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correlation) between Indonesia and the eastern
Pacific. Other north-south climatic oscillations
were identified in the North Atlantic (Azores vs.
Iceland, known as the North Atlantic Oscillation)
and the North Pacific (Alaska vs. Hawaii). In the
phase of the Southern Oscillation when there is
high pressure over the eastern Pacific, westward-
flowing central Pacific surface waters, with a
consequent upwelling of cold water, plankton-
rich, off the coast of South America, are associated
with ascending air giving heavy summer rains over
Indonesia. Periodically, weakening and breakup of
the eastern Pacific high pressure cell leads to
important consequences. The chief among these
are subsiding air and drought over India and
Indonesia and the removal of the mechanism of
the cold coastal upwelling off the South American
coast with the consequent failure of the fisheries
there. The presence of warm coastal water is
termed 'El Niño'. Although the central role played
by lower latitude high pressure systems over the
global circulations of atmosphere and oceans is
well recognized, the cause of the east Pacific
pressure change which gives rise to El Niño is not
yet fully understood. There was a waning of
interest in the Southern Oscillation and associated
phenomena during the 1940s to mid-1960s, but
the work of Berlage (1957), the increase in the
number of Indian droughts during the period
1965-1990, and especially the strong El Niño
which caused immense economic hardship in
1972, led to a revival of interest and research. One
feature of this research has been the thorough
study of the 'teleconnections' (correlations
between climatic conditions in widely separated
regions of the earth) pointed out by Sir Gilbert
Walker.
paleoclimatology, the study of past climates, were
developed in the 1960s to 1970s. The astronom-
ical theory to explain the great ice ages of the
Pleistocene proposed by Croll (1867), and
developedmathematicallybyMilankovitch (1920),
seemed to conflict with evidence for dated climate
changes. However, in 1976, Hays, Imbrie and
Shackleton recalculated Milankovitch's chronol-
ogy using powerful new statistical techniques
and showed that it correlated well with past
temperature records, especially for ocean paleo-
temperatures derived from isotopic (
18
0/
16
0) ratios
in marine organisms recorded in ocean cores. The
idea behind Milankovitch forcings is that periodic
changes in the eccentricity of the earth's orbit,
the tilt of the earth's axis and the timing of the
equinoxes cause variations in the amount of solar
radiation received at different times of the year
over different parts of the surface. As is now widely
accepted, the major ice ages as well as intervening
interglacials over about the past two million years
reflect influences of these Milankovitch cycles and
attendant climate feeedbacks that amplify change.
Paleoclimate information from ocean cores and
terrestrial sources is complemented by ice cores
obtained from the Greenland and Antarctic ice
sheets, ice caps in Canada and elsewhere. As well
as documenting climate links with Milankovitch
cycles, these records provide evidence of rapid,
large-scale shifts in climate. The longest ice core
record currently available from Dome C in the
eastern Antarctic spans 800,000 years and shows
that interglacials before 450,000 years ago were
weaker (less warm) than subsequently. Recon-
structed temperature records from ice cores
are obtained from oxygen isotope ratios (
18
O).
Samples of past atmospheres trapped as bubbles
in ice cores also document close links between
climate and atmospheric carbon dioxide concen-
trations, and show convincingly that present-day
concentrations of this greenhouse gas are higher
than at any time during at least the past 800,000
years.
Other paleoclinatic information is obtained
from annual tree rings that reflect growing season
H PALEOCLIMATES
Before the middle of the twentieth century thirty
years of records was generally regarded as
sufficient in order to define a given climate. By
the 1960s the idea of a static climate was recog-
nized as being untenable. New approaches to
