Geoscience Reference
In-Depth Information
Montreal Protocol agreements to curtail production and
use substitutes (see Figure 2.8B). Although their con-
centration is <1 ppbv, CFCs account for nearly 10 per
cent of the greenhouse effect. They have a residence
time of 55 to 130 years in the atmosphere. However,
while the replacement of CFCs by hydrohalocarbons
(HCFCs) can reduce significantly the depletion of
stratospheric ozone, HCFCs still have a large green-
house potential.
Ozone
(O
3
) is distributed very unevenly with height
and latitude (see Figure 2.3) as a result of the complex
photochemistry involved in its production (A.2, this
chapter). Since the late 1970s, dramatic declines in
springtime total ozone have been detected over high
southern latitudes. The normal increase in stratospheric
ozone associated with increasing solar radiation in
spring apparently failed to develop. Observations in
Antarctica show a decrease in total ozone in September
to October from 320 Dobson units (DU) (10
-3
cm at
standard atmospheric temperature and pressure) in the
1960s to around 100 in the 1990s. Satellite measure-
ments of stratospheric ozone (Figure 2.9) illustrate
the presence of an 'ozone hole' over the south polar
region (see Box 2.2). Similar reductions are also evident
in the Arctic and at lower latitudes. Between 1979
and 1986, there was a 30 per cent decrease in ozone at
30 to 40-km altitude between latitudes 20 and 50°N
and S (Figure 2.10); along with this there has been
an increase in ozone in the lowest 10 km as a result of
anthropogenic activities. Tropospheric ozone represents
about 34 DU compared with 25 pre-industrially. These
changes in the vertical distribution of ozone concen-
tration are likely to lead to changes in atmospheric heat-
ing (Chapter 2C), with implications for future climate
trends (see Chapter 13). The global mean column total
decreased from 306 DU for 1964 to 1980 to 297 for
1984 to 1993 (see Figure 2.3). The decline over the past
twenty-five years has exceeded 7 per cent in middle and
high latitudes.
The effects of reduced stratospheric ozone are partic-
ularly important for their potential biological damage
to living cells and human skin. It is estimated that a 1 per
cent reduction in total ozone will increase ultraviolet-B
radiation by 2 per cent, for example, and ultraviolet
radiation at 0.30 µm is a thousand times more damag-
ing to the skin than at 0.33 µm (see Chapter 3A). The
ozone decrease would also be greater in higher latitudes.
However, the mean latitudinal and altitudinal gradients
of radiation imply that the effects of a 2 per cent UV-B
Figure 2.8
Concentration of: (A) nitrous oxide, N
2
O (left scale),
which has increased since the mid-eighteenth century and
especially since 1950; and of (B) CFC-11 since 1950 (right scale).
Both in parts per billion by volume (ppbv).
Source
: After Houghton
et al
. (1990 and 2001).
400
350
300
250
200
150
1967-71
1989
2001
100
50
Jul
Aug
Sep
Oct
Nov
Dec
Figure 2.9
Total ozone measurements from ozonesondes over
South Pole for 1967 to 1971, 1989, and 2001, showing deep-
ening of the Antarctic ozone hole.
Source
: Based on Climate Monitoring and Diagnostics Laboratory,
NOAA.
increase in mid-latitudes could be offset by moving
poleward 60 km or 100 m lower in altitude! Recent polar
observations suggest dramatic changes. Stratospheric
ozone totals in the 1990s over Palmer Station,
Antarctica (65°S), now maintain low levels from
September until early December, instead of recovering
in November. Hence, the altitude of the sun has been
higher and the incoming radiation much greater than in
previous years, especially at wavelengths
0.30 µm.
However, the possible effects of increased UV radiation
on biota remain to be determined.
Aerosol
loading may change due to natural and
human-induced processes. Atmospheric particle con-
≤
