Background: Climate is affected by the long-term balance between the solar irradiance absorbed by the Earth-ocean-atmosphere system and the IR radiation exchanged within that system and emitted to space. Thus, key observations include incident and reflected solar irradiance and the spectrally resolved IR radiance emitted to space that carries the spectral signature of IR climate forcing and the resulting response of that climate system. Given the recognized imperative to develop long-term, high-accuracy time series with global coverage of critical climate variables, CLARREO addresses the objective of establishing global, highly accurate, long-term climate records that are tied to international standards maintained in the United States by the National Institute of Standards and Technology (NIST). In addition, to achieve societal objectives that require a long-term climate record, it is essential that the accuracy of the core benchmark observations be verified against absolute standards on-orbit by fundamentally independent methods.

Science Objectives: Four elements constitute the CLARREO science strategy:

• Absolute spectrally resolved IR radiance is measured with high accuracy (0.1 K 3σ brightness temperature) by downward-directed spectrometers in Earth orbit. Both the radiative forcing of the atmosphere resulting from greenhouse-gas emissions and aerosols and the response of the atmospheric variables are clearly observable in the spectrally resolved signal of the outgoing radiance. Similarly, large differences among model projections of temperature, water vapor, and cloud distributions imply, for each model, different predicted changes in absolute, spectrally resolved radiation. The spectrum of IR radiance, if observed accurately and over the full thermal band, carries decisive diagnostic signatures in frequency, spatial distribution, and time.

• Solar radiation, reflected from the Earth-atmosphere system back to space, constitutes a powerful and highly variable forcing of the climate system through changes in snow cover, sea ice, land use, and aerosol and cloud properties. Systematic, spatially resolved observations of the time series of the absolute spectrally resolved flux of near-ultraviolet, visible, and near-IR radiation returned to space by the Earth system tied to NIST standards in perpetuity underpin a credible climate record of the changing Earth system. In combination with establishment of the absolute spectrally resolved solar irradiance reflected from the Earth-atmosphere system to space, it is essential to continue the long-term, high-accuracy time series of incident solar irradiance.

• Global Navigation Satellite System (GNSS) radio occultation offers an ideal method for benchmarking the climate system because much of the infrastructure for this active limb-sounding technique already exists, or soon will, in the form of the U.S. Global Positioning System (GPS) and the European Galileo satellites; because orbiting GNSS receivers are comparatively inexpensive; and because the technique is a measurement of frequency shift against a time standard and is thus directly traceable to international standards. GNSS radio occultation profiles the refractive properties of the atmosphere by observing the timing delay of GNSS signals induced by the atmosphere as the ray descends into the atmosphere in a limb-sounding geometry. The index of refraction is directly related to pressure, temperature, and watervapor concentration in such a way that the refractive index can be easily simulated from model output. Moreover, both GNSS and absolute, spectrally resolved radiance in the thermal IR are accurate to 0.1 K traceable to SI (Systeme Internationale) standards on-orbit and therefore represent independent, absolute records that, for the first time, allow the determination of systematic error in the climate record.

• CLARREO would serve as a high-accuracy calibration standard for use by the broadband CERES instruments on-orbit. In addition, the suite of IR operational sounders launched on NPP and NPOESS could use CLARREO to establish SI-traceable accuracy on-orbit, establish an independent analysis of time-dependent bias in calibrated radiance, and form a basis for intercomparison of all operational sounders now and in the future.