2
Plan Review

The NOAA Working Group Report (Appendix B) outlines the TCTE mission. This mission is the NOAA approach to address the impending gap in TSI measurements that is expected to occur with the imminent end of SORCE and the late 2016 to early 2017 launch of TSIS on the JPSS FF-1 mission. This approach evolved from two studies conducted by Greg Kopp and Judith Lean for NOAA.1 In its review of the TCTE proposal, the Committee considered the information contained in all three documents (the NOAA Working Group Report, Study A, and Study B). Studies A and B arose from the involvement of Kopp and Lean in a LASP/NRL/NIST team that successfully competed for a 3-year award from the NOAA Climate Data Record (CDR) Program in 2009. Following the GLORY launch failure, Kopp and Lean offered to conduct the two studies as part of their contribution to that project. The NOAA Working Group Report reviewed by the Committee was an abbreviated summary of these two reports plus a very brief description of the TCTE mission and even briefer description of the accommodation of the TIM on the STPsat-3. Much of the analysis that frames the rationale for TCTE concerned the impact of a gap in the TIM data record that started with the TIM launched on SORCE in 2003 (January 25). The NOAA data record as perceived, however, extends back to earlier satellite measurements as summarized below. The perspective of the impending gap on this longer record was not quantitatively addressed in the studies.

TOTAL SOLAR IRRADIANCE DATA RECORD

An approximate two-year gap occurred between ACRIM I and ACRIM II (see upper panel of Figure 2.1).2 This hiatus in the ACRIM data record offers an ideal test case to gain some understanding of the influence of a gap on the construction of a TSI climate data record. Over the past several years, different procedures have been developed by three different groups to address this issue and to construct a composite time series (see lower panels of Figure 2.1) using ERBE data or the Nimbus HF data (sometimes augmented by TSI models based on proxy data) to fill the gap. Clearly the details of how the data are combined matter. Comparison between the composites of the change of levels of irradiance during solar

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1 The two Kopp and Lean studies, Uncertainties Spanning Potential SORCE/TIM to JPSS/TIM Gap (2011) and The Solar Climate Data Record: Scientific Assessment of Strategies to Mitigate an Impending Gap in Total Solar Irradiance Observations between the NASA SORCE and NOAA TSIS Missions (2013), will be referred to as Study A and Study B, respectively. Both are provided in Appendix C.

2 ACRIM is the Active Cavity Radiometer Irradiance Monitor instrument and is part of NASA’s Earth Observing System program.



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2 Plan Review The NOAA Working Group Report the TIM on the STPsat-3. Much of the (Appendix B) outlines the TCTE mission. analysis that frames the rationale for TCTE This mission is the NOAA approach to concerned the impact of a gap in the TIM address the impending gap in TSI data record that started with the TIM measurements that is expected to occur with launched on SORCE in 2003 (January 25). the imminent end of SORCE and the late The NOAA data record as perceived, 2016 to early 2017 launch of TSIS on the however, extends back to earlier satellite JPSS FF-1 mission. This approach evolved measurements as summarized below. The from two studies conducted by Greg Kopp perspective of the impending gap on this and Judith Lean for NOAA.1 In its review of longer record was not quantitatively the TCTE proposal, the Committee addressed in the studies. considered the information contained in all three documents (the NOAA Working Group Report, Study A, and Study B). TOTAL SOLAR IRRADIANCE DATA Studies A and B arose from the involvement RECORD of Kopp and Lean in a LASP/NRL/NIST team that successfully competed for a 3-year An approximate two-year gap occurred award from the NOAA Climate Data Record between ACRIM I and ACRIM II (see upper (CDR) Program in 2009. Following the panel of Figure 2.1).2 This hiatus in the GLORY launch failure, Kopp and Lean ACRIM data record offers an ideal test case offered to conduct the two studies as part of to gain some understanding of the influence their contribution to that project. The of a gap on the construction of a TSI climate NOAA Working Group Report reviewed by data record. Over the past several years, the Committee was an abbreviated summary different procedures have been developed by of these two reports plus a very brief three different groups to address this issue description of the TCTE mission and even and to construct a composite time series (see briefer description of the accommodation of lower panels of Figure 2.1) using ERBE data or the Nimbus HF data (sometimes augmented by TSI models based on proxy 1 data) to fill the gap. Clearly the details of The two Kopp and Lean studies, Uncertainties Spanning Potential SORCE/TIM to JPSS/TIM how the data are combined matter. Gap (2011) and The Solar Climate Data Record: Comparison between the composites of the Scientific Assessment of Strategies to Mitigate an change of levels of irradiance during solar Impending Gap in Total Solar Irradiance Observations between the NASA SORCE and 2 NOAA TSIS Missions (2013), will be referred to ACRIM is the Active Cavity Radiometer as Study A and Study B, respectively. Both are Irradiance Monitor instrument and is part of provided in Appendix C. NASA’s Earth Observing System program. 7

