An introduction to climate change - Climate Change: Biological and Human Aspects - page 28

imminent and that this global cooling might even threaten civilisation. (This concern
was fuelled further by sulphur aerosol pollution in the early and mid-20th century.)
More research was required into the strengths of the various factors influencing (that
is, forcing) climate. It was realised in the 1980s that, on balance, the factors were
warming our planet. Factors such as the current human generation of greenhouse
gases were adding to natural warming processes and so were greater than the various
cooling factors. The question that remained was how great a warming could we expect
from our fossil fuel generation of carbon dioxide and how would this compare against
the current range of other climatic factors? This was the subject of the first IPCC
report, published in 1990.
1.6 Non-greenhouseinluencesonclimate
Milankovitch variation and the changing sunlight-reflecting properties (called
albedo) of ice caps (and indeed the solar-reflecting differences of any surface)
demonstrate that non-greenhouse considerations do play a part in climate change.
They can, through feedback cycles, either increase or decrease the magnitude of
change. They can also superimpose their own variability imprint on climate.
However, it is now accepted by nearly all within the climate community that the
anthropogenic addition of greenhouse gases is the key factor determining current
global warming. But this does not mean that these other, non-greenhouse, factors are
not taking place and have no effect on climate.
One such factor in particular has caused some controversy, although it is generally
accepted to have a minor effect compared to current anthropogenic climate change.
This is the variation in solar output, or changes in the Sun's intensity, which has had
a major part to play over the three billion years or so of evolution of the Earth's
biosphere. As we shall see in Chapter 3, the Sun, being a main-sequence star, is
growing significantly warmer over a scale of hundreds of million to billions of years.
This has considerable implications when it comes to elucidating the evolution of
the biosphere. However, on shorter timescales of hundreds or thousands of years it
is comparatively (but not entirely) stable. On even shorter timescales the 11-year
sunspot cycle has an impact. But this last effect is small, causing a variation in
irradiance of only 0.08%, which is too small to have much effect: such changes may
affect the global climate by 0.02-0.4 ◦ C. Changes in sunspot activity do seem to tie
in with similar patterns of change in global temperature, but these are superimposed
on larger climate changes determined by other factors (Foukal et al., 2004).
The question of whether the Sun is largely responsible for current global warming
came about because the Sun became slightly more active during the 20th century.
During much of this time the Earth's temperature rose and fell almost simultaneously
with changes in solar activity. There is also the question of the so-called Little Ice
Age and solar activity, and we will return to this in Chapters 4 and 5.
Since 1978 we have been able to take space-borne measurements of solar output
and correlate them with sunspot activity, for which we have previously only had an
observational record going back a few centuries. The relationship is not clear but there