventures on EOS, TOPEX/Poseidon, and RADARSAT-1—and more generally through programs such as the International Global Observing System, the International Polar Year, and CLIVAR. Moreover, it is now relatively common for flight agencies to offer announcements of opportunity to the international science community as the agencies attempt to maximize the payoff of each flight project. The potential advantages of collaborations are obvious, but realizing the advantages can be complicated by a number of factors. Instruments built by one partner may not be designed to the exact requirements of another partner, and technology-transfer restrictions may prevent the exchange of important technical details about the instruments. Restrictions on access to data and software vary from country to country, as do approaches to calibration and validation. Joint ventures between government flight agencies and commercial partners can result in serious complications with data cost, availability, and distribution. With that in mind, international partnerships should be fostered only where synergy between instrument capabilities and science requirements is strong, where there is free and easy access to data, and where there is transparency in the process of analyzing data so that analysis algorithms are freely available. Improving Climate Modeling Through the Application of New Satellite Measurements Interaction between the climate modeling and satellite remote-sensing communities is too limited (NRC, 2001c). Existing data sets are underused by the climate-modeling community. The CLIVAR Climate Process Teams Program for in situ measurements is designed specifically to understand processes poorly handled by the climate models and provides a framework that could be adopted for a similar effort involving satellite measurements. The panel recommends a new cross-agency effort to foster a more fertile crossover between those collecting, managing, and analyzing satellite observations and the modeling groups. Such a program must be well managed and funded. Success will require improved, coincident in situ observations such as those traditionally carried out by DOE, DOD, NSF, and other agencies, but these should be augmented to include dedicated field programs that address specific scientific questions related to the proposed missions. Workforce A successful and robust plan for improving climate prediction must include the education of the workforce, including the engineers who design sensor systems and the geoscientists who interpret data. A close interaction between those groups to assess evolving needs is essential. A concerted effort should be made to fund universities and national laboratories for training graduate students and postdoctoral researchers for this purpose.

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Data Management and Distribution A successful climate science program will require a robust data system for satellite measurements as outlined in previous NRC reports (NRC, 2000a, 2004, 2005a). In particular, computer systems must be designed to facilitate reprocessing, archiving, and distribution of NPOESS data. Many of the recommendations made in an NRC report on climate data records (NRC, 2004) apply to satellite-based observation. Three points deserve particular attention: there should be easy access to data; metadata should be available to document sensor performance history and data-processing algorithms and to allow reprocessing to adjust for bias, drift, and other errors in the datasets; and representatives of various components of the climate community should be actively involved in data generation and stewardship decisions. Those procedures will enhance and expand data access by researchers and climate service providers and foster the development of value-added products for the climate services discussed earlier. BIBLIOGRAPHY Ackerman, A., O.B.Toon, D.E.Stevens, A.J.Heymsfield, V.Ramanathan, and E.J.Welton. 