Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 59

place but was not reflected in changes in atmospheric carbon dioxide concentra-
tions.
Another problem with relating atmospheric carbon dioxide to climate is that if the
climate is changing and plants are absorbing more carbon dioxide, or rotting and
producing more, or it is getting wetter and wetlands are expanding, thus pushing up
methane concentrations, then it takes time for this to be reflected to any great degree
in the atmospheric concentration. True, atmospheric carbon dioxide varies by 5 parts
per million by volume (ppmv) every 6 months outside of the central tropics due to
seasonal vegetation changes, but carbon dioxide concentration changes associated
with glacial-interglacial transitions are of the order of 150 ppmv. If the hemispheric
carbon pump can only cope (optimistically) annually with less than 10 ppmv then
climate changes associated with greater carbon dioxide change than this fall outside of
this window. In part related to this, there is the problem that the atmospheric residence
time of a carbon dioxide molecule is of the order of a century or two. Consequently,
climate change episodes not primarily driven by atmospheric carbon dioxide (such as
those driven by changes in ocean circulation) and which occur quickly, cannot easily,
if at all, be discerned from the atmospheric gas record. This is yet another reason why
it is best to use as many as possible indicators of climate so as to understand what is
really going on.
2.3.4 Dustasanindicatorofdry-wethemisphericclimates
Dust blown many hundreds of miles is an indicator of the dryness of the climate. In
a wet climate rain is more likely to wash dust out of the air than in a dry climate.
Indeed, we know that warm interglacials are globally far wetter than the cool glacials.
As noted above, this is because in a warmer world there is more oceanic evaporation,
hence precipitation, than in a cooler one. Of course, to discern this indicator one needs
a recording location that is hundreds of miles from the nearest large source of dust.
Such locations include the middle of ice caps and, as mentioned, one in particular -
Greenland - lends itself to the building up of ice layer by layer each year and has ice
strata going back hundreds of thousands of years. Measuring the dust trapped in the
ice can be done by measuring the ice's electrical conductivity. (Conductivity being
the reciprocal of resistance.) The more dust there is in the ice then the less conductive
it becomes as the dust neutralises the acidic ions that facilitate conductivity. As with
dendrochronology, providing dating can be accurately achieved, where there is clear
layering it is similarly possible to discern conductivity levels on close to an annual
basis. As we shall see in Chapter 4, electrical conductivity measurement analysis of
the Greenland ice core has revealed dry-wet (hence presumably cold-warm) climatic
flickers of roughly a year or two, and certainly far less than a decade (see Figure 4.12b).
2.4 Otherindicators
The above sections deal with many of the key indicators (with a focus on the biolo-
gically related ones) used to determine past climates. However, it is important to note