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The CLARREO (Climate Absolute Radiance and Refractivity Observatory) is a high priority NASA Decadal Survey mission recommended by the National Research Council in 2007.[1] The CLARREO mission is intended to provide a metrology laboratory in orbit for the purpose of accurately quantifying and attributing Earth's climate change (see List of climate research satellites). If launched at the earliest opportunity,[2] CLARREO's observations could be used to detect the largest of climate trends above natural variability by the year 2039.[3] Then it may go on to further test, validate, and improve prediction by climate models. The mission also might provide the first orbiting radiometers with accuracy sufficient to serve as reference calibration standards for other spaceborne sensors,[4] making climate trends apparent in their data sets also within a 30 year time frame.

The mission concept[edit]

CLARREO is currently in an extended planning phase or "Pre-Phase A," where the mission and science teams are funded to continue advancing the science of CLARREO, explore alternative implementation strategies, and reduce technical risk.[5]

Below is the mission concept presented at the Mission Concept Review in November 2010.[6] CLARREO was then envisioned to consist of four observatories on two dual-manifested launches on Minotaur IV+ vehicles.

  • Four Observatories, two dual-manifested launches on Minotaur IV+ vehicles
    • July 2018: Two infrared observatories, each with GNSS-RO
    • May 2020: Two reflected solar observatories
  • 609 km polar orbits (90 degree inclination)

Now due to reduced funding the International Space Station (ISS) is being investigated as a possible platform. Although the ISS orbital inclination of 51.65 degrees negates the possibility of global climate change monitoring by CLARREO, it does provide opportunities to compare measurements with those made by other satellites in higher orbits.

The science behind CLARREO[edit]

CLARREO could make highly accurate decadal change observations that are traceable to International Systems of Units (SI) standards. At solar wavelengths this is intended to be confirmed after launch using comparison of actual data to theoretical simulations of lunar/solar radiance generated within a hi fidelity sensor model.[7] The Earth observations then made by CLARREO have sensitivity to the most critical but least understood climate radiative forcings, responses, and feedbacks, such as:

  • Infrared spectra to infer temperature and water vapor feedbacks, cloud feedbacks, and decadal change of temperature profiles, water vapor profiles, clouds, and greenhouse gas radiative effects
  • GNSS-RO to infer decadal change of temperature profiles
  • Solar reflected spectra to infer cloud feedbacks, snow/ice albedo feedbacks, and decadal change of clouds, radiative fluxes, aerosols, snow cover, sea ice, and land use[8]

Reference intercalibration[edit]

Fig. 2. CLARREO RS measurements (in red) are not matched in time, but are in viewing geometry to cross-track observations made earlier by VIIRS (in green). Approximate on-orbit data matching is enabled by CLARREO's ability to point its RS instrument in two dimensions.

Current satellite-based sensors are not designed to meet the accuracy requirements of CLARREO. Many sensors used for climate measurements were designed to meet operational weather needs and are not optimized for climate sampling. These sensors, along with older instruments designed for climate, lack the on-board ability to test for systematic errors on orbit. The CLARREO mission will meet these goals through careful consideration of the instrument design[citation needed], calibration traceability at all stages of development and operation, with spectral, spatial and temporal sampling focused specifically on the creation of climate records. Then after development of new <0.15% accurate cross-calibration methodologies[citation needed], CLARREO may serve as an in-orbit standard to provide reference intercalibration for missions like the broadband Clouds and the Earth's Radiant Energy System (CERES), operational sounders including the Cross-track Infrared Sounder (CrIS) and Infrared Atmospheric Sounding Interferometer (IASI), and imagers such as the Visible Infrared Imaging Radiometer Suite (VIIRS) and Advanced Very High Resolution Radiometer (AVHRR).[9]

CLARREO selection[edit]

The 2007 National Research Council (NRC) Decadal Survey report,[1] "Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond," provides the basis for the future direction of NASA’s space-based Earth observation system. Missions were ranked according to scientific merit, contributions to long-term observational records, societal benefits, affordability, and technological readiness. The four missions recommended for earliest implementation by NASA were classified as “Tier 1” missions and included CLARREO. The NRC Decadal Survey concluded that the single most critical issue for current climate change observations was their lack of accuracy and low confidence in observing the small climate change signals over decade time scales. CLARREO observations of climate change on decadal scales address this issue by achieving the required levels of accuracy and traceability to SI standards for a set of observations sensitive to a wide range of key climate change observations.

Decadal Survey recommendations represent the community's input on the future direction of space-based Earth science; therefore, NASA will continue to engage the scientific community to refine mission requirements during the planning for CLARREO.

The CLARREO Team[edit]

CLARREO was originally recommended as a joint NASA/NOAA mission[10] where NOAA would contribute the total and spectral solar irradiance measurements and the Earth energy budget climate data records by flying the Total Solar Irradiance Sensor (TSIS) and the Clouds and the Earth’s Radiant Energy System (CERES) sensors. The NASA portion involved the measurement of spectrally resolved thermal IR and reflected solar radiation at high absolute accuracy. However, recent events have put such allocations in question.[11]

A NASA team led by Langley Research Center with contributions from other NASA Centers, government organizations, academia, and NASA HQ developed a mission concept that passed its Mission Concept Review (MCR) on 11/17/10 .[12] The team’s successful completion of this suggested CLARREO might proceed into Phase A and then begin to prove its new design concepts, but this has been delayed indefinitely.[13]

Societal benefits of an Improved Climate Observing System[edit]

Fig. 3. Length of data set required to observe GCM predicted Cloud Radiative Forcing Trends with 95% confidence (e.g. earliest CMIP3 climate trend detection date by CLARREO+CERES is launch date of 2023, plus length of observed trend).

Due to natural variability and a launch date at least 25 years after that of other missions, the CLARREO team predict that it will not provide the data necessary for decisions on public policy concerning climate change until around ten years after say the existing CERES observing system (see Fig. 3). However they have also calculated that if better informed decisions could be made[citation needed] around 15–20 years prior to arrival of useful CLARREO + CERES results, it would provide a large economic benefit to the United States and the world (estimated to be about 12 Trillion dollars at 3% discount rate over the next 40 – 60 years[14]). Reduction of climate prediction uncertainties impacts civil Government and military planning (i.e., Navy bases), disaster mitigation, response, and recovery (i.e., insurance industry), and U.S. international policy decisions.


  1. ^ a b National Research Council, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. National Academies Press, Washington, D.C., 426pp, 2007.
  2. ^
  3. ^ Wielicki et al., "Achieving Climate Change Absolute Accuracy," Bull. Amer. Meteor. Soc., 1829 pp. 1519-1539, 2013.
  4. ^ N. Fox et al., "Accurate radiometry from space: an essential tool for climate studies," Phil. Trans. R. Soc. A., 369 pp. 4028-4063, 2011.
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  6. ^ "CLARREO Mission Overview 2011 (21 January 2011)". Retrieved 18 July 2012.  |first1= missing |last1= in Authors list (help)
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  14. ^ R. Cooke, B.A. Wielicki, D.F. Young, M. Mlynczak, "Value of Information for Climate Observing Systems," Environ. Syst. Decis., 12 pp., 201, DOI 10.1007/s10669-013-9451-8.

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