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8 Review of NOAA WG Report on Long-term Sa w W L atellite Total S Solar Irradian Observatio nce on FIGU 2.1 Uppe Panel The daily averaged values of TS from radio URE er d d SI ometers on dif fferent space platfoorms since Noovember 1978 HF on Nim 8: mbus7, ACRIM I, ERBE, AC M CRIM II, VIR RGO, ACRIM M III, an TIM on SO nd ORCE. The da are plotted as published by the corre ata d d esponding ins strument teams Lower Pane The PMOD ACRIM an IRMB com s. els D, nd mposite TSI as daily values plotted in s different colors to indicate the data sources used in the composite. SOU d u URCE: PMOD (ACRIM is D. s the Active Cavity Radiometer Ir R rradiance Monitor, ERBE i the Earth R is Radiation Bud dget Exper riment, VIRG is the Vari GO iability of sola Irradiance a Gravity O ar and Oscillations).

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Plan Review 9 TABLE 2.1 Climate Data Record Requirements for the JPSS/TIM. SOURCE: NOAA Working Group Report. Parameter CDR Requirement Absolute Accuracy 0.01% (100 ppm; 0.14 Wm-2) Stability (long-term precision) 0.001%/yr (10 ppm/yr; 0.014 Wm-2/yr) Noise (short-term precision) 0.001% (10 ppm; 0.014 Wm-2) Note: 1 ppm ~ 0.0014 Wm-2 minimum periods can be taken as a measure of COMMITTEE FINDINGS AND overall precision of the record. The differences CONCLUSIONS between the three reconstructions alone are greater than the 10 ppm/yr precision specified in Question 1: Does the plan appropriately Table 2.1 and have resulted in considerable reflect the scientific content of the debate as to the Sun’s role in warming over the decade 1986 to 1996 and misinterpretation of the commissioned papers? influence of the Sun on recent global warming. Findings Because the stringency of the CDR 1. The Committee found that the plan requirements (Table 2.1) severely limit faithfully followed the Kopp and Lean solutions for filling a gap, the Committee studies. The plan also displayed an determined that it was important to take a admirable degree of nimbleness in step back and revisit these CDR reacting to a pressing need to fill an requirements themselves. The traceability of impending CDR gap. The solution the TSI requirements appears to arise out of presented was a creative, rapid, and low- an understanding of the variability of the cost response that exploited the Sun rather than from an understanding of availability of an existing engineering Earth climate system variability and change instrument model, and heritage in (Kopp, 2011). The climate-driven engineering, mission architecture, and requirements first began to emerge in Solar data analysis. Studies A and B Influences on Global Change (NRC, 1994) themselves also provide a useful analysis and can be traced through a series of that furthers our understanding of the documents (Box 2.1). Since then much more performance of existing space-borne TSI has been learned about the variability of measurements. Earth’s energy imbalance (Loeb et al., 2012) and about how to define requirements Given published information on instrument tailored for climate change detection accuracy and stability available as of the (Wielicki et al., 2013). Box 2.2 later in this dates of the Kopp and Lean studies and the chapter revisits the TSI CDR requirements NOAA working group report, the formulated within the context of this new Committee considers the review of gap- understanding. filling alternatives to be fair. While the plan did reflect the scientific content of Studies A

