Historical Geology/Paleoclimatology: introduction
In this article we shall take a brief overview of what paleoclimatology is before getting down to specifics.
What is paleoclimatology?
As the name suggests, paleoclimatology is the study of ancient climates. Climate can be defined as broad trends in the weather: the fact that it rained in Seattle yesterday is merely weather; the fact that it often rains in Seattle is climate.
By definition, paleoclimatology must include the whole of climatology, plus some techniques for finding out about past conditions. Now, climatology is a complicated science, because the climate is composed of many parts which interact with one another in complex ways. A full introduction to the subject would have to deal with the influence and relations of such factors as the salinity of the ocean, the biological productivity of the land, the composition of the atmosphere, the effects of chemical weathering on climate, the effects of climate on chemical weathering, the deposition of coal, the short and long term effects of volcanoes, variations in the Earth's orbit, and dozens of other considerations. To explore all these factors in the detail they really deserve would so unbalance this textbook as to make it require a new title along the lines of Historical Geology Plus Everything You Ever Wanted To Know About The Climate.
Instead, we shall focus more on methods for finding out what the climate was like in the past than on understanding the underlying principles that drive the climate.
The gold standard of measurement would be instrumental measurement, such as is performed with a thermometer, a barometer, a rain gauge, etc. However, such measurements have only been made in the recent past, and obviously geologists have to look elsewhere for paleoclimatic data: they have to look at the sedimentary and fossil records.
In paleoclimatology, the relationship between the geological record and the facts is not so straightforward as in other areas of geology: there is nothing that stands in the same simple relationship to (for example) ancient temperatures as aeolian sand does to ancient ergs, or fossils do to ancient organisms. Instead, geologists have found it necessary to develop proxies. A proxy might be defined as something we can measure which is not the thing we'd actually like to measure, but which bears a known relationship to it. A proxy can be justified in various ways: by testing that it presently indicates what we think it indicates, by observation of the natural world or of laboratory experiments; on theoretical grounds that argue that the proxy ought to indicate what we think it indicates; by comparison to proxies in which we already feel confident; or any combination of these considerations. We shall look at a selection of these proxies in subsequent articles.
As we have noted above, the climate is complicated, climatic models are complicated, and to keep from doubling the length of this book we shall have to treat climate models more or less as black boxes containing the accumulated knowledge of climatologists.
That said, the construction of climate models is of interest to us: we can use models to figure out what the climate should have been like in the past, and then compare this with the evidence we have for what the climate actually was like. If the two match up, then this increases our confidence in both our proxies and our models.
This also gives us a check on our reconstruction of past geological events. For example, if geologists tell us that there used to be a mountain range along the coast of a continent, and if climate models tell us that the mountain range should have created a rain shadow desert on its leeward side, then if we find evidence of a desert in the right place with the right date, this increases our confidence in the models and in the geologists' claim that there used to be a mountain range where they say it was. In this way paleoclimatology can not only draw evidence from the other fields in historical geology, but can also contribute evidence to them.