In this section variations in total solar irradiance and how these translate into radiative forcing are described and potential mechanisms for amplification of solar effects are discussed. The detection of solar effects in observational records is covered in Chapters 2 and 12.

6.11.1.1 The observational record

Figure 6.4: Measurements of total solar irradiance made between 1979 and 1999 by satellite, rocket and balloon instruments (http://www.pmodwrc.ch/solar_const/solar_const.html).

The fundamental source of all energy in the climate system is the Sun so that variation in solar output provides a means for radiative forcing of climate change. It is only since the late 1970s, however, and the advent of space-borne measurements of total solar irradiance (TSI), that it has been clear that the solar “constant” does, in fact, vary. These satellite instruments suggest a variation in annual mean TSI of the order 0.08% (or about 1.1 Wm^-2) between minimum and maximum of the 11-year solar cycle. While the instruments are capable of such precision their absolute calibration is much poorer such that, for example, TSI values for solar minimum 1986 to 1987 from the ERB radiometer on Nimbus 7 and the ERBE experiment on NOAA-9 disagree by about 7 Wm^-2 (Lean and Rind, 1998). More recent data from ACRIM on UARS, EURECA and VIRGO on SOHO cluster around the ERBE value (see Figure 6.4) so absolute uncertainty may be estimated at around 4 Wm^-2. Although individual instrument records last for a number of years, each sensor suffers degradation on orbit so that construction of a composite series of TSI from overlapping records becomes a complex task. Figure 6.4 shows TSI measurements made from satellites, rockets, and balloons since 1979.

Willson (1997) used ERB data to provide cross-calibration between the non-overlapping records of ACRIM-I and ACRIM-II and deduced that TSI was 0.5 Wm^-2 higher during the solar minimum of 1996 than during solar minimum in 1986. If this reflects an underlying trend in solar irradiance it would represent a radiative forcing² of 0.09 Wm^-2 over that decade compared with about 0.4 Wm^-2 due to well-mixed greenhouse gases. The factors used to correct ACRIM-I and ACRIM-II by Willson (1997) agree with those derived independently by Crommelynk et al. (1995) who derived a Space Absolute Radiometric Reference of TSI reportedly accurate to ± 0.15%. Fröhlich and Lean (1998), however, derived a composite TSI series which shows almost identical values in 1986 and 1996, in good agreement with a model of the TSI variability based on independent observations of sunspots and bright areas (faculae). The difference between these two assessments depends critically on the corrections necessary to compensate for problems of unexplained drift and uncalibrated degradation in both the Nimbus 7/ERB and ERBS time series. Thus, longer-term and more accurate measurements are required before trends in TSI can be monitored to sufficient accuracy for application to studies of the radiative forcing of climate.

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