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10 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation BOX 2.1 Traceability of Total Solar Irradiance Measurement Requirements Requirements for climatically useful TSI measurements can be traced to discussions in Solar Influences on Global Change (NRC, 1994). The National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Integrated Operational Requirements Document (IORD-I; NPOESS, 1996) specified threshold (minimum success) and objective (goal) accuracies of 0.1% and 0.035% and precisions of 0.002% yr-1 and 0.0005% yr-1, respectively. An NRC workshop (NRC, 2000) changed the accuracy objective to 0.01%, restated precision threshold and objective as 0.002% and 0.001% respectively, and added stability threshold and objective values of 0.002% yr-1 and 0.0005% yr-1, respectively. These values were formalized in IORD-II (NPOESS 2001). A multiagency workshop (Ohring, 2007) confirmed the accuracy objective of <0.01% and recommended a stability objective of <0.001% yr-1. A report by Datla et al. (2009) quoted the less stringent threshold values in IORD-I for accuracy and stability. Given the inconsistency of the values, transition from research to operational requirements, unclear justifications for the values, and the emergence of a Climate Data Record Project at NOAA, a workshop was held in 2011 to sort out these issues (LASP, 2011). When NPOESS was restructured in 2010, NOAA became responsible for JPSS. The original Level 1 requirements for the TSI-measuring part of the TSIS package selected to fly on JPSS-1 stated minimum success accuracy and stability of 0.35% and 0.035% yr-1 and goals of 0.01% and 0.001% yr-1, respectively (Viereck and Denig, 2011). The present Level 1 threshold requirement values are listed in Table 2.1; objective values are one-half of the values in Table 2.1 (Viereck and Denig, 2011). Justification for the values was originally driven by instrument capability but is now primarily based on present understanding of long- term solar variability and a need to detect <0.1% long-term TSI changes in a century (Kopp, 2011). and B, as presented it lacked certain essential Question 2: Does the potential alternate information needed to determine the method in the plan maintain the integrity strength of the proposal. No reliability of the data record? information (probability of TCTE surviving until the launch of JPSS FF-1) for the key Findings: mission elements was provided. There was 2.a Study B clearly argues that the CDR no budget and no clear timeline of the requirements can only be met when data funding or continued support from the Air overlap occurs at both ends of the gap. Force was offered. These aspects of the Because no reliability estimates of Committee’s review are discussed in greater mission components or information detail in response to Question 2 below. about mission funding or Air Force support were provided, it was not

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Plan Review 11 possible to assess the real likelihood of provided to the Committee by LASP, the the gap being filled. The Committee drop in probability of TCTE survival concluded that the plan is unable to from one to two years, two to three ensure the integrity of the data record years, and three to four years are all the because as presented it is a 1.5 year plan same: about 0.86 times the previous to fill a 3+ year gap. year’s value. This is typical of spacecraft 2.b The launch of TCTE is currently and instrument reliability estimates. scheduled for October 30, 2013, which Thus, the Committee can estimate the will likely ensure overlap with SORCE. probability of overlap if the launch of Although not presented to the JPSS/TSIS is delayed beyond 2017. If the Committee, it appears the probability is launch slips to 2018, the probability of high of SORCE operating beyond the overlap drops to 0.48. If it slips to 2019, launch of TCTE, thus providing critical the probability of overlap drops further overlap at the front end of the gap. This to 0.41.3 determination was based on information 2.d The NOAA Working Group report did provided by the Spring 2013 SORCE not provide information on funding senior review proposal made available to support for the TCTE mission and the Committee by NASA (Woods, stated that NOAA is working with the 2013). A 1.5 year collection of data on Air Force to ensure operation of the orbit however leaves a gap of more than STPSat-3/TCTE mission for as long as a year between the stated end of TCTE possible. Continued funding and and beginning of the TSIS on JPSS FF-1. cooperation from the Air Force will be 2.c Although the Committee was not necessary for continued data collection briefed on what elements of the mission and maintaining the integrity of the data limit the lifetime of TCTE, it was able to record. determine that there is high probability that the single-string spacecraft could Question3: Does the plan adequately operate beyond 3 years. Independent summarize the strengths and weaknesses information provided to the Committee of the proposed approach? by the spacecraft provider indicated that the single string baseline STPSat-3 type Finding: bus reliability for 3 years on-orbit is 3 Taken together, the three documents above 0.80 and drops to 0.75 for 4 years provided a balanced discussion of on orbit (personal communication to strengths and weakness of the proposed Committee Chair). Estimates provided method to fill the TSI gap and to the Committee by LASP indicate that the TIM instrument reliability is 0.80 at 3 years and 0.74 at 4 years. Thus the 3 The probability of TCTE maintaining data likelihood of achieving 4 years of data collection for four years is 0.56. The probability on orbit and thus overlap with the of survival drops by a factor of about 0.86 each JPSS/TSIS (scheduled to launch in late year. Thus the probability of TCTE surviving five 2016 to early 2017) is 0.56, or slightly years (until 2018) is 0.56 × 0.86 = 0.48. The better than 50%. In the reliability data probability of TCTE surviving six years (until 2019) is 0.48 × 0.86 = 0.41.

