If climate models accurately reproduce climate sensitivity, one way to ascertain whether they do so for the correct physical reasons is to divide climate response according to feedbacks. CLARREO should be capable of doing so because individual feedbacks have distinctive spectral signatures in the thermal infrared and visible wavelengths. GNSS occultation should play an important role because of its insen-sitivity to clouds: it might resolve the cloud-surface temperature ambiguity inherent to sounding in the thermal infrared. We apply optimal fingerprinting techniques to spectral infrared and microwave refractivity as produced offline by many sophisticated climate models. This should tell us the relative contribution of various data types to testing various climate feedbacks, what accuracy is needed for each data type, and how long we should expect to wait before a satisfactory test can be applied to climate models' predictions.
A feedback can be determined by trend analysis by dividing the trend in outgoing radiation due to a specific thermodynamical variable or constituent concentration by the trend in surface air temperature:
with dFlw the change in downward radiation at the tropopause due to a change in thermodynamic variable or constituent concentration i. In order to estimate the feedback, one must be able to estimate dF1LW/di observationally as well as dT/dt. Moreover, observations in the thermal infrared allow one to detect radiative forcing by anthropogenic greenhouse gases, A Frad. [Presently, an exploration of the possibility of testing climate models has been done only for longwave radiation and not yet for shortwave radiation; hence, we restrict our discussion to the longwave.]
If a variable perturbs the tropopause radiation field, then it has an associated feedback, and because changes in variables lead to unique changes in the infrared spectrum at the tropopause, careful observation of the evolution of the tropopause radiation field should constrain the feedbacks of the climate system. In fact, in most cases individual feedbacks have unique fingerprints in the spectra of outgoing longwave and shortwave radiation. CLARREO, in measuring the outgoing longwave radiation, can uniquely discern the longwave feedbacks because each has a unique signature in the thermal infrared spectrum. How long a timeseries of CLARREO-like data is necessary before climate models' realizations of the climate feedbacks can be tested remains an open question.
In Fig. 1 we show the spectral infrared signatures of tropospheric temperature change, stratospheric temperature change, tropospheric water vapor increase, and
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