Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 37

Principal indicators of past climates

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If palaeoclimatologists had their wildest dreams come true then they would undou-
btedly travel back in time to make meteorological recordings before humans existed,
let alone thermometers. If only life would be so obliging. Instead, preserved indica-
tions of past climate have to be identified, calibrated and then used to infer usually
just one aspect of the climate, and frequently just at one locality. Such a preserved
indication provides us with a single proxy measurement.
Many measurements are needed from many proxies to build up a climatic picture
of the past. Indeed, there are also many types of climate proxies and the principal
ones will be reviewed in this chapter. The products of proxy analyses are often
termed palaeoclimatic data. Palaeoclimatic data commonly come with considerable
uncertainty in one or more of the following regards: the inferred temperature, the
inferred time and the space to which it applies. It is important to emphasise this
and to recognise that most proxy data rarely ever give a direct measure of a single
meteorological parameter. For example, tree-ring thickness is affected by numerous
factors, not just climate. These include: the aspect in which the tree is found, be it in
a valley or on a hillside, and whether it is south- or north-facing; soils, which affect
nutrients; and local geology, which affects water availability.
As said, there are many palaeoclimatic proxies. Not only do they each come
with their own vagaries as to their relationship with climate, but the passing of
time also blurs information. For example, a sediment may contain an isotope or a
biomolecule that relates to temperature, but the sedimentation process itself may not
have been uniform with time, or the sediment may itself be re-worked by biological
and/or geological processes (such as burrowing organisms and/or strata movement,
respectively). It is as if this topic not only contained information in the form of words
on the page, but that the type of paper and even the page order had changed between
the time of my writing it to you reading it. An example of this sediment re-working is
known as smearing, which is the process of fossils being transported downwards into
older sediments (backward smearing) or upwards into younger sediments (forward
smearing).
Another example of how external factors can affect the accuracy of proxy dating
is exemplified in 14 C dating analysis. The radioactive 14 C isotope is formed in the
upper atmosphere by the action of cosmic rays (mainly from the Sun) on nitrogen and
carbon atoms in the atmosphere. Broadly, new 14 C atoms are produced at a rate of 2
atoms cm − 3 s − 1 . This assumes that cosmic ray (or solar) activity is constant with time.
This is not so. However, there is sufficient homogeneity for some meaningful work,
and steps have been taken to calibrate palaeo-carbon-14 (see the previous chapter and
section 2.1.1, on isotopic dendrochronology).

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