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12 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation recognized fully the limitations of the length. With overlaps, provided by proposed TCTE gap filler. In developing intervening measurements or models, the the plan, the analysis of the stability drift between the two TIM measurements uncertainty estimates for ACRIM3, can be estimated and the uncertainty VIRGO, and PREMOS in comparison to reduced. Figure 2.2 illustrates how longer SORCE/TIM was essential in the overlaps and shorter gaps enhance this formulation of TCTE. The comparative effect; for example with a gap of 3 year stability of different instruments and duration an overlap of more than 0.5 years empirical models summarized in Table will improve uncertainty over that which 2.2 are based on comparisons of the would be obtained by relying on absolute instrument data records relative to those accuracy alone. The improvement is limited of the SORCE/TIM and from changes for short (< ~10 days) overlaps by noise in between each instrument’s successive the measurements. Figure 2.3 provides data versions. The Committee was analogous illustrations of alternative gap- however aware that it was only being filling methods. presented a TIM-based gap-filling Because filling the TSI data gap with the concept and then only the TCTE TCTE is not assured, it was prudent to concept. The Committee considered examine other options.4 In particular, material from the NOAA Working models of TSI based on proxy data (e.g., Group Report, Kopp and Lean Studies A sunspot darkening and facular brightening) and B, the SORCE and ACRIM-3 NASA can, in principle, be used to fill a gap Senior Review proposals, existing depending on the demonstrated literature on TSI observations, and past performance of the model, duration of the changes in TSI records needed to correct gap, and availability of high-quality proxy instrument artifacts. Overall, the data. In Study B several models were arguments in favor of TIM as the constructed based on linear regression of current best reference were the most various combinations of proxy data to the compelling. The TIM instruments for TIM TSI observations (2003-2012). The best example show one-third the on-orbit agreement was an NRL model based on a degradation of that for ACRIM, and sunspot blocking index derived from reduced systematic noise during quiet ground-based white light images and a Mg II sun periods. The use of TIM as the spectral line ratio index derived from reference leads directly to the results in SORCE/SOLSTICE observations. This Table 2.2 and Figure 2.2. model had a correlation coefficient of 0.961 and thus fits 92.4% of the observed TSI Figure 2.2 illustrates how the measurement variance (Study B). The uncertainty in TSI introduced by the presence of a gap is influenced by a number of parameters. Based only on the absolute 4 It is beyond this Committee’s charge to accuracy of the instruments, shown in prioritize other options (e.g., proxy data models Figure 2.2 as the dashed line at ~360 ppm versus other space instruments). However, the representing the accuracy of SORCE/TIM, Committee does note that NOAA would be wise the uncertainty does not depend on gap to utilize all available data resources to fill the gap.

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TABL 2.2 Instrum LE ment and Mo Performa odel ance Estimate SOURCE: Kopp and Lean Study B. es. Instrumen nt Stated Accur racy (ppm) Stability ((ppm/yr) Noise (ppm) ) SOR RCE/TIM 3550 100 4 b ACR RIMSat/ACRI IM3 10000 71 34 SoHO/VIRGO 25000 299 28 PICA ARD/PREMO OS 3000 799 <52 JPSSS/TCTE/TIM 3550 100 50a JPSSS/TSIS/TIM 1000 100 10 NRL Model L -NAA- 48 38 SFO Model -NAA- 88 19 a Effe ective noise du to orbital sampling at ti ue s imes of high s solar activity. Instrument n noise is less than 10 ppm. n b The ACRIM Sen Review Pr e nior roposal (Willson, 2013) sta that ACR ates RIM3 results h have demmonstrated a trraceability un ncertainty of < 5 ppm/yr an that LASP/ nd /TRF calibrat tion will resul lt in ac ccuracy impro ovement. If th performa hese ance improve ements prove to be true, AC CRIM could be a viable alterna ative for filling the data gap p. FIGU 2.2 General effects con URE ntributing to uncertainties in filling a TSI data gap. S Stability uncerrtainties incre with gap duration, lim ease miting accurac measureme noise limi the cy; ent its uncerrtainties at sho gap durati ort ions. Longer overlap durattions with pri (SORCE/T ior TIM) and follow wing (JPSS/TI instrumen improve knowledge of stability of, a agreemen with, IM) nts k f and nt interv vening gap-fil lling instrume or model, decreasing u ent uncertainties. SOURCE: NO OAA Work king Group Re eport. (This figure, with ac f ccompanying explanation is in Appendi B.) g ix 13

