The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 139

section 4.6.2), as the Earth was beginning to emerge into the current interglacial
(Holocene), is strongly represented in the Greenland ice cores but only weakly in its
Antarctic counterparts. This suggests that some climatic-forcing events were driven
by factors located in one hemisphere rather than the other, even though glacials and
interglacials overall are planetary in nature.
Looking at ice cores in more detail reveals that the temperature changes are not
smooth (the indication of crossing a 'critical transition' and a climate threshold).
There are a number of factors causing this. Two of the main ones are changes in
ocean circulation and also, not surprisingly, greenhouse gas concentrations.
First, as previously noted, ocean circulation is a very important mechanism for
transporting heat away from the tropics and so it is worth examining aspects of this
in a little more detail. In recent geological times, in the land-dominated northern
hemisphere (which loses heat quickly), one of the major currents transferring heat
from the tropics has been the North Atlantic Drift (or Gulf Stream). This runs from
east of the Gulf of Mexico, up the south-east coast of the USA and north east across
the Atlantic up to Iceland as well as the British Isles and the western coasts of
Scandinavia. In winter months this warms the western edge of northern Europe so
that it is 10-15 ◦ C warmer there at latitudes of 40-60 ◦ N compared with Russia and
much of Canada: the heat transported from the tropics is considerable. On its journey
north evaporation takes place so that by the time the water has reached Icelandic
latitudes the current is more concentrated in salts than other parts of the ocean. All
this time, as well as evaporating, the current is shedding heat (and hence moderating
winters in north-west Europe) and so cooling down. At Icelandic latitudes the North
Atlantic Drift is cooler and saltier and so it sinks. But the current does not end there.
This action of dense (cool and saltier) water sinking is one of the main drivers of a
global current that extends from the surface to beneath the oceans. It forms a global
conveyor driven by thermohaline gradients ( thermo being Greek for heat and haline
meaning salt). The conveyor runs either on the surface or beneath the major oceans and
importantly connects the North and South Atlantic with the Pacific (see Figure 4.7).
In 1992 Wallace Broecker of Columbia University argued that the global thermo-
haline conveyor might be disrupted if one or more of its driving components were
disabled. One way to disrupt the conveyor would be to dilute the saltier sea water at
points where it sank. Adding fresh water at these points could do this. If this were
done in the North Atlantic then the water would not sink and indeed block further
water coming up from the south. Of course, the North Atlantic gyre would not cease,
as Coriolis forces from the Earth's rotation would remain. Instead, the northern part of
the North Atlantic Drift would be displaced south and so cease to warm north-western
Europe in winter but further warm Spain and North Africa. Broecker suggested that
the switching off of the conveyor would result in a sudden cooling of north-western
European in winters.
Briefly jumping forward to 2010, the European (Spanish, British and German)
team of researchers led by Cesar Negre and Rainer Zahn solved a conundrum of what
had been seemingly contradictory evidence by using isotopic data that they obtained
from the South Atlantic and comparing it with data from the North Atlantic. They
showed that the abyssal currently southerly current reversed during glacial times.
(Figure 4.7 is a rough sketch as to how the Broecker thermohaline circulation works
in our warm interglacial.)