Geoscience Reference
In-Depth Information
PIONEERS OF CLIMATIC CHANGE
RESEARCH
box 13.1
significant
20th-c. advance
In the late nineteenth century, it was widely accepted that climatic conditions were described by long-term averages or
normals
. The longer the record, the better would be the approximations to the long-term mean. Geologists and a few
meteorologists were aware that climates of the past had been very different from the present and sought explanations
of Ice Ages in astronomical and solar variations. Two classic works by C. E. P. Brooks -
The Evolution of Climate
(1922)
and
Climates of the Past
(1926) - provided a remarkably comprehensive picture of changes through geologic time and
set out the possible factors, external and internal to the earth's climate system. It was not until the 1950s to 1960s,
however, that awareness grew of important decadal-century scale climatic fluctuations. Historical weather records and
proxy climatic data began to be assembled. Pioneers in historical climatology included Gerald Manley and Hubert Lamb
in England, Herman Flohn in Germany, LeRoy Ladurie in France, and Murray Mitchell and Reid Bryson in the United
States. In the 1970s attention turned initially to the possibility of a renewed Ice Age and then to concern over the
greenhouse effect of increasing carbon dioxide concentrations in the atmosphere. The possibility of global cooling arose
from two main sources; the first was paleoclimatological evidence that previous interglacial conditions lasted for only
about 10,000 years and already the post-glacial Holocene period was of that length. A conference titled 'The Present
Interglacial - how and when will it end?' took place at Brown University, Providence, RI, in 1972 (G. Kukla, R. Matthews
and J. M. Mitchell). A second factor was concern over the role of aerosols in reducing incoming solar radiation. The early
1970s also saw an increase in the extent of northern hemisphere snow cover (Kukla, 1974). Almost simultaneously,
however, the first conferences on carbon dioxide and greenhouse warming were taking place! The occurrence of abrupt
climatic shifts during the last Late Pleistocene and Holocene began to be identified in the 1970s to 1980s. Most notable
is the 1000-year-long severe cooling known as the Younger Dryas that occurred around 11,000 years ago.
Interest in past climates was driven by the concept that 'the past is the key to the future'. Hence, efforts were made
to document conditions during historical times and the remote geological past, when global climate varied over a much
wider range of extremes, and to determine their causes.
variation about the mean (the standard deviation, or
interquartile range), the extreme values, and often the
shape of the frequency distribution (see Note 1). A
change in climate can occur in several different ways
(Figure 13.1). For example, there may be a shift in the
mean level (B), or a gradual trend in the mean (C). The
variability may be periodic (A), quasi-periodic (B)
or non-periodic (C), or alternatively it may show a
progressive trend (D). It is necessary first to determine
whether such changes are real or whether they are an
artefact of changes in instrumentation, observational
practices, station location or the surroundings of the
instrumental site, or due to errors in the transcribed data.
Even when changes are real, it may be difficult to ascribe
them to unique causes because of the complexity of the
climate system. Natural variability operates over a wide
range of timescales, and superimposed on these natural
variations in climate are the effects of human activities.
B CLIMATE FORCINGS AND
FEEDBACKS
The average state of the climate system is controlled
by a combination of
forcing
factors external to the
system (solar variability, astronomical effects, tectonic
processes and volcanic eruptions) and internal radiative
forcings (atmospheric composition, cloud cover). There
are also anthropogenically induced changes (in atmos-
pheric composition, surface land cover) and
feedback
effects
(such as changes in atmospheric water vapour
content or cloudiness caused by global temperature
changes). Figure 13.2 illustrates the complexity of the
climate system. It is useful to try and assess the magni-
tude of such effects, globally and regionally, and the
timescales over which they operate.