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14 Review of NOAA WG Report on Long-term Sa w W L atellite Total S Solar Irradian Observatio nce on FIGU 2.3 Estim URE mated uncertainties as a fun nction of gap and overlap dduration for g gap-filling altern natives: ACRIM (top left), VIRGO (top right), PREM (bottom left), and NR model M3 , p MOS m RL (botto right). In all four cases, longer gap durations and shorter overl times incr om , d d lap rease uncerrtainties. SOU URCE: Kopp and Lean Stud A. (This fi a dy companying explanation i igure, with acc is in Appendix C.) SOLSTICE data wi not outlive data from ill Stud B. Ball et al (2011) comp dy l. pared TSI TIM so models bas on non-SO s sed ORCE data moddeled with SATIRE-S (base on sunspot ed t are reelevant. The best correlation reported in n imag and magn ges netic field meaasurements) Study B for such a model was 0.942, which y with observed TIM data. For th period h M he fits 88 5 of the TIM variance. 8.7% T 20033-2009 they foound a correla ation of 0.984 4 The Committe considered two T ee (97% of variance fitted). Chapm et al. % man publis shed compari isons not included in (201 2), using a di ifferent set of ground-based d prox observation for the period 2003- xy ns 2010 found a slig 0, ghtly smaller ccorrelation off 5 This value was calcu ulated by the committee. It is c s 0.974 (95% of var 4 riance fitted) w TIM with simply the square of the correlation coefficient y f n data . These two empirical mod have dels report in Study B. This value, the coefficient of ted e f recen been com ntly mpared with t PMOD the determ mination, gives the percentage of the total s e com posite TSI ind Ball et al (2012) for dex. l. variati that is expl ion lained by the model m the p period 1996-2 2008 obtained a correlation d n (Wack kerly et al., 2008).

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Plan Review 15 of 0.981 and Chapman et al. (2013) found approach the CDR values required for 0.96 for 2003-2010. detecting solar variability. The TCTE It appears that by using different proxy provides an early test of this. It will be the data sources than those of Study B, the first TIM to fly that was calibrated against agreement between empirical models of TSI the TRF (the PREMOS-A active cavity and observations may be improved so that radiometer instrument was calibrated to 280 ≥95% of the TSI variance can be fit with a ppm in 2010), and thereby potentially one of model. Combining such models with the the most absolute accurate measurements of instrumental approaches suggested in the TSI to date. The comparison of the TCTE Working Group Report would greatly TIM TSI value with all the others will improve the chances of successfully bridging provide a highly accurate tie point, and will a TSI gap. However, as noted in Study B, validate the SORCE TIM and PREMOS there is no certainty that the sources of values. This provides additional incentive to proxy data for the models will be available fly the mission, even if it does not fill the during the TSI gap. Declining funding, age, gap. and looming closures threaten both ground- and space-based sources of synoptic solar Question 4: Do the background documents observations. Nor are resources readily and plan together fully explore the available to improve the quality of or add implications of loss of, or changes in, new sources of solar data useful for TSI measurement on the understanding of modeling. Earth’s climate system and processes? A hypothetical alternative method for future gap filling not explicitly discussed Finding: adequately by the plan is reliance on 4 The Committee was not initially absolute accuracy. Now that LASP has the convinced that the requirements as TSI Radiometric Calibration (TRF) facility posed (Table 2.1) represent whereby it can provide a reported pre-flight requirements that were derived from absolute calibration of the TIM instruments those relevant to understanding climate to about the 200 ppm to 300 ppm level change. The Committee’s research on (personal communication from G. Kopp to the source of the requirements given the Committee), it seems that not only the suggests they derive from empirical TSIS TIM, but also the TCTE TIM and all knowledge of solar variability and subsequent TIMs (assuming maintenance of instrumental capability and are less the TRF scale), should be calibrate-able to related to the energetics of the Earth this level. The Committee notes that these system (Box 2.1). To determine the results have not been peer reviewed, but if implications of these requirements on this uncertainty can in fact be achieved and the understanding of the Earth’s climate could be improved in the future to 100 ppm, system, the Committee considered two we could be more tolerant of gaps, because different pathways for setting these comparison of any pair of sufficiently long requirements based on climate (to overcome short-term random effects) sensitivity, rather than on solar segments of non-overlapping data, each with variability or instrumental capability. absolute uncertainty at this level, would These are summarized in Box 2.2.

