Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 42

Today there are a number of long, annual-resolution records of tree growth from
temperate regions, determined by the aforementioned method of piecing together
overlapping growth-ring patterns from successively older timbers. One of the length-
iest is the oak chronology in central Germany that runs continuously back to 8480 .

This is even longer than the bristlecone pine chronology from the western USA, which
has been available since the 1960s and which provides a record back to 6200

bc

.
There are some eight chronologies that span the last seven millennia; one from
northern Russia, one from western North America and six European ones covering
Ireland, England, Germany and Fennoscandia. The rate of development of dendro-
chronological databases in the latter half of the 20th century is illustrated by the
creation of the Irish oak chronology by researchers at Queen's University Belfast.
Here thousands of samples of oak were collected from buildings and archaeological
sites. Samples from living trees acted as a chronological anchor. By 1977 a con-
tinuous chronology ran back to 885

bc

and by 1982 a continuous 7272-year Irish
chronology had been determined. By 1984 sufficient correlation between Irish and
German dendrochronological records had been ascertained for the announcement of
the European oak chronology covering seven millennia.
It is important to remember that greenhouse gases are not the only factors for-
cing climate. For example, trees are sufficiently sensitive that dendrochronology can
identify the years of cooling following major volcanic eruptions (see Figure 2.1).
There are notable declines in tree-ring growth associated with large volcanic erup-
tions, such as Tambora (1815) and Krakatau (1883). The dust from these eruptions
entered the stratosphere and resulted in spectacular sunsets for a few years, as well as
global cooling and disrupted harvests (see Chapter 5). Acid levels in ice cores (such as
the Greenland cores discussed later in this chapter) suggest past super-eruptions and
these too have been connected to years of poor tree-ring growth. Such corroborative
evidence is important not only to ensure that the climate inference is accurate but
also to help provide further detail. For instance, several dendrochronological records
suggest a twin climate event in the 6th century: one in the year 536 and something
worse in the early 540s. The ice-core record is not nearly so clear; although there
is evidence of some volcanic activity at that time there is nothing of the magnitude
suggested by dendrochronology. However, one possibility is that the Earth may have
accrued cometary debris around that time (Baille, 2003). Nonetheless, despite its lim-
itations (such as its lack of effectiveness in the tropics) dendrochronology remains
the only terrestrial biotic record detailing climatic change with annual detail covering
the Holocene.

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2.1.2 Isotopicdendrochronology

As will be discussed in more detail, isotope analysis can be used to infer climate
change. Neutrally charged isotopes of the same element have the same number of
protons and electrons as each other but a different number of neutrons. Different
isotopes of the same element therefore exhibit the same chemical properties but have
different atomic weight and hence different physical properties. Most oxygen in the
atmosphere (99.76%) consists of

16 O (with an atomic weight of approximately 16)