immediately to provide records of absolute values for all time of a number of carefully selected observables that define climate forcings and climate responses. Since radiative forcings and climate responses are highly wavelength dependent, high spectral resolution is needed to isolate the spectral signatures of the relevant processes and components. This produces additional challenges since accuracy and calibration difficulty increases as spectral resolution increases (stray light, instrument profile function, wavelength calibration, signal to noise, matching to available standards). Key parameters for which benchmark measurements are crucial include among others sea level altimetry, solar irradiance, global positioning system (GPS) index of refraction, ozone and CO2 concentrations, and spectrally resolved, absolute radiance to space.

Establishing and validating the accuracy and precision of a geophysical quantity involves tracking instrument calibrations from the laboratory to deployment and throughout mission lifetimes to reduce systematic errors on orbit. This requires considerable additional effort and commitment to an experimental strategy designed to reveal systematic errors and drifts through independent cross checks, open inspection, and continuous interrogation. It involves simultaneous observations of related and similar quantities using both similar and differing radiometric techniques. Regular calibrations are needed, for example, using the Sun, Moon, known land scenes, or on-orbit sources or detectors. Since the forcings and responses that determine any one particular climate state involve a distribution about a mean, the ensemble must be properly characterized and quantified so that changes in the mean can be identified reliably. Ultimately, the specification of the forcings and responses must be integrated to test climate forecast models.

Achieving radiometric accuracy and traceability requires new programs and techniques to advance the current state of metrology and transfer these advances to the determination of radiative climate forcings and effects. This has motivated an alliance of NPOESS and the National Institute of Standards and Technology (NIST), but there is a need to accord this a high priority with sufficient time and funds. It is unlikely that many of the climate variables measured by NPOESS will be directly traceable to an absolute standard. A recent NIST workshop (Ohring et al., 2004) to address this challenge notes that “measuring the small changes associated with long-term global climate change from space is a daunting task. Satellite instruments must be capable of observing atmospheric temperature trends as small as 0.1°C per decade, ozone changes of as little as 1 percent per decade and variations in the Sun’s output as tiny as 0.1 percent per decade.” NIST is developing new facilities to meet the metrology challenges of future climate-related observations. Of particular relevance is the Spectral Irradiance and Radiance Calibrations with Uniform Sources (SIRCUS),



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