Geoscience Reference
In-Depth Information
had fallen to about 200 Dobson units in September to
October (see Figure 2.9), compared with 400 units in
the 1970s. In the extreme years (1993 to 1995), record
minima of 116 DU have been recorded at the South Pole.
It has been estimated that, because of the slowness of the
global circulation of CFCs and of its reaction with ozone,
even a cut in CFC emissions to the level of that in 1970
would not eliminate the Antarctic ozone hole for at least
fifty years. Winter ozone depletion also occurs in the
Arctic stratosphere and was well marked in 1996 and
1997, but absent in 1998. Localized mini-holes are fairly
common, but extensive holes are rare even in cold
stratospheric winters. It seems that whereas the Antarctic
vortex is isolated from the mid-latitude circulation, the
Arctic vortex is more dynamic so that transport of ozone
from lower latitudes makes up much of the loss.
Predictions regarding the future climatic effects of
changes in atmospheric composition will be treated in
section F, this chapter.
The role of tropospheric aerosols in climate forcing
and the magnitude of such effects are poorly known
(see Figure 13.14). There are four key aerosol types (see
Chapter 2A.4), and these have a variety of effects:
The indirect effects of cloud condensation nuclei from
anthropogenic sources are undetermined. Nevertheless,
a ±15 per cent change of CCN within marine stratus
clouds, which cover about 25 per cent of the earth, could
change the global energy balance by ±1 W m
-2
.
Indirect anthropogenic factors, such as increasing
population pressures leading to overgrazing and forest
clearance, may increase desertification which also
contributes to the increase of wind-blown soil. The
'dust-bowl' years of the 1930s in the United States and
the African Sahel drought since 1972 illustrate this, as
well as dust transported from western China across the
Pacific to Hawaii, and from the Sahara westward across
the North Atlantic (see Plate 5).
The presence of particles in the atmosphere increases
the backscatter of short-wave radiation, thereby increas-
ing the planetary albedo and causing cooling, but the
effect on infra-red radiation is one of surface warming.
The net result is complicated by the surface albedo.
Man-made aerosols cause net warming over snow and
ice and most land surfaces, but cooling over the oceans,
which have a low albedo. Natural aerosols probably
cause general cooling. The overall effect on global
surface temperature remains uncertain.
Changes in surface albedo occur naturally with sea-
son, but climatic forcing is also caused by anthropogenic
vegetation changes. Human effects on vegetation
cover have a long history. Burning of vegetation by
Aborigines in Australia has been traced over the past
50,000 years, while significant deforestation began
in Eurasia only during Neolithic times (
c
. 5000
BP
),
as evidenced by the appearance of agricultural species
and weeds. Deforestation expanded in these areas
between about
AD
700 and 1700 as populations slowly
grew, but it did not take place in North America until
the westward movement of settlement in the eighteenth
and nineteenth centuries. During the past half-century
extensive deforestation has occurred in the tropical
rainforests of Southeast Asia, Africa and South
America. Estimates of current tropical deforestation
suggest losses of 10
5
km
2
/year, out of a total tropical
forest area of 9
1
Black carbon
- absorbs solar radiation; changes the
vertical temperature gradient.
2
Water-soluble inorganic species
(SO
2
, NO
3
, NH
4
) -
backscatter of direct beam solar radiation, indirect
effect of CCN on cloud albedo and cloud droplet
lifetime.
3
Condensed organic species
- as (2).
4
Mineral dust
- as (1), (2) and absorption/emission
of infra-red radiation.
The global mean forcing exerted by the principal
aerosols for 1750 to the present (see Figure 13.14) is
approximately as follows:
•
sulphate aerosols -0.4 W m
-2
;
•
biomass-burning aerosols -0.1 W m
-2
(fossil fuel
organic carbon) and + 0.2 Wm-
2
(fossil fuel black
carbon);
10
6
km
2
. This annual figure is more
than half the total land surface at present under irri-
gation. Forest destruction causes an increase in albedo
of about 10 per cent locally, with consequences for
surface energy and moisture budgets. However, the
large-scale effect of deforestation in temperate and
tropical latitudes on global surface albedo is estimated
to be ≤0.001. The radiative forcing associated with
•
mineral dust aerosols are in the range 0.5 to -0.6 W
m
-2
for the direct effect but 0 to -2.0 W m-
2
for the
indirect effects on cloud condensation nuclei (CCN)
for water clouds.
It should be emphasized that about 88 per cent of the
total aerosol (see Table 2.2) input is of natural origin.