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16 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation Box 2.2 TSI Accuracy Requirements for Understanding Anthropogenic Climate Change Approach I: The analysis below is based on the arguments for the measurement requirements of the CLARREO mission (Wielicki et al., 2013) and revolves around certain assumptions of future climate change. Dangerous climate change is internationally agreed to be ~ 2 °C over the long term (Copenhagen Accord, 2009). Best current estimate of equilibrium (long term) climate sensitivity is approximately 3 °C for an anthropogenic forcing of ~ 4 Wm-2 associated with a nominal doubling of carbon dioxide (Andrews et al., 2012). Thus a 2 °C warming corresponds to a radiative forcing of roughly 2.7 Wm-2 [(2 °C/3 °C)*4 Wm-2]. Because separation of natural from anthropogenic radiative forcing is required for adequate scientific understanding of the Earth’s climate system and processes, long term radiative forcing due to changing TSI should be known to at least 10 percent of the level of dangerous climate change with 95 percent confidence (2σ) in order to clearly separate anthropogenic signal from natural variability (e.g. Wielicki et al., 2013).a This suggests a knowledge of TSI changes to levels that can cause less than or equal to 0.13 Wm-2 (1σ) in radiative forcing of the climate system. Changes in TSI relate to changes in climate radiative forcing by ΔF = 0.7 ΔS /4, where ΔS is the change in TSI. The factor of 4 is the ratio of the Earth’s cross-sectional area to its surface area and the factor of 0.7 is the global average solar absorption of Earth (1 - albedo). Or equivalently, ΔS = 5.7 ΔF. This implies that the long term change in TSI should be known to 5.7 * 0.13 = 0.74 Wm-2 or less. The time interval for this long term trend in TSI should be roughly that over which current anticipated anthropogenic radiative forcing would reach the 2 °C warming level. Climate model simulations of doubled CO2 radiative Coincidentally, the outcome of the affect understanding of Earth’s climate calculations made by the Committee because TSI, and its variations, play a agrees with the pre-defined fundamental role in determining global requirements in Table 2.1. Hence, given average temperature. However, there is that the plan and the background increasing evidence that variations in solar documents together made their ultraviolet (UV) radiation contribute to recommendations based on these regional and seasonal climate (Gray et al., requirements, the Committee considers 2010). In fractional terms UV changes are that these documents, to an appreciable much larger than those in TSI and these extent, explored the implications of loss directly affect the temperature and of, or changes in, TSI measurements on composition of the stratosphere, where UV the understanding of Earth’s climate is predominantly absorbed. To fully system and processes. appreciate the potential of the Sun in regional climate change it is therefore This Committee was charged to address essential that measurements of spectrally- how the loss of TSI data specifically would resolved radiance are maintained in parallel

