Page 45 - Earth's Climate Past and Future

CHAPTER 2 • Climate Archives, Data, and Models 21

FIGURE 2-5 Ice archives Ice is an
1–10 km Snow important archive of many climate
signals. Ice cores retrieve climate
100 m
Ice core records extending back (A) thousands
Accumulation
of years in small mountain glaciers to
as much as (B) hundreds of
Flow
thousands of years in continent-sized
ice sheets.
Ablation

A Mountain glaciers

1000 km
1 km Snow

Ice core
Accumulation
Iceberg
calving Annual layers
Ablation
Flow Thinning and
stretching

B Continental ice sheets

that experience large seasonal climate changes (see 2-2 Dating Climate Records
Figure 2–6).
In clear sunlit waters at tropical and subtropical lati- Climate records in older sedimentary archives are dated
tudes, corals form annual bands of CaCO that hold by a two-step process. First, scientists use the technique
3
several kinds of geochemical information about climate of radiometric dating to measure the decay of radioac-
(see Figure 2–6). Individual corals may live for time tive isotopes in rocks. (Isotopes are forms of a chemical
spans of years to tens or hundreds of years. element that have the same atomic number but differ in
Within the last few thousand years, humans have mass.) Dates are obtained on hard crystalline igneous
kept historical archives of climate-related phenomena. rocks that once were molten and then cooled to solid
Examples include the time of blooming of cherry trees form. In the second step, dates obtained from the
in Japan, the success or failure of grape and grain har- igneous rocks provide constraints on the ages of sedi-
vests in Europe, and the number of days with extensive mentary rocks that occur in layers between the igneous
sea ice in regions such as Iceland and Hudson Bay in rocks and form the main archives of Earth’s early
Canada. These records precede (and in most cases over- climate history.
lap) the instrumental records of the last 100 to 200 Radiometric Dating and Correlation Radiometric
years. The first thermometers for measuring climate dating is based on the radioactive decay of a parent
appeared in the eighteenth century, but human ingenu- isotope to a daughter isotope. The parent is an unstable
ity has now created instruments to measure climate radioactive isotope of one element, and radioactive decay
remotely from space (Figure 2–7). transforms it into the stable isotope of another element