Page 51 - Earth's Climate Past and Future

CHAPTER 2 • Climate Archives, Data, and Models 27

Time scales examined in this book look. Using biotic proxies to reconstruct past climates
Tectonic (II) over longer tectonic time scales often requires a reliance
on the general resemblance of past forms to their mod-
Orbital (III)
ern counterparts either in general appearance or in
Deglacial/millennial (IV) specific features that can be measured.
Historical/instrumental (V) Because fossil remains of plants tend to be more
1 Byr 1 Myr 1000 yrs 1 yr numerous than those of animals in geologic records
Time span of record from continents, vegetation plays a central role in the
reconstruction of ancient climates. Often the presence
3
7
2
10 9 10 8 10 10 6 10 5 10 4 10 10 10 1 yr
of a single critical temperature-sensitive form is useful as
a climate indicator. For example, warmer climates tens
Instrumental
of millions of years ago are inferred from the presence of
palmlike trees at high northern latitudes (Figure 2–12).
Historical
For the younger continental record, climate scien-
Tree rings tists more commonly use the relative abundance of
climate-sensitive vegetation indicated by pollen assem-
Ice cores blages deposited in sediments (Figure 2–13). Minute
pollen grains are produced in vast numbers by vegeta-
Lake sediments tion, distributed mostly by wind, and deposited in lakes,
Coral reefs

Ocean sediments

Continental coastal sediments

10 8 10 7 10 6 10 5 10 4 10 3 10 2 10 1 1 yr 1 mo
Resolution of record (years)

FIGURE 2-11 Resolution of climate records Climate
archives vary widely in the length of the records they contain
and in the degree of resolution they yield. A log scale
(changing by powers of 10) is needed to show all geologic time
in a single plot. (Adapted from J. C. Bernabo, Proxy Data: Nature’s
Record of Past Climates [Washington, DC: National Oceanic and
Atmospheric Administration, 1978].)

The two climate proxies that are most commonly