2000. Reduction of tropical cloudiness by soot. Science 288:1042–1047. Albrecht, B.A. 1989. Aerosols, cloud microphysics, and fractional cloudiness. Science 245:1227–1230. Anderson, J.G., J.A.Dykema, R.M.Goody, H.Hu, and D.B.Kirk-Davidoff. 2004. Absolutely, spectrally-resolved, thermal radiance: A benchmark for climate monitoring from space. J. Quant. Spectrosc. Radiat. Transfer 85:367–383. Andreae, M.O., C.D.Jones, and P.M.Cox. 2005. Strong present-day aerosol cooling implies a hot future. Nature 435(7046):1187– 1190, doi:10.1038/nature03671. Behrenfeld, M., J.T.Randerson, C.R.McClain, G.C.Feldman, S.O.Los, C.J.Tucker, P.G.Falkowski, C.B.Field, R.Frouin, W.E.Esaias, D.D.Kolber, and N.H.Pollack. 2001. Biospheric primary production during an ENSO transition. Science 291:2594–2597. Behrenfeld, M., E.Boss, D.A.Siegel, and D.M.Shea. 2005. Carbon-based ocean productivity and phytoplankton physiology from space. Global Biogeochem. Cy. 19:GB1006, doi:10.1029/2004GB002299. Bengtsson, L., G.Robinson, R.Anthes, K.Aonashi, A.Dodson, G.Elgered, G.Gendt, R.Gurney, M.Jietai, C.Mitchell, M.Mlaki, A. Rhodin, P.Silvestrin, R.Ware, R.Watson, and W.Wergen. 2003. The use of GPS measurements for water vapor determination. B. Am. Meteorol. Soc. 84:1249–1258. Breon, F.M., and P.Goloub. 1998. Cloud droplet effective radius from spaceborne polarization measurements. Geophys. Res. Let. 25:1879–1882. Breon, F.M., J.C.Buriez, P.Couvert, P.Y.Deschamps, J.L.Deuze, M.Herman, P.Goloub, M.Leroy, A.Lifermann, C.Moulin, F. Parol, G.Seze, D.Tanre, C.Vanbauce, and M.Vesperini. 2002. Scientific results from the Polarization and Directionality of the Earth’s Reflectances (POLDER). Adv. Space Res. 30(11):2383–2386. CCSP (Climate Change Science Program). 2003. Strategic Plan for the U.S. Climate Change Science Program. CCSP, Washington, D.C. Available at http://www.climatescience.gov/Library/stratplan2003/final/ccspstratplan2003-all.pdf. Curry, J.A., A.Bentamy, M.A.Bourassa, D.Bourras, E.F.Bradley, M.Brunke, S.Castro, S.H.Chou, C.A.Clayson, W.J.Emery, L. Eymard, C.W.Fairall, M.Kubota, B.Lin, W.Perrie, R.A.Reeder, I.A.Renfrew, W.B.Rossow, J.Schulz, S.R.Smith, P.J.Webster, G.A.Wick, X.Zeng. 2004. SEAFLUX. B. Am. Meteorol. Soc. 85:409–424. Dubayah, R., J.B.Blair, J.L.Bufton, D.B.Clark, J.Jájá, R.Knox, S.B.Luthcke, S.Prince, and J.Weishampel. 1997. The Vegetation Canopy Lidar Mission. Pp. 100–112 in Land Satellite Information in the Next Decade II: Sources and Applications. ASPRS, Washington, D.C. Ekstrom, G., M.Nettles, and V.C.Tsai. 2006. Seasonality and increasing frequency of Greenland glacial earthquakes. Science 311:1756. Eyring, V., N.R.P.Harris, M.Rex, T.G.Sheperd, D.W.Fahey, G.T.Amanatidis, J.Austin, M.P.Chipperfield, M.Dameris, P.M.de F. Forster, A.Gettleman, H.F.Graf, T.Nagashima, P.A.Newman, S.Pawson, M.J.Prather, J.A.Pyle, R.J.Salawitch, B.Santer, and D.W.Waugh. 2005. A strategy for process-oriented validation of coupled chemistry-climate models. B. Am. Meteorol. Soc. 86(8):1117–1133. Forest, C.E., P.H.Stone, and A.P.Solokov. 2006. Estimated PDF of climate system properties including natural and anthropogenic forcings. Geophys. Res. Lett. 33:L01705, doi:10.1029/2005GL023977. Fu, L-L., and E.R.Rodriguez. 2004. High-resolution measurement of ocean surface topography by radar interferometry for oceanographic and geophysical applications. Pp. 209–224 in State of the Planet: Frontiers and Challenges, AGU Geophysical Monograph 150, IUGG Vol. 19 (R.S.J.Sparks and C.J.Hawkesworth, eds.). American Geophysical Union, Washington, D.C. GCOS (Global Climate Observing System). 2003. The second report on the adequacy of the global observing systems for climate in support of the UNFCCC. GCOS-82, WMO Tech. Doc. 1143.

