Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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teleconnection that tends to synchronize climatic events over the Northern Hemisphere
(Kiefer et al., 2001; Garidel-Thoron & Beaufort, 2001). A correlation of the terrestrial
and oceanic events is confirmed by the SST fluctuations over the Medieval Warm Period
– Little Ice Age transition (Keigwin, 1996).
Monsoons. The monsoon intensities are of primary importance for the tropical biotic
production providing a major sink for the atmospheric CO 2
. The controlling factors for
monsoons are summer insolation (astronomically driven by precession cycles: Rossignol-Strick
et al., 1998; Mommersteeg et al., 1995) and the tropical SST (a decrease in the
monsoon index associates with strong El Niños, below), as well as the trade wind velocities.
Monsoon fluctuations are thus linked to deep-water circulation, in particular, NADW
production as a contributor to equatorial upwellings.
While deep-water circulation establishes a teleconnection between the polar and
tropical marine climates, the tropical monsoons couple them to the terrestrial climates.
Incidentally, the world-wide effect of NADW is indicated by a negative correlation of
East Asian monsoons with Dansgaard–Oeschger stadials (Garidel-Thoron & Beaufort,
2001). The millennial vegetation changes in southern California, inflicted by fluctuations
of winter monsoon (Heusser & Sirocko, 1997), seem related to this coupling
mechanism.
Conversely, a reduced NADW production, a shoaling of oceanic heat conveyor with
sea-level or a barrier for deepwater circulation would have a decoupling effect. Climatic
modelling for the last glacial maximum, 21 k.y., gives the tropical SST about 2.2°C lower
than at present (Weaver et al., 1998) while the continents were cooler by 3 -7°C (Behling
et al., 2001). Over the warming trend, the SST might lag behind the land temperatures.
Actually, a weakening of monsoons during the glacial maxima – strengthening
during the early interglacials and Holocene are inferred from both sedimentological and
palynological data (Faibridge, 1986; Hooghiemstra, 1988; Dupont & Hooghiemstra, 1989;
Maxwell, 2001).
Yet the rainforest dynamics over the glacial/interglacial cycles was affected by a
combination of factors, including a displacement of trade-wind belts. Incidentally, both a
drop of terrestrial productivity (recorded by the contemporary pollen rain) and a decrease
in clastic runoff (recorded by a lower clay content in the contemporary marine
sediments) at the glacial maxima might have been due to a diversion of moist easterlies
by strong monsoons. Offshore palynology indicates a negative correlation of the African
monsoon intensity and the northeast trade wind velocities over the glacial cycles (Hooghiemstra,
1988). A latitudinal shift of the jet stream is also inferred in relation to vegetation
changes in the winter-wet zone (southern California: Heusser & Sirocko, 1997), where
the relatively humid phases of conifer expansion correlate with glacial maxima.
El Niño. Perhaps a not fully adequate but still instructive short-term model of longterm
oceanic/climate change is provided by El Niño, a quasiperiodic rise of SST caused
by the wind-driven surface current fluctuations in turn related to oscillations of atmospheric
pressure, the “southern oscillations”, in the Pacific and Austral-Asiatic region