Geoscience Reference
In-Depth Information
tenths of a degree Celsius within a few years. Neither is
it easy to explain the existence of periods of relatively
stable global temperatures (e.g. 1861 to 1920 and the
mid-1940s to early 1970s) at times of continuing
increases of greenhouse gas concentration. Nevertheless,
the general warming since the mid-twentieth century
and the most recent climatic events seem to be a response
to the changes in atmospheric composition noted in
Chapter 2A.4 (rather than to other forcing mechanisms
such as solar variations). In addition, it is recognized that
the course of climatic change is undoubtedly compli-
cated by little-known global mechanisms such as the
inertial effects of the oceans and the possibly periodic
effects of deep ocean currents. Overall, there remains a
strong impression that natural climatic variability may
from time to time be merely offsetting an inexorable and
increasingly menacing anthropogenic increase of global
temperature.
significant increase in the frequency of mild south-
westerly airflow over the British Isles between about
1900 and 1930, as reflected by the average annual
frequency of Lamb's westerly airflow type (see Chapter
10A.3). For 1873 to 1897, 1898 to 1937, 1938 to 1961
and 1962 to 1995 the figures are 27, 38, 30 and 21 per
cent, respectively. Coinciding with the westerly decline,
cyclonic and anticyclonic types increased substantially
(Figure 13.13). The decrease in westerly airflow during
the last thirty-year interval, especially in winter, is
linked with greater continentality in Europe. These
regional indicators reflect a general decline in the overall
strength of the mid-latitude circumpolar westerlies,
accompanying an apparent expansion of the polar
vortex.
Marked climatic fluctuations have occurred in the
North Atlantic sector in association with the dominant
phase of the North Atlantic Oscillation. This was mostly
negative between the 1930s and 1970s but returned after
1980 to the mainly positive phase (giving enhanced
westerly flow) that dominated the first two decades
of the twentieth century. The winters 1995 to 1996 and
1996 to 1997, however, interrupted a run of mild winters
in northern Europe.
Other coupled atmosphere-ocean anomalies may
affect climatic trends on a global scale. For example,
the occurrence and intensity of the warm phases of
ENSO events are estimated to have increased the global
mean temperature by about 0.06°C between 1950 and
1998.
D POSSIBLE CAUSES OF RECENT
CLIMATIC CHANGE
Remarkably, the causes of the observed climatic
changes over the past few centuries are no better under-
stood than that of the last glacial cycle. A multiplicity
of possible explanations exists and, indeed, more than
one factor is likely to be involved. It is useful to try to
distinguish between the natural causes that have existed
throughout the earth's history and those attributable to
human activities. However, in many instances - such
as the production of tropospheric aerosols - the two
operate together.
2 Energy budgets
The key to these atmospheric variations must be linked
to the heat balance of the earth-atmosphere system
and this forces us to return to the fundamental energy
considerations discussed in Chapter 3. The evidence
for fluctuations greater than 0.5 per cent in the 'solar
constant' is inconclusive, although significant variations
apparently do occur in the emission of high-energy par-
ticles and ultraviolet radiation during brief solar flares.
All solar activity follows the well-known Schwabe
cycle of approximately eleven years, which is usually
measured with reference to the period between sunspot
maximum and minimum (see Figure 3.2). Recent solar
cycles show that the solar irradiation varies by about 1.1
W m-
2
, with a corresponding air temperature fluctuation
of ≤0.1°C. As was pointed out in Chapter 3A.1, sunspot
darkening is accompanied by increased emission from
1 Circulation changes
The immediate cause of the climatic fluctuations in the
early twentieth century appears to be the strength of
the global wind circulation. The first thirty years of the
century saw a pronounced increase in the vigour of
the westerlies over the North Atlantic, the northeast
trades, the summer monsoon of South Asia and the
southern hemisphere westerlies (in summer). Over the
North Atlantic, these changes consisted of an increased
pressure gradient between the Azores high and the
Icelandic low, as the latter deepened, and also between
the Icelandic low and the Siberian high, which spread
westward. These changes were accompanied by more
northerly depression tracks, and this resulted in a
