Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 44

of several grams to be measured in about a day. However, detecting radioactive decay
is only one way of detecting (inferring) 14 C. A mass spectrometer enables direct
detection of 14 C and 12 C and a similar accuracy can be obtained using a spectrometer
with a few milligrams of sample in a few hours.
Isotopic dendrochronology has provided a lynchpin for both dendrochronology
and 14 C dating. First, 14 C enables dendrochronologists to date a sample of wood to
within a few years or decades. Further accuracy may then be obtained by matching the
sample to the corresponding regional dendrochronological record. Second, because
14 C production is not constant due to changes in solar radiation it is possible to
use the established dendrochronological record as a base against which to determine
variations in atmospheric 14 C production. Notably, two 14 C-calibration curves have
been produced: one by researchers from the University of California, La Jolla, using
the dendrochronological bristlecone pine data set, and the other by those at Belfast
University in Northern Ireland using oak. In particular, the results show that 14 C
production fluctuates with a periodicity of around 200 years. Overall, for the past
6000 years both curves show that 14 C production decreased a little from 6000 years
ago to 2500 years ago before a marginal recovery. As 14 C production relies largely
on solar radiation this helps inform as to alterations in the solar contribution to
climate change over time. This can then be added into the mix of the various factors
influencing (forcing) climate and so (because we know the atmospheric concentrations
of the isotopes) help us estimate the greenhouse component over time.
2.1.3 Leafshape(morphology)
In 1910 two explorers (Irving Bailey and Edmund Sinnott) noticed that the shapes
of leaves appeared to relate to the climate of a given area. The most noticeable trait
was that trees that grew in warm areas had large leaves with smooth edges, whereas
trees that grew in cold places tended to have smaller leaves with very serrated edges
(Bailey and Sinnott, 1916; Ravilious, 2000). This was largely forgotten until the
1970s, when a US researcher, Jack Wolfe, wondered whether fossilised leaves could
reveal something of the climate if they were compared with their modern counterparts.
He collected leaves from all over North America, recording aspects of their shape
and relating them to regional weather records for the previous 30 years. Then, using
statistical analysis, he was able to compare fossil leaf assemblages with those of
modern leaves and come up with past average temperature and rainfall values for the
regions and times in which the fossil leaves once grew. For example, he predicted that
the temperature in north-eastern Russia some 90 million years ago (mya) was roughly
9 ◦ C warmer than it is today (Wolfe, 1995). Other early work following Bailey and
Sinnott, such as by R. Chaney and E. Sanborn in 1933, showed that leaf size and apex
shape were also related to climate. Apart from the Wolfe database standard for North
America, other databases have been developed. For instance, in the late 1990s Kate
Ravilious worked on developing a similar database for the UK. These databases can
be used in techniques such as leaf margin analysis (LMA) to infer temperature and
leaf area analysis (LAA) to infer precipitation. LMA relates to the aforementioned
changes in leaf shape with temperature, whereas LAA relates to changes in leaf area
with precipitation level; for instance, plants with access to plenty of water can afford