12.2.3.1 Natural climate forcing
Since the SAR, there has been much progress in attempting to understand the
climate response to fluctuations in solar luminosity and to volcanism. These
appear to be the most important among a broad range of natural external climate
forcings at decadal and centennial time-scales. The mechanisms of these forcings,
their reconstruction and associated uncertainties are described in Chapter
6, and further details of the simulated responses are given in Chapter
8, Section 8.6.3.
Volcanic forcing
The radiative forcing due to volcanic aerosols from the recent El Chichon and
Mt. Pinatubo eruptions has been estimated from satellite and other data to be
-3 Wm-2 (peak forcing; after Hansen et al., 1998). The forcing associated
with historic eruptions before the satellite era is more uncertain. Sato et
al. (1993) estimated aerosol optical depth from ground-based observations over
the last century (see also Stothers, 1996; Grieser and Schoenwiese, 1999). Prior
to that, reconstructions have been based on various sources of data (ice cores,
historic documents etc.; see Lamb, 1970; Simkin et al., 1981; Robock and Free,
1995; Crowley and Kim, 1999; Free and Robock, 1999). There is uncertainty of
about a factor of two in the peak forcing in reconstructions of historic volcanic
forcing in the pre-satellite era (see Chapter 6).
Solar forcing
The variation of solar irradiance with the 11-year sunspot cycle has been assessed
with some accuracy over more than 20 years, although measurements of the magnitude
of modulations of solar irradiance between solar cycles are less certain (see
Chapter 6). The estimation of earlier solar irradiance
fluctuations, although based on physical mechanisms, is indirect. Hence our
confidence in the range of solar radiation on century time-scales is low, and
confidence in the details of the time-history is even lower (Harrison and Shine,
1999; Chapter 6). Several recent reconstructions estimate
that variations in solar irradiance give rise to a forcing at the Earth’s
surface of about 0.6 to 0.7 Wm-2 since the Maunder Minimum and about
half this over the 20th century (see Chapter 6, Figure
6.5; Hoyt and Schatten, 1993; Lean et al., 1995; Lean, 1997; Froehlich and
Lean, 1998; Lockwood and Stamper, 1999). This is larger than the 0.2 Wm-2
modulation of the 11-year solar cycle measured from satellites. (Note that we
discuss here the forcing at the Earth’s surface, which is smaller than
that at the top of the atmosphere, due to the Earth’s geometry and albedo.)
The reconstructions of Lean et al. (1995) and Hoyt and Schatten (1993), which
have been used in GCM detection studies, vary in amplitude and phase. Chapter
6, Figure 6.8 shows time-series of reconstructed solar and volcanic forcing
since the late 18th century. All reconstructions indicate that the direct effect
of variations in solar forcing over the 20th century was about 20 to 25% of
the change in forcing due to increases in the well-mixed greenhouse gases (see
Chapter 6).
Reconstructions of climate forcing in the 20th century indicate that the net
natural climate forcing probably increased during the first half of the 20th
century, due to a period of low volcanism coinciding with a small increase in
solar forcing. Recent decades show negative natural forcing due to increasing
volcanism, which overwhelms the direct effect, if real, of a small increase
in solar radiation (see Chapter 6, Table
6.13).
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