6.11 Solar Forcing of Climate
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 Total Solar Irradiance
220.127.116.11 The observational record
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.