Geoscience Reference
In-Depth Information
Another reason given for the suddenness of the warming after the LGM is that
changes in the Broecker thermohaline conveyor played a significant part. But were
the changes in the planet's surface and deep-ocean circulation initiated in the northern
hemisphere? There has been considerable attention given to the circulation's North
Atlantic pump, but there are other possibilities (for example, see the discussion of
the MOC earlier in this section). One, according to a Canadian team led by Andrew
Weaver (2003), is that a pulse of comparatively warm water could have originated
from Antarctica, warmed due to Milankovitch and methane factors. Had a southern
hemisphere event affected the global conveyor then - because the conveyor is global -
this would have affected the northern hemisphere. The jury is still out, but we should
not dismiss the possibility that factors (note the plural) in both hemispheres played
their part.
Finally in this subsection on temperature changes during the glacial we should look
at annual events within the timeframe of several years. There are a number of climate
cycles in this scale that impact on meteorology (more related to weather than climate).
Of these, the one of greatest significance today is the ENSO: it determines things
like droughts and moist times on the east coat of South America and the strength of
the monsoon in Asia, among other climate-related/meteorological phenomena. You
will remember the discussion of the ENSO in the warmer Pliocene and how ENSO
state might be an analogue for what we might expect in a future warmer Earth (see
section 4.3), so what was happening to the ENSO in cooler glacial times? To cut to the
chase, it seems that in cooler glacial times ENSO events were slightly less common
and, when they did occur, led to less variable meteorological events. This is perhaps
not that surprising: after all, in cooler glacials there is less energy in the biosphere
for ocean evaporation and the generation of temperature gradients. The evidence
for this includes that by researchers led by Christian Wolff and Gerald H. Haug in
2011. As said, the ENSO affects many parts of the globe and indeed interannual
rainfall variations in equatorial East Africa are tightly linked to the ENSO, with
more rain and flooding during El Ni no and droughts in La Nina years, both having
severe impacts on human habitation and food security (see parts of sections 7.3.1
through to 7.4.2). The Wolff-Haug team reported evidence from an annually laminated
lake sediment record from south-eastern Kenya for interannual to centennial-scale
changes in ENSO-related rainfall variability during the last three millennia suggesting
that there were reductions in both the mean rate and the variability of rainfall in East
Africa during the last glacial period. Furthermore, climate model simulations support
forward extrapolation from these lake sediment data that future warming will intensify
the interannual variability of East Africa's rainfall. In effect, it would seem that if
the frequency of ENSO events in a given number of years, and their intensity, were
lower in cooler glacial times, then they would be more frequent and more intense in
a future anthropogenically warmed world.
4.5.4 Biologicalandenvironmentalimpactsofthelastglacial
The depth of the last glacial saw increased aridity over much of the planet. With ice
caps extended and the sea level lowered, there was a considerable migration of species.
The former induced migration to lower latitudes and the latter facilitated migration
 
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