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Plan Review 17 forcing typically use a roughly 1 percent increase in CO2 per year for a 70 year doubling time (Cubasch et al., 2001). This suggests that the rough time scale for the 2 °C warming for our metric is then (2 °C/3 °C)*70 years or ~ 50 years. A long term trend in TSI to be discernable above anthropogenic climate forcing is then 0.74 Wm-2 / 50 years = 0.015 Wm-2/yr. Since TSI ~ 1361 Wm-2 (Kopp and Lean, 2011), then a 1000 ppm/yr (0.1 percent) trend is 1.4 Wm-2/yr, and the 0.015 Wm-2/yr trend corresponds to an 11 ppm/yr trend in TSI. This value of 11 ppm/yr is similar to the current JPSS requirement of 10 ppm/ yr stability for TSI observations. Another way to look at the above requirement is that the 50 year change in TSI should be uncertain to less than 0.75 Wm-2 (1σ) which is equivalent to ~500 ppm. For the JPSS 100 ppm absolute accuracy requirement (1σ), the absolute accuracy requirement is ~ 1/5th the total trend uncertainty desired over 50 years. This level of accuracy would also provide a climate record that is robust to data gaps, unlike our current record. Approach II: A second approach for setting the requirement relevant to climate change studies is to place the TSI change within the context of the global net radiation. Current estimates of ocean heat storage indicate that the global net radiation is ~ 0.5 Wm-2 (Loeb et al., 2012). TSI is one component of global net radiation, and its absolute accuracy should be no worse than 10 percent of global net radiation (2σ), or 0.025 Wm-2 (1σ). Using the same relationship between TSI and radiative forcing of the Earth’s climate, this results in a TSI absolute accuracy requirement for global net radiation of (0.025)(5.7) = 0.15 Wm-2, or ~100 ppm (0.01 percent) in TSI. The JPSS requirement is 100 ppm for absolute accuracy and thus is consistent with a requirement based on energy imbalance and long term ocean heat storage. a 2σ represents two standard deviations from the mean of a Gaussian normal distribution. Approximately 95% of the distribution is contained within two standard deviations of the mean. This is a commonly used confidence boundary in statistics (Wackerly et al., 2008). with those of TSI. of solar variability in influencing Earth’s The TSI CDR has two components. climate. Thus the focus was directed to the There is the shorter, but more accurate shorter, more accurate TIM era record and record of the TIM era with SORCE/TIM- no real discussion of the full TSI CDR and level data quality. There is also the full (33+ its stewardship was offered. Although this year) space-based TSI measurement record. existing satellite data record, dating back to This longer record, although not of the 1978, currently fails to meet the stability quality of the TIM era record, is still requirements of Table 2.1, the Committee important to preserve. The NOAA Working believes it is also important to maintain the Group Report focused on the impending TSI stewardship of the entire record and place gap that is to occur because of end of life of the current TIM era data and impending SORCE and the failure of the GLORY gaps within the context of the longer data mission. Much of the working group report record. NOAA’s plan does not ensure emphasized the importance of maintaining continuity for the TIM-era record, but is an unbroken record of TSI to inform more likely to ensure continuity of the full ongoing debates regarding the potential role 33 year record.

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18 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation ADDITIONAL THOUGHTS NOAA found was severely constrained by cost, schedule, and programmatic While the Committee was charged with challenges. The TCTE solution is not evaluation of NOAA's response to an optimal in a scientific sense given roughly 50 impending gap in TSI observations for percent likelihood of successful overlapping climate research, NOAA's response and its observations in the best case scenario of an appropriateness can only be understood in on-time JPSS FF-1 launch combined with an light of how it fits into the larger context of absence of any programmatic issues in climate observations. Unlike for weather NOAA and Air Force collaboration on observations, there is no U.S. or extending the TCTE mission life from 2 to 4 international climate observing system. As a years. Yet in the context of the lack of a result, monitoring of climate change climate observing system, the TCTE solution including TSI is a necessarily ad hoc, high can be considered optimal within the risk, and loosely coordinated activity across constraints present. We should expect the 13 U.S. agencies of the U.S. Global similar issues in the future with many of the Change Research Program (USGCRP). The approximately 50 essential climate variables. USGCRP has the responsibility for climate There have been some recent reports that change research, but has no authority over show a recognition of this challenge, in agency actions, nor budget to deal with particular the U.S. NSTC "National Strategy observing system issues like those that arose for Civil Earth Observations" (April, 2013), for TSI when the NASA Glory mission and the international "Strategy Towards an launch vehicle failed March 3, 2011. Glory Architecture for Climate Monitoring from was to be the end of NASA leading TSI Space" (January, 2013). Development of a observations from space, with the NOAA national and international climate observing JPSS weather satellite system beginning the system could be the long term solution to next set of observations in 2016. As a result climate monitoring challenges like TSI. of the launch failure, the solution that

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Appendixes

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