HR: 11:00h
AN: GC42A-03 INVITED    [Abstracts]
TI: The Total Solar Irradiance Record and Its Continuity
AU: * Willson, R C
EM: rwillson@acrim.com
AF: Columbia University, 12 Bahama Bend, Coronado, CA 92118, United States
AB: Continuous time series of total solar irradiance (TSI) observations have been constructed from the set of redundant, overlapping TSI measurements made by satellite experiments during the past 29 Years. One, the ACRIM composite [Willson & Mordvinov, 2003], displays a significant upward trend in TSI of 0.04 percent per decade during solar cycles 21-23. Another, the PMOD composite [Frohlich & Lean, 1998], displays no significant trend using different combinations of TSI data sets, computational philosophy and assumptions. The potential significance of solar variability as a climate forcing makes it important to determine which TSI composites best represents the measurement database. Two types of experiments have provided TSI satellite data: self-calibrating, precision TSI monitors and Earth radiation budget (ERB) experiments. TSI monitors provide much higher accuracy and precision and are capable of self-calibrating the degradation of their sensors, providing enhanced data traceability. The ERB experiments are designed to provide less accurate and precise TSI ‘boundary value' results for ERB modeling and cannot self-calibrate sensor degradation. The optimum composite TSI time series utilizes TSI monitor results where available. However, a two year gap in the TSI monitoring record between the ACRIM1 and ACRIM2 experiments (1989 - 1991) would have prevented compilation of a continuous record over the 29 years of satellite observations were it not for the availability of ERB results during the gap. The relationship between ACRIM1 and ACRIM2 results across the ACRIM gap can be derived using the overlapping ERB data sets: the Nimbus7/ERB and/or the ERBS/ERBE. These two choices are embodied in the construction of ACRIM and PMOD composites, respectively. The ACRIM composite uses the results for its constituent databases published by the experiment science teams and relates ACRIM1 and ACRIM2 using overlapping Nimbus7/ERB comparisons. The PMOD composite uses a different subset of the satellite TSI database, the ERBS/ERBE ACRIM gap ratio and modifies published Nimbus7/ERB and ACRIM1 results to conform its time series to the predictions of TSI proxy models. There are a number of differences between the ACRIM and PMOD composites but the most important is the trend during solar cycles 21 - 23. The absence of a trend in the PMOD composite has been shown to be an artifact of uncorrected degradation of ERBE results during the gap. The ERBS/ERBE database was significantly affected by uncorrected degradation throughout its observational lifetime and provides a less precise ACRIM gap ratio than the Nimbus7/ERB results. TSI proxy models are not competitive in precision or accuracy with satellite observations. Their use in constructing the PMOD composite convolutes the relatively high uncertainty of the model with the observational data and is therefore less likely to represent the extant TSI observational database with the greatest fidelity. Modifications of TSI results by PMOD obfuscates the relationships between the original observations' SI calibrations and traceabilities and the composite's representation of the TSI time series. The ACRIM TSI composite's use of original results from satellite experiment science teams likely provides the most accurate representation of the extant TSI satellite measurement database and therefore, of TSI variability knowledge. The TSI record has been sustained by overlapping, redundant experiments using their level of measurement precision to sustain longer term traceability. This TSI monitoring strategy is essential for continuity in the future because the uncertainty of current satellite sensors (~ 0.1 %) is an order of magnitude too large to detect subtle long term TSI variations of potential climate change significance.
UR: http://www.acrim.com
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1622 Earth system modeling (1225)
DE: 1626 Global climate models (3337, 4928)
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 7538 Solar irradiance
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting