Page 320 - Earth's Climate Past and Future

296 PART V • Historical and Future Climate Change

BOX 16–1 TOOLS OF CLIMATE SCIENCE
Analyzing Tree Rings

t chosen sites, a dozen or more trees are sampled by The first step in tree ring analysis is to remove the
Ataking small radial cores (about 0.5 cm in diameter). gradual effects of aging. Trees grow wider rings when they
The investigators date each tree by comparing the are young than at maturity, and investigators have to elim-
sequences of tree rings back in time, beginning from the inate this growth effect before focusing on the effects of
year the cores are taken. Taking multiple cores lets scien- year-to-year changes in climate.
tists detect annual rings that may be missing or falsely pre- The next step is to relate the sequence of tree ring
sent within a particular core because of local damage or measurements to nearby instrumental records of climate.
disease. Deliberate sampling of old-age trees produces In geographically remote areas often chosen for tree ring
sequences spanning several centuries. Width and density studies, instrumental records may cover only the last 50 to
changes in the tree rings are measured in each core, and at most 100 years, and thus they overlap only with the
records from dozens of cores are averaged to create a sin- lattermost part of the tree ring sequence. By examining the
gle representative signal for each site. correlations between the width or density of the tree rings

Coring for tree ring studies (A) To study
tree rings, scientists drill into trees at sites
where trees are under moderate stress
because of cold or dryness. (B, C) The
cores extracted are small in diameter
compared to the trees. (Courtesy of G. C.
Jacoby, Lamont-Doherty Earth Observatory
of Columbia University.)

A B C

developed from trees distributed over the entire circum- occurred around the Arctic during the Little Ice Age,
Arctic region (Figure 16–7B) shows cool conditions in which was clearly not a time of unrelieved cold. The
the late seventeenth and early eighteenth centuries, some warming of the Arctic in the mid-twentieth century
warming in the middle and late eighteenth century, a reached values unique for the 320 years of record, but
deeper cooling in the early to mid-nineteenth century, a temperatures in the late twentieth century were similar
slow but substantial warming after 1850, a brief cooling to those in the 1700s.
between 1950 and 1970, and a small warming since 1980. Another important region for tree ring studies is
The 320-year trend in Figure 16–7B covers the Central Asia. Because Asia is the largest continent, its
middle and end of the Little Ice Age and the instrumen- climate is less subject to the moderating effects of ocean
tally recorded warming of the twentieth century. Esti- water than other regions. Climate signals derived from
mated regional temperature variations larger than 1°C tree rings in Mongolia (Figure 16–7C) show a trend