Botkin Environmental Science Earth as Living Planet 8th txtbk

20.5 How We Study Climate 439
Log in Native American home
Dead tree
Living tree
FIGURE 20.12 Dendrochronology,
the study of tree growth rings, can be
used as an indicator of past climate.
The spacing (relative volume of wood) of
rings and the isotopic content ( 14 C, for
example) of wood can provide information
about past rainfall and solar activity.
Old
Tree cores
~ 1 / 4 inch
diameter
1850 1860 1870 1890
Old
?
Correlation of rings extends
record back in time
?
?
1900 1910
7
2
?
Young
1920 1930
3
7
7 Young
7
Tree ring annual rings,
contain carbon, carbon-14,
and other chemicals
which may provide additional insights into possible causes
of climate change. Ice cores are obtained by drilling into
the ice (Figure 20.11). The age of glacial ice back to about
800,000 years is estimated by correlating ice accumulation
rates linked to the geologic record of climate change from
other proxy sources
Tree Rings
The growth of trees is influenced by climate, both
temperature and precipitation. Many trees put on one
growth ring per year, and patterns in the tree rings—their
width, density, and isotopic composition—tell us something
about the variability of the climate. When conditions
are good for growth, a ring is wide; when conditions
are poor, the ring is narrow. Tree-ring chronology, known
as dendrochronology, has produced a proxy record of climate
that extends back over 10,000 years (Figure 20.12).
Sediments
Biological material, including pollen from plants, is
deposited on the land and stored for very long periods
in lake, bog, and pond sediments and, once transported
downstream to the coast, in the oceans. Samples may
be taken of very small fossils and of chemicals in the
sediments, and these may be interpreted to study past
climates and extend our knowledge back hundreds of
thousand years (Figure 20.13). Pollen is useful because
(1) the quantity of pollen is an indicator of the relative
abundance of each plant species; (2) the pollen can be
dated, and since the grains are preserved in sedimentary
layers that also might be dated, we can develop a chronology;
and (3) based on the types of plants found at
different times, we can construct a climatic history.
FIGURE 20.13 Scientist examining a sediment core taken by
drilling into the seafloor.