The means
exist to reconstruct the climate beyond historic documentation.
Through paleoclimatology, seasonal precipitation, and average temperature
can be extrapolated over the span of the proxy indicator.
Through the Principle of Uniformitarianism, and the idea that biological
responses in the past are the same as the present, it is possible to use
scientific methods to accurately reconstruct climate change (Bradley,
1999)
. The only limitations of
paleoclimatology are the availability and integrity of samples.
The
process of reconstructing the climatological conditions of the first few
centuries AD in Germania allows the building a model of modern agricultural
responses, determining the social structure of the inhabitants, interpreting the
data compiled from various “proxies” (indicators) of climate, and applying
the paleoclimatic interpretation to antiquity.
The botanical responses are assumed to be consistent in both antiquity
and modern times, presuming that the conditions observed are similar in both
eras (Fritts, 1976).
Fritts
(1976) describes the process of dendroclimatological reconstruction through six
phases: (1) plant process, (2) operational environment, (3) microclimate, (4)
weather, (5) climate state, and (6) macroclimate (Fritts, 1976)
. Upon understanding one
phase, the subsequent phases become comprehensible.
The (1) plant process deals with the physiological processes of the
plants and the way in which their response to climatic variation is made
evident.
Upon
understanding of the plant processes, the (2) operational environment can be
examined. The operational
environment examines the plant as a whole, concentrating on the plant and its
response to climate variables. When
the operational environment of the plants is understood, accurate reconstruction
of regional climate is achievable. This
is due to the Principle of Uniformity in the order of nature. The responses of vegetation to (3) climate conditions in the
present are assumed the same in antiquity.
Potentially, the variation in plant growth, or any other (4)
environmental variable that can be interpreted in terms of instrumental data,
are proxies of climate.
When
examining ancient history, the last two of Fritts’ phases are the most
important: the (5) climate state and (6) macroclimate.
The climate state uses the information gathered about weather conditions
to assess the climatic conditions of the entire region over a period of months
or years, as in this study of Germania. The
study of macroclimate examines the climate state over many years and generalizes
the information (i.e., the macroclimate of Germania in AD 200 to AD 400 was a
period of drought and poor agricultural conditions although there were certainly
years, and perhaps even decades of favorable conditions interspersed in these
centuries.)
Retrospection
is the key to understanding climate indicators in biological data sets, such as
flora and fauna, as well as meteorological data. Biological data can be used as climate indicators due to the
consistency of their response under the similar climate conditions.
However, biological data gathered in very recent times is difficult to
relate to climate variation due to Global Warming and advances in modern
agriculture.
The
overlapping of several climate indicators helps ameliorate the complications of
variable interpretations rendered from individual climate indicators.
For example, a single dendrochronological sample can command several
valid interpretations, all of which are attached to the growth of a particular
tree species in a region, but the use of several dendrochronological studies
and/or other climate indicators may increase accuracy of interpretation.
Biological
indicators, such as tree rings and pollen, are the foundations of
paleoclimatological studies, because they examine the plant process and
operational environment phases, which are the building blocks of all
interpretation. Biological proxies
can be highly predictable and therefore subject to scientifically based
reconstruction. It is also useful
to incorporate stable isotopic data and sedimentary indicators (e.g., riverbed
sediments or ice core samples) to enhance biologically based interpretations of
past climate. It is dangerous to
rely upon a single proxy, even dendrochronology, to interpret climate change.
It would be as if the historian relied upon a single document to
reconstruct the history of a people.
In
antiquity, biological indicators are important for identifying the timing and
severity of agricultural anomalies. Dendrochronological
reconstruction, ice cores, and varves are among the few indicators that can
potentially reveal year-to-year changes in agricultural yields and growth
potential. There are several
indicators, such as palynology and lake and ocean sediments, that can be used to
reconstruct vegetation, temperature, and rainfall, but they are often limited to
general seasonal changes and lack the annual precision of dating afforded by
tree rings. Still other types of
data: speleothems, variation of oxygen isotopes in coral, radiocarbon records of
sunspot activity, peat cellulose studies, and other types of isotopic data that
can reveal climate changes over decades, centuries, and millennia (Linacre,
1992)
. These different types of
indicators, revealing data chronologically accurate to a single year or to an
approximate date prove useful in reconstructing ancient climates.