138 Climate changein the twenty-firstcenturyand beyondDoes the Sun’s output change?
Some scientists have suggested that all climate vari-
ations, even short-term ones, might be the result of
changes in the Sun’s energy output. Such sugges-
tions are bound to be somewhat speculative because
the only direct measurements of solar output that are
available are those since 1978, from satellites out-
side the disturbing effects of the Earth’s atmosphere.
These measurements indicate a very constant solar
output, changing by about 0.1% between a maxi-
mum and aminimum in the cycle of solar magnetic
activity indicated by the number of sunspots.
It is known from astronomical records and from
measurements of radioactive carbon in the atmo-
sphere that this solar sunspot activity has, from time
to time over the past few thousand years, shown
large variations. Of particular interest is the period
known as the Maunder Minimum in the seventeenth
century when very few sunspots were recorded.^21
Studies of the recent measurements of solar out-
put correlated with other indicators of solar activ-
ity, when extrapolated to this earlier period, suggest
that the Sun was a little less bright in the seventeenth
century, perhaps by about 0.4% or about 1 W m−^2 in
the average solar energy incident on the Earth’s sur-
face. This reduction in solar energy may have been
a cause of the cooler period at that time known as
the ‘Little Ice Age’. Careful studies have estimated
that since 1850 the maximum variations in the solar
energy incident on the Earth’s surface are unlikely
to be greater than about 0.5 W m−^2 (Figure 6.12).
This is about the same as the change in the energy
regime at the Earth’s surface due to about a ten
years’ increase in greenhouse gases at the current
rate.Radiative forcing (W m−^2)Figure 6.12Radiative forcing due to variations in the energy input from the Sun as estimated by Lean
et al.(1995) and by Hoyt and Schatten (1993).