The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 159

cores (fairly distant from biomass burning) is reasonably constant from 11 000 to
15 000 years ago so the ECM record reflects acid-neutralising calcium dust, which
comes from the erosion of calcium carbonate rocks such as limestone or chalk. How-
ever, there is another factor: the presence of dust reflects the amount of precipitation.
The more rain, the more acid-neutralising dust is washed out of the atmosphere.
Dust is less sensitive than ammonia to washing out of the air and so, in the absence
of nearby extensive vegetation, dust is a good indicator of regional precipitation
as water vapour will tend not to take it out of the air, although rain will. Put the
aforementioned data all together and the Greenland ice-core ECM record reflects
regional precipitation. Furthermore, this is one more example of the climatic relevance
of dust.
As we have already frequently noted, the warmer the planet the more ocean evap-
oration, hence rain: increased temperature and rain go together. Consequently dust,
which would only be present when it is dry, is in turn a secondary indicator of hemi-
spheric temperature (Taylor et al., 1993). More exactly, as temperature is not being
directly reflected, the level of dust indicates whether the Earth is in a glacial (low
rainfall/high dust) as opposed to an interglacial (high rainfall/low dust) mode. Finally,
from Chapter 2 we know that deuterium is an indicator of regional climate. Given
this, it possible to see how the panels of Figure 4.12 relate to climate.
Some 15 000 years ago the Earth was warming after the LGM. However, this
warming was not permanent and cooler conditions returned, mainly in the northern
hemisphere. The temporary warm period itself was comprised of two shorter warm
periods, or interstadials, called the Allerød and Bølling, which in turn were followed
by a cool period, the Younger Dryas, from 12 850 to 11 650 years ago before there was
a warming again to the present Holocene interglacial. The results of a high-resolution
deuterium (see section 2.3.1) Greenland ice-core analysis by an international team led
by Jørgen Peder Steffensen concluded in 2008 that the northern hemisphere climate
mode initially switched mode. The warming transition 14 700 years ago is the most
rapid and occurs within a remarkable 3 years while the warming transition at 11
700 years ago lasted 60 years, as revealed by Greenland cores. This is fast! (And of
relevance to the 'climate surprise' discussion at the end of Chapter 6.)
Returning to Figure 4.12a, it is an enlargement of part of Figure 4.2 from an Ant-
arctic Vostok ice core and shows that the Younger Dryas was not so strongly reflected
in the southern hemisphere: in other words, the Younger Dryas was predominantly
a northern hemisphere event. Looking closely at Figures 4.12a and b it can be seen
that the Antarctic cooling in fact began roughly 1000 years before the Younger Dryas
in the northern hemisphere. So, although the two events are linked, they are differ-
ent and the cooling in Antarctica is known as the Antarctic Cold Reversal. There
is other evidence that the northern hemisphere's Younger Dryas cooling had less
impact south of the equator, such as in the pollen-grain record from lake sediments
in southern Chile (Bennett et al., 2000). However, not all the evidence is so minimal,
and little impact compared to the northern hemisphere is not the same as no impact.
Other pollen evidence from Chilean lakes (Moreno et al., 2001) demonstrates that
there was enough alteration of the climate for some significant ecosystem change in
the southern hemisphere. Coral evidence from the south-western Pacific suggests a
sea-surface cooling somewhere between 3.2 and 5.8 ◦ C. Furthermore, the Younger