Notes 13950 000 years the solar radiation incident in summer on the polar regions
will be unusually constant so that the present interglacial is expected to
last for an exceptionally long period.^22 Suggestions therefore that the
current increase of greenhouse gases might delay the onset of the next
ice age are unfounded.
These orbital changes only alter thedistributionof incoming solar
energy over the Earth’s surface; the total amount of energy reaching
the Earth is hardly affected by them. Of more immediate interest are
suggestions that the actual energy output of the Sun might change with
time. As we mentioned in Chapter 3 (see Figure 3.8) and as is described
in the box above, such changes, if they occur, are estimatedto be much
smaller than changes in the energy regime at the Earth’s surface due to
the increase in greenhouse gases.
There have also been suggestions of plausible indirect mechanisms
whereby effects on the Sun might influence climate on Earth. Changes
in solar ultraviolet radiation will influence atmospheric ozone and hence
might affect climate. There is a possibilitythat the galactic cosmic
ray flux, modified by the varying Sun’s magnetic field, could influ-
ence cloudiness and hence climate. From studies of these possible
mechanisms there is as yet insufficient evidence of significant climate
effects.^23
Another influence on climate comes from volcanic eruptions. Their
effects, lasting typically a few years, are relatively short-term compared
with the much longer-term effects of the increase of greenhouse gases.
The recent large volcanic eruption of Mount Pinatubo in the Philippines
which occurred in June 1991 has already been mentioned (Figure 5.21).
Estimates of the change in the net amount of radiation (solar and thermal)
at the top of the atmosphere resulting from this eruption are of about
0.5 W m−^2. This perturbation lasts for about two or three years while
the major part of the dust settles out of the atmosphere; the longer-term
change in radiation forcing, due to the minute particles of dust which
last in the stratosphere for somewhat longer, is much smaller.
To summarise this chapter:
The increase in greenhouse gases is by far the largest of the factors
which can lead to climate change during the twenty-first century.
The likely climate changes for a range of scenarios of greenhouse gas
emissions have been described in terms of global average temperature
and in terms of regional change of temperature and precipitation and
the occurrence of extremes.
The rate of change is likely to be larger than any the Earth has seen at
any time during the past 10 000 years.