Solar irradiance provides the only significant source of energy input to the climate system and its variability has the potential to either mitigate or exacerbate anthropogenic change. Maintaining an unbroken record of Total Solar Irradiance (TSI) is critical in resolving ongoing debates regarding the potential role of solar variability in influencing Earth’s climate.
Space-borne instruments have acquired TSI data since 1978. Currently, the best calibrated and lowest noise source of TSI measurements is the Total Irradiance Monitor (TIM) onboard NASA’s Solar Radiation and Climate Experiment (SORCE). These TIM-era data are of higher quality than the older data in the full record. Thus, the TSI climate data record (CDR) has two components. There is the shorter, but more accurate record of the TIM era and the full (33+ year) space-based TSI measurement record. Both are important and require preservation.
SORCE is well past its design life and is encountering significant battery degradation. The Total Solar Irradiance Sensor (TSIS), a dual-instrument package that will be flown on the Joint Polar Satellite System (JPSS) Free Flyer 1 (FF-1) mission to continue the TSI record, is not scheduled to launch until late 2016 or early 2017, creating the potential for a data gap. Unfortunately, NASA’s Glory spacecraft, which carried a TIM that would have provided a gap filler, failed to reach orbit on March 4, 2011. Without a mitigation plan, the continuity of both data records (the TIM-era and full record) is threatened.
In examining options to avoid a gap in the record between SORCE and JPSS TSIS measurements, scientists at the Laboratory for Atmospheric and Space Physics (LASP), working with NASA and NOAA officials, identified an opportunity to include a TSI Calibration Transfer Experiment (TCTE) module, which includes a TIM sensor and electronics box, on an upcoming Air Force STPSat-3 launch. In early 2013, a NOAA Working Group developed a plan that describes how the agency anticipates mitigating the measurement gap through the TCTE mission.
The Committee found that the 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.
The CDR requirements can only be met when TCTE data overlap occurs at both ends of the gap (with both SORCE/TIM and JPSS FF-1/TSIS). The focus of the NOAA plan was on the shorter, more accurate TIM-era record, and the Committee concluded that the plan is unable to ensure the integrity of the TIM-era data record because as presented it is a 1.5 year plan to fill a 3+ year gap. The launch of the TCTE is currently scheduled for October 30, 2013, which will
likely ensure overlap with SORCE. A 1.5 year collection of data on orbit however leaves a gap of more than a year between the stated end of TCTE and beginning of the TSIS on JPSS FF-1. Based on data furnished by the spacecraft provider and LASP, the Committee found the likelihood of achieving overlap with the JPSS/TSIS is 0.56, or slightly better than 50 percent. If the launch of the JPSS FF-1 is delayed, the probability of overlap will decrease. Although the TCTE mission will not ensure continuity of the TIM-era data record, it is more likely to ensure continuity of the full, lower quality data record. The Committee does note that NOAA would be wise to utilize all available data resources (e.g., TCTE, other instruments, proxy models) to fill the gap.
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 TCTE. The Committee was not initially convinced that the CDR requirements as posed represent requirements that were derived from those relevant to understanding climate change. The Committee’s research on the source of the requirements given suggests they derive from empirical knowledge of solar variability and instrumental capability and are less related to the energetics of the Earth system. To determine the implications of these requirements on the understanding of the Earth’s climate system, the Committee considered two different pathways for setting these requirements based on climate sensitivity, rather than on solar variability or instrumental capability. Coincidentally, the outcome of the calculations made by the Committee agrees with the pre-defined CDR requirements. Hence, given that the recommendations were based on these requirements, the Committee considers that the 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.