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ATTACHMENT Table 9.A.1 lists the Panel on Climate Change and Variability’s summary of the status of space-based, and in some cases supporting ground-based, observations of critical climate variables. The table’s limitations are described in the section “Observational Needs and Requirements” toward the beginning of this chapter. TABLE 9.A.1 Climate Change and Variability Panel’s Summary of Status of Major Climate Variables and Forcing Factors Measurement (GCOS)a Strategy Current Status Follow-on (2010–2020) RFI Responseb Total solar irradiance (1.2) Direct measurement SORCE launched 2003; Glory (TIM only) 2008 NPOESS TSIS-GFE 25, 30, 47, 52 Earth radiation budget Multispectral imager combined with broadband radiometers Scene identification, top of the atmosphere fluxes. MODIS/CERES on Terra (2000), Aqua (2002) VIIRS/ERBS on NPOESS, C1 (2013) Mission 2 9, 17, 18, 25, 30, 52, 59, 76 Surface radiation budget Multispectral imager combined with broadband radiometers Scene identification, top of atmosphere fluxes, radiative transfer modeling MODIS/CERES on Terra (2000), Aqua (2002) VIIRS/ERBS on NPOESS, C1 (2013) Mission 2     Surface-based radiometers ARM, BSRN, CMDL, SURFRAD sites, sparsely located       Tropospheric aerosols (1.3): geographic and vertical distribution of aerosols, optical depth, size, shape, single-scattering albedo Multispectral imagers Provide optical depth, some inference of size over oceans and dark surfaces AVHRR since 1981 (NOAA 7), currently on NOAA 16, 17, 18 VIRS on TRMM (1997) MODIS, MISR on Terra (2000) MODIS on Aqua (2002) VIIRS follow-on to MODIS on NPP, NPOESS 3, 7, 25, 30, 35, 45, 52, 61, 77 UV radiometer-imager Provide optical depth, some inference of absorption for elevated aerosol layers OMI on AURA (2004) OMPS on NPP (2008) OMPS on NPOESS Mission 1     Polarimeters Provide optical depth, size, shape, single-scattering albedo POLDER on PARASOL (2005) APS on Glory (2008) limited to subsatellite ground track APS on NPOESS Mission 1     Lidar Provide vertical profile of aerosol concentration, some inference of size and shape CALIPSO (2006) Mission 1     Surface multispectral radiometers AERONET, ARM VIIRS on NPOESS Mission 1     Surface and Earth broadband flux measurements CERES on Terra (2000), Aqua (2004) combined with BSRN, ARM, SURFRAD sites ERBS on NPOESS Mission 2  

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Measurement (GCOS)a Strategy Current Status Follow-on (2010–2020) RFI Responseb Stratospheric aerosol properties, optical depth, size, shape, single-scattering albedo (1.3) Limb and solar occultation measurements Profile of aerosol extinction HIRDLS on Aura, infrared radiometer SAGE II on ERBS (1984–2006) SAGE III on Meteor (2002–2006) SciSat (Canadian-U.S.) None 63 Limb-scattered light Profile of aerosol optical depth   OMPS on NPP (2009), NPOESS     Lidar Vertical profile of aerosol concentration, some inference of size and shape CALIPSO (2006) Mission 1 3, 57, 110, 111 Cloud properties (1.2): geographic and vertical distribution, water-droplet effective radius, ice-cloud crystal habitat and size, mixed-phase cloud water/ice ratio and hydrometeor size and visible optical depth, cloud liquid and ice water amounts Multispectral imagers Properties of single effective cloud layer AVHRR since 1981 (NOAA 7), currently on NOAA 16, 17, 18, inferences of hydrometeor size, but not phase VIRS on TRMM MODIS on Aqua and Terra provide inference of hydrometeor phase VIIRS on NPP, NPOESS provides inference of hydrometeor phase 2, 66, 110, 111 Multiple-view radiometers, polarimeters MISR on Terra, cloud altitude from stereo imaging POLDER on PARASOL, hydrometeor size and phase from polarimetry APS on Glory (2008), phase from polarimetry APS on NPOESS, hydrometeor phase from polarimetry   15-µm sounders, imagers Cloud-layer pressure for effective single-layered cloud system, even for optically thin cirrus HIRS on NOAA 16, 17, 18 MODIS on Terra, Aqua AIRS on Aqua (2002) CrIS on NPP (2008) CrIS on NPOESS     Microwave imagers Microwave inference of cloud liquid water over oceans SSM/I on DMSP TMI on TRMM AMSR-E on Aqua CMIS on NPOESS     Lidar Upper boundary, extinction for optically thin clouds with polarization, particle phase CALIPSO (2006) Mission 1     Cloud radar Cloud boundaries,vertical distribution of liquid water, rates of drizzle when precipitation is light CloudSat (2006) Mission 1   Ozone: stratosphere, troposphere (1.3) UV radiometer-imager Provides tropospheric column ozone, coarse vertical-resolution profiles of stratospheric ozone OMI on Aura (2004) OMPS Nadir on NPP (2009), OMPS Nadir on NPOESS     UV limb scanner Provides vertical profile of stratospheric concentration OMPS on NPP OMPS Limb on NPP (2009), NPOESS  

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Measurement (GCOS)a Strategy Current Status Follow-on (2010–2020) RFI Responseb Trace gases controlling ozone (HCI, N2O, CH4, H2O, HNO3) Infrared sounders Provides vertical profiles of tropospheric, stratospheric ozone HIRDLS on Aura TES on Aura also provides limb viewing (not being used after 2005) AIRS on Aqua (2002) None 61   Microwave limb sounding Provides vertical profile of stratospheric ozone MLS on Aura None 61 CO2 (1.3) Near-IR spectrometer High-precision column concentrations of CO2 OCO (2008); goal is to achieve accuracies sufficient to allow determinations of sources and sinks. Surface-based networks (WMO GAW, NOAA, AGAGE) None 3, 20   Infrared sounders AIRS on Aqua (2002) None 8 CH4 (1.3) Infrared spectrometer High-precision column concentrations of CH4 TES on Aura Surface-based networks (WMO GAW, NOAA, AGAGE) None 95   Infrared sounders AIRS on Aqua (2002) None 8 Land-surface cover and surface albedo (3) (snow cover, glaciers, ice caps covered later) Multispectral imagery Vegetation index, inference of surface albedo AVHRR on NOAA 16, 17, 18: inferences of atmospherically corrected spectral albedos MODIS on Terra (2000), Aqua (2002) Landsat series VIIRS on NPP (2009), NPOESS 38   Hyperspectral imagery Vegetation types, land cover Hyperion (EO-1) Mission 1   Temperature (1.2): vertical profiles Infrared, microwave sounders Vertical profiles of layer temperatures HIRS/MSU since 1979 currently on NOAA 16, 17, 18 SSM/I on DMSP (1995, 1997, 1999) AIRS/AMSU on Aqua (2002) CrIS, ATMS on NPP (2009), NPOESS 5, 8, 10, 41, 43, 48, 92   GPS radio occultation Vertical profiles with resolution of about 0.5-1 km near surface GPS on CHAMP (2000), COSMIC (2007) Mission 2     Surface network Radiosonde temperature profiles, WMO sonde network (1959)      

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Measurement (GCOS)a Strategy Current Status Follow-on (2010–2020) RFI Responseb Watervapor (1.2): column amounts, vertical profiles Microwave imaging Column water-vapor amounts over oceans SSM/I on DMSP polar satellites (1995, 1997, 1999) ATMS on NPP (2009), CMIS on NPOESS     Multispectral imagery Column amounts from near-IR watervapor channels MODIS on Terra (2000), Aqua (2002) None     Infrared sounders Water-vapor layer amounts at relatively coarse vertical resolution in troposphere HIRS data from 1979 (TIROS-N), currently on NOAA 16, 17, 18 CrIS on NPP (2009), NPOESS 3, 5, 8, 9, 10, 92, 99   High-spectral-resolution infrared radiometers Water-vapor layer amounts at finer vertical resolution in troposphere AIRS on Aqua (2002) TES on Aura (2004) CrIS on NPP (2009), NPOESS     Infrared, microwave limb-scanning radiometers Water-vapor layer amounts in upper troposphere, stratosphere TES, MLS on Aura (2004) None     GPS-radio occultation Profiles of temperature, water vapor with up to about 0.5-km vertical resolution near surface CHAMP (2000), COSMIC (2006) Mission 2     Surface network Radiosonde water-vapor profiles, WMO sonde network (1959)       Fire disturbance (3) Near-IR thermal imagery High-spatial-resolution detection of fire hotspots AVHRR data from 1981 (NOAA 7), currently on NOAA 16, 17, 18 MODIS on Terra (2000), Aqua (2002) VIIRS on NPP (2009), NPOESS   Land biomass, fraction of photosynthetically active radiation (FAPAR) (3) Multispectral imagery Index of vegetation, inference of FAPAR AVHRR data from 1979 (NOAA 6), currently on NOAA 16, 17, 18 MODIS on Terra (2000), Aqua (2002), SeaWiFS VIIRS on NPP (2009), NPOESS Mission 1   Radar Land cover from C-band radar backscatter RADARSAT 1 (1995), RADARSAT 2 (2007), data commercially available None   Glaciers, sea ice, ice caps (3) Multispectral imagery Area coverage AVHRR data from 1979 (TIROS-N), currently on NOAA 16, 17, 18 MODIS on Terra (2000), Aqua (2002) VIIRS on NPP (2009), NPOESS 44, 87, 111   Microwave imagers Area coverage SSM/I on DMSP (1995, 1997, 1999) AMSR-E on Aqua, TMI on TRMM (1997) CMIS on NPOESS     Radars Ice area and flow, sea-ice thickness from topography RADARSAT 1 (1995), RADARSAT 2 (2007), data commercially available Mission 3     Lidar Ice elevation GLAS on ICESat (2003) Mission 1     Gravity satellite Ice mass when combined with measure of topography GRACE (2002) GRAGE follow-on  

