Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 38

Radioactive decay dating using different radioactive isotopes from associated geo-
logical strata is one common way of pinpointing a climate proxy in time. The first
recorded use of radioactivity for dating took place 2 years before the discovery of
isotopes by Frederick Soddy in 1913, just 15 years after the discovery of radioactivity
and 9 years after Ernest Rutherford explained the phenomenon of radioactive decay!
Arthur Holmes' classic paper from June 1911 demonstrated how uranium decay
could be used to date rocks millions of years old, a far greater timescale than can
be used with the aforementioned 14 C dating, because 14 C has a much shorter decay
half-life.
Because widespread direct instrumental climatic measurements have only been
available for the past 100 years or so we are dependent on proxy indicators combined
with dating techniques for earlier information. To ensure their reliability, reconstruc-
tions based on these proxies must be validated by comparison with instrumental
records during periods of overlap. Sometimes direct comparison is not possible and
so this must be done indirectly by relating to another overlap with another proxy, and
so commonly a number of proxies are used simultaneously.
Another problem is that all palaeoclimatic proxies only relate to a certain time
period. Glacial terminal moraines largely tell us about the maximum extensions of
ice, hence glacial maximums and not glacial beginnings . Furthermore, the aspect
of the climate being indicated may only relate to part of the year. For instance,
tree-ring data are only applicable to periods when trees grow and so largely relate
to warm-season conditions and do not give an indication as to the severity of win-
ters. (Of course, there are often exceptions and here, to continue with our cur-
rent example, cold-season tree-ring information is available but largely limited to
species from semi-arid or Mediterranean environments.) Such restrictions, in this
case seasonal specificity, may give a misleading picture of large-scale temperature
changes. For example, the climatic response to volcanic forcing can lead to oppos-
ite temperature changes in the summer (with cooling) and winter (warming) over
continents.
It is for all these reasons that many proxies are required. Sets of these in turn are
used to determine indices against which it may be possible to ascertain large-scale
climatic changes. For example, one such set is used to determine what is called the
North Atlantic Oscillation (NAO) index (see section 5.1.5, Holocene summary).
However, no matter how each proxy is subsequently used there is considerable
need to add to our knowledge of existing proxies and to identify new ones. This is
both to establish further palaeoclimatic data and for corroboration.
The good news is that although as many climate proxies as possible are required to
build up a picture of past climates, they do all begin to paint the same picture. So, on
one hand there is uncertainty, but on the other, by looking at more and more proxies
(in both number and type), there is increasing certainty.
Climatic proxies are many but can be divided into two principal groups: biotic
proxies (terrestrial and marine) and abiotic proxies (physical and abiotic geological).
We will look at a few examples of the principal types of each of these, but because
this text concerns the biology of climate change we will focus mainly on those that
are in some way biological.