Geoscience Reference
In-Depth Information
phases of ENSO events are estimated to have
increased the global mean temperature by about
0.06°C during 1950-1998. The very high global
average temperature of 1998 can be linked to the
strong El-Niño event of the same year.
A long-lived El-Niño-like pattern is the Pacific
Decadal Oscillation (PDO) of North Pacific SST
variability. It has undergone major 20-30 year cycles
- cool (negative) from 1890-1924 and 1947-1976
(West Pacific warm, East Pacfic cool) and warm
(positive) from 1925-1946 and 1977-mid-1990s
(Wesst Pacific cool, East Pacific warm). Its causes
are not yet known. The 15-30 year Interdecadal
Pacific Oscillation similarly affects the northern
and southern Pacific.
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.
Climate model simulations suggest that enhanced
solar irradiance during a sunspot maximum, with
a corresponding increase of about 1.5 percent 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 over the past 300 years, determined from tree
ring data and the approximately 22-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 min-
imum (i.e., alternate 11-year sunspot minima). A
clear mechanism is not established, however.
2 Solar variability
The ultimate driver of the climate system is of
course the sun. The well-known solar cycle of
approximately 11 years is usually measured with
reference to the period between sunspot maxima
and minima (see
Figure 3.2
). As shown in satellite
records available since 1980 and as discussed
earlier, irradiation varies by a modest 1W m
-2
over
the 11-year cycle (the radiation flux averaged
across the top of the atmosphere is only 25 percent
as large). As pointed out in Chapter 3A.1,
the explanation is that sunspot darkening is
accompanied by increased emission from faculae
that is 1.5 times greater than the darkening effect.
The 11-year cycle corresponds to global air
temperature fluctuation of <0.1
3 Volcanic activity
Links between climate variability and volcanic
activity are clear (see
Box 13.3
). As discussed,
surface cooling driven by increased amounts of
volcanic dust and sulfate aerosols in the stratos-
phere occurs one to two years after major explosive
events. The effects of the eruption of Mt. Pinatubo
in the Philippines in June 1991 (
Plate 13.3
) may
be seen in
Figure 13.7A
as lower global average
temperatures in 1992 and 1993 compared to
surrounding years. Regionally the impacts were
larger. Surface temperatures over the northern
continents were up to 2
C.
What of longer term variations? Strong global
warming since 1980 cannot be attributed to solar
activity, since the satellite data show no discernible
trend. On the other hand, based on reconstruc-
tions, solar variability may account for perhaps
half of the warming between 1860 and 1950.
Variations in solar irradiance may also offer a
partial explanation of the Little Ice Age.
It is suggested by David Rind of NASA that
the direct solar forcing on climate may pale in
comparison with the potential for solar forcing to
trigger interactions involving a variety of feedback
processes. There appear to be regional patterns in
°
C below average in sum-
mer 1992 but, owing to impacts on atmospheric
circulation patterns, up to 3
°
C above average in
the winters of 1991-1992 and 1992-1993 (see
Box
13.3
). As noted earlier, given the short timescale of
the forcing, prolonged cooling would require a
chain of eruptions events; such a series of events
may help to explain the 'Little Ice Age'. The period
1883-1912 also saw frequent eruptions (see
Figure
2.12
). Conversely, reduced volcanic activity after
1914 may have contributed in part to the early
twentieth-century warming.
°