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Measurement (GCOS)a Strategy Current Status Follow-on (2010–2020) RFI Responseb Permafrost, seasonally frozen ground (3)         Snow cover (and snow water equivalent) (3) Radars combined with microwave radiometers Combination of area, roughness, topography to provide snow-water equivalent RADARSAT 1 (1995), RADARSAT 2 (2007), data commercially available No planned follow-on 10, 14, 19, 56 Groundwater (3) Microwave imagers Soil moisture except for areas covered by ice-snow and heavily forested areas SSM/I on DMSP (1995, 1997, 1999) AMSRE on Aqua (2002) CMIS on NPOESS 19, 96   Gravity satellite Large-scale groundwater (requires in situ auxiliary observations) GRACE (2003) GRACE follow-on   Lake levels (3) High-resolution multispectral imagery Lake areas Landsat 7 (1999) LDCM     Radars Lake area RADARSAT 1 (1995), RADARSAT 2 (2007), data commercially available No planned follow-on     Lidar Water-surface elevation GLAS on ICESat (2003) Mission 1   River discharge (3) High-resolution imagery Lake, river areas Landsat 7 (1999) LDCM     Lidar altimeter River levels ICESat (2002) Mission 1     Radar Lake, river areas RADARSAT 1 (1995), RADARSAT 2 (2007), data commercially available No planned follow-on   Leaf-area index (LAI) (3) Multispectral imagers Vegetation index AVHRR, data since 1981 (NOAA 6), currently on NOAA 16, 17, 18 MODIS on Terra (2000), Aqua (2002) MISR on Terra (2000) SeaWiFS (1997) VIIRS on NPP (2008) VIIRS on NPOESS     High-spatial-resolution multispectral imagers Vegetation index at higher spatial resolution Landsat 7 (1999) ASTER on Terra (2000) EO-1 LDCM Mission 1   Sea level Altimeter Ocean sea-level height Jason 1 (2001) GFO ALT on NPOESS Mission 4 GRACE follow-on 56   SAR radars Area of coastal zones RADARSAT 1 (1995), RADARSAT 2 (2007), data commercially available None  

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Measurement (GCOS)a Strategy Current Status Follow-on (2010–2020) RFI Responseb Sea state (2.1), surface wind (1.1) Microwave imagers Surface windspeed SSM/I on DMSP (1995, 1997, 1999) AMSR-E on Aqua (2002) CMIS on NPOESS 56, 98   Scatterometer Surface wind vector QuikSCAT (1999) ASCAT on MetOp) ASCAT (MetOp) Mission 4   Ocean color (2.1) Multispectral imagers with UV-blue capabilities Surface-leaving radiances SeaWiFS (1997) MODIS on Terra (2000), Aqua (2002) VIIRS on NPP (2009), and NPOESS 21, 86 Ocean surface (2.1) and sub-surface temperature (2.2) Multispectral imagery Sea-surface temperature AVHRR, data since 1981 (NOAA 7), currently on NOAA 16, 17, 18 VIRS on TRMM (1997) MODIS on Terra (2000), Aqua (2002) VIIRS on NPP (2009), and NPOESS     Infrared-microwave sounders Sea-surface temperature AVHRR on NOAA 16, 17, 18 AIRS, AMSR-E on Aqua (2002) MODIS on Aqua (2002), Terra (1999) MODIS CrIS/ATMS on NPP (2009), CMIS on NPOESS     Expendable profiling floats Profiles of temperature, temperature at depth of neutral buoyancy, surface ARGO floats     Ocean surface (2.1), subsurface salinity (2.2) Microwave radiometer and scatterometer Surface salinity, ocean roughness   AQUARIUS (2010)     Expendable profiling floats Profiles of salinity, salinity at depth of neutral buoyancy ARGO floats     Ocean surface (2.1), subsurface currents (2.2) Altimeter Ocean-surface height from which currents derived Jason 1 (2001) ALT on NPOESS Mission 4     Gravity satellite Subsurface or barotropic mass shifts (computed in conjunction with surface altimeter measurements) GRACE (2002) GRACE follow-on     Expendable profiling floats Position drift at depth of neutral buoyancy (and surface with some caveats) ARGO floats     Subsurface phytoplankton (2.2)         Precipitation (1.1) Microwave imagers Rainfall rate over oceans SSM/I on DMSP (1995, 1997, 1999) TMI on TRMM (1997) AMSR-E on Aqua (2002) CMIS on NPOESS, GPM (2012)     Precipitation radar Vertical structure of rain rates TRMM (1997) GPM (2012)     Cloud radar Rate for light drizzle CloudSat (2006) Mission 1   aNumbers in parentheses refer to the essential climate variables listed in Appendix 1 of CGOS (2003). bAn indexed list of RFI responses is given in Appendix E.