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3.3. Palaeoclimate Reconstruction from Proxy Data

Climate varies over different time scales, from years to hundreds of millions of years, and each periodicity is a manifestation of separate forcing mechanisms (section 2.4). In addition, different components of the climate system change and respond to forcing factors at different rates; in order to understand the role such components play in the evolution of climate it is necessary to have a record considerably longer than the time it takes for them to undergo significant changes (Bradley, 1985).

Palaeoclimatology is the study of climate and climate change prior to the period of instrumental measurements. A geological chronology of Earth history is provided in the appendix, which will provide a useful time frame for the material discussed both in the remainder of this chapter and in chapter 5.

Instrumental records span only a tiny fraction (<10-7) of the Earth's climatic history and so provide a inadequate perspective on climatic variation and the evolution of the climate today and in the future. A longer perspective on climate variability can be obtained by the study of natural phenomena which are climate-dependent. Such phenomena provide a proxy record of the climate.

Many natural systems are dependent on climate, and from these it may be possible to derive palaeoclimatic information from them. By definition, such proxy records of climate all contain a climatic signal, but that signal may be weak and embedded in a great deal of random (climatic) background noise. In essence, the proxy material has acted as a filter, transforming climate conditions in the past into a relatively permanent record. Deciphering that record is often a complex business.

Table 3.1. Principle sources of proxy data for palaeoclimatic reconstructions

Glaciological (Ice Cores)
              Oxygen isotopes
              Physical properties
              Trace element & microparticle concentrations

Geological
  A. Sediments
    1. Marine (ocean sediment cores)
      i) Organic sediments (planktonic & benthic fossils)
              Oxygen isotopes
              Faunal & floral abundances
              Morphological variations
      ii) Inorganic sediments
              Mineralogical composition & surface texture
              Distribution of terrigenous material
              Ice-rafted debris
              Geochemistry
    2. Terrestrial
              Periglacial features 
              Glacial deposits & erosional features
              Glacio-eustatic features (shorelines)
              Aeolian deposits (sand dunes)
              Lacustrine deposits/varves (lakes)
  B. Sedimentary Rocks
              Facies analysis
              Fossil/microfossil analysis
              Mineral analysis
              Isotope geochemistry

Biological
              Tree rings (width, density, isotope analysis)
              Pollen (species, abundances)
              Insects

Historical
              meteorological records
              parameteorological records (environmental  indicators)
              phenological records (biological indicators)

The major types of proxy climatic data available are listed in Table 3.1. Each proxy material differs according to: a) its spatial coverage; b) the period to which it pertains; and c) its ability to resolve events accurately in time (Bradley, 1985). Some proxy records, for example ocean floor sediments, reveal information about long periods of climatic change and evolution (107 years), with a low-frequency resolution (103 years). Others, such as tree rings are useful only during the last 10,000 years at most, but offer a high frequency (annual) resolution. The choice of proxy record (as with the choice of instrumental record) very much depends on what physical mechanism is under review. As noted, climate responds to different forcing mechanisms over different time scales, and proxy materials will contain necessary climatic information on these to a greater or lesser extent, depending on the three factors mentioned.

Other factors that have to be considered when using proxy records to reconstruct palaeoclimates include the continuity of the record and the accuracy to which it can be dated (Bradley, 1985). Ocean sediments may provide continuous records for over 1 million years (Ma) but typically they are hard to date using existing techniques. Usually there is an uncertainty of +/- 5% of the record's true age. Ice cores are easier to date but may miss layers from certain periods due to melting and wind erosion. Glacial deposits are highly episodic in nature, providing evidence only of discrete events in the past. Different proxy systems also have different levels of inertia with respect to climate, such that some systems may vary exactly in phase with climate whereas others lag behind by as much as several centuries (e.g. Bryson & Wendland, 1967).

Like climate construction from instrumental records, palaeoclimate reconstruction may be considered to proceed through a number of stages (Hecht et al., 1979). The first stage is that of proxy data collection, followed by initial analysis and measurement. This results in primary data. The next stage involves the calibration of the data with modern climate records. In this, the uniformitarian principle is assumed, whereby contemporary climatic variations form a modern analogue for palaeoclimatic changes. It is important to be aware, however, of the possibility that palaeo-environmental conditions may not have modern analogues (Sachs et al., 1977). The calibration may be only qualitative, involving subjective assessment, or it may be highly quantitative. The secondary data provide a record of past climatic variation. The third stage is the statistical analysis of this secondary data. The palaeoclimatic record is now statistically described and interpreted, providing a set of tertiary data.

Table 3.1 is obviously not exhaustative. Bradley (1985) offers an excellent review of the various proxy methods and techniques employed to reconstruct QuaternarySee Appendix. palaeoclimatic change, whilst Frakes (1979) provides a useful commentary on the evidence for pre-Quaternary climates spanning most of geologic time. In the following sections, some of the more widely used proxy techniques will be reviewed. In all of the accounts, attention should be paid to the issues of reliability, dating, interpretation and meaning for all forms of climate reconstruction.