Climate change - Atmosphere, Weather and Climate - page 394

faculae that is 1.5 times greater than the darkening
effect. There may also be a longer term variation of solar
irradiance, based on data from sun-like stars in states
resembling the Maunder sunspot minimum (1645
to 1715). These variations in solar irradiance appear to
have largely determined global temperature fluctuations
until the mid-nineteenth century, and about half of
the warming between 1860 and 1950. Since the 1970s,
anthropogenic forcing has accounted for half of the
changes, solar forcing around 30 per cent and internal
variability 10 to 20 per cent. The mechanisms involved
in solar forcing are unclear. It is suggested by David
Rind that, over a wide range of timescales, direct solar
forcing is a relatively minor component of climate
change compared with the its potential triggering of
interactions involving a variety of feedback processes.
There appear to be regional patterns in the temperature
response to solar variability with the largest signals in
low latitudes where there are large insolation totals and
over oceans where the albedo is low. Hence, maximum
responses are likely to occur over eastern tropical ocean
areas. Recent GCM simulations suggest that enhanced
solar irradiance during a sunspot maximum with a corre-
sponding increase of about 1.5 per cent in column ozone
modifies the global circulation; the Hadley cells weaken
and the subtropical jet streams and Ferrel cells shift
poleward. A statistical relationship has also been found
between the occurrence of droughts in the western
United States during the past 300 years, determined
from tree ring data and the approximately twenty-two-
year double (Hale) cycle of the reversal of the solar
magnetic polarity. Drought areas are most extensive in
the two to five years following a Hale sunspot minimum
(i.e. alternate eleven-year sunspot minima). A mecha-
nism is not established, however.
Changes in atmospheric composition may also have
modified the atmospheric heat budget. The presence
of increased amounts of volcanic dust and sulphate
aerosols in the stratosphere is one suggested cause of
the 'Little Ice Age'. Major eruptions can result in a
surface cooling of perhaps 0.5°C for one or two years
after the event (see Box 13.3). Hence frequent volcanic
activity would be required for persistently cooler condi-
tions. Conversely, it is suggested that reduced volcanic
activity after 1914 may have contributed in part to the
early twentieth-century warming. New interest in this
question has been aroused by eruptions of El Chichón
(March 1982) and Mount Pinatubo (June 1991) (see
Chapter 2A.4). Surface temperatures over the northern

continents were up to 2°C below average in summer
1992 but up to 3°C above average in the winters of 1991
to 1992 and 1992 to 1993. Volcanic forcing is hard to
determine because there have been few well-observed
events.
The role of low-level aerosols is also complex. These
originate naturally, from wind-blown soil and silt,
as well as from atmospheric pollution due to human
activities (industry, domestic heating and modern
transportation) (Table 2.2). Their net radiative effect is
negative due to scattering, but the magnitude is poorly
determined (see below).
Striking evidence of cumulative human effects
on the global energy budget is found in the increase
in ocean heat content. The world ocean between the
surface and 3000-m depth gained ~18 10 22 J between

1955 and 1996 based on ocean temperature data. The
quantitative significance of this number is apparent
from Figure 12.1. By comparison, changes in the heat
content of the atmosphere and of land ice and sea
ice over the same period were an order of magnitude
smaller. Model results imply that the change in ocean
heat content was attributable to greenhouse gas-induced
global warming. These anthropogenic factors are dis-
cussed below.

3 Anthropogenic factors

The growing influence of human activities on the
environment is being increasingly recognized, and
concern over the potential for global warming caused by
such anthropogenic effects is growing. Four categories
of climatic variable are subject to change (Table 13.2)
and will now be considered in turn.
Changes in atmospheric composition associated with
the explosive growth of world population, industry and
technology have been described in Chapter 2A.4, and it
is clear that these have led to dramatic increases in the
concentration of greenhouse gases. The tendency of
these increases is to increase radiative forcing and global
temperatures; the percentage apportionment of radiative
forcing of these greenhouse gas increases since the
pre-industrial era is summarized in Table 13.3, together
with the associated ranges of uncertainty and levels of
confidence assigned to each factor. The radiative forcing
effect of the minor trace gases is projected to increase
steadily. Up to 1960, the cumulative CO 2 contribution
since AD 1750 was about 67 per cent of the calculated
1.2 Wm -2 forcing, whereas for 1980 to 1990 the CO 2