Expert Report

Review of NOAA Working Group Report on Maintaining the Continuation of Long-Term Satellite Total Irradiance Observations (2013)

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Since 1978, space-borne instruments have provided scientists with an unbroken data record on Total Solar Irradiance (TSI)--the input of solar energy to the climate system. This long-term record is critical for resolving ongoing debates regarding the potential role of solar variability in influencing Earth’s climate. However, the continuity of the data is at risk. One set of TSI data collection instruments is now well beyond its design life and suffering from battery degradation problems, and a mission to launch a new instrument package on the JPSS FF-1 is not scheduled until late 2016 or early 2017. Meanwhile NASA’s Glory spacecraft, which would have carried instruments to help avoid a data gap, failed to reach orbit on March 4, 2011.

In early 2013, a NOAA Working Group developed a plan that describes how the agency could mitigate the data gap by including a TSI collection instrument on an upcoming Air Force spacecraft launch. This report reviews that plan, finding that the proposed solution is not optimal in a scientific sense, given that in the best case scenario there is only a 50 percent likelihood of the proposed gap-filling mission being able to extend longer than its 1.5 year timeframe and collect data for 4 years, and thus overlap with the launch of the new instrumentation planned for late 2016 to early 2017. However, with the lack of other options or a dedicated climate observing system, the solution can be considered optimal within the constraints present. The committee also notes that similar issues with many of the approximately 50 other essential climate variables can be expected in the future.

Key Messages

  • The NOAA Working Group plan faithfully followed the scientific content of two studies conducted by Greg Kopp and Judith Lean for NOAA. The solution presented was a creative, rapid, and low-cost response that exploited the availability of an existing engineering instrument model, and heritage in engineering, mission architecture, and data analysis.
  • To ensure continuity of the TSI data record, the proposed mission will need to overlap with both data collected using current methods, and those collected using the new instrument package scheduled for launch in 2016 or 2017. The Committee concluded that the plan is unable to ensure the integrity of the data record because as presented it is a 1.5 year plan to fill a 3+ year gap. However, the NOAA plan focused on the shorter, more accurate data record that began with the current collection instrument. The Committee found that the plan is more likely to ensure the integrity of the longer, but less accurate, data record that began in 1978.
  • Taken together, the NOAA Plan and the Kopp and Lean studies provided a balanced discussion of strengths and weakness of the proposed method to fill the TSI gap and recognized fully the limitations of proposed mission.
  • The Committee considers that the NOAA Working Group plan, to an appreciable extent, explored the implications of loss of, or changes in, TSI measurements on the understanding of Earth’s climate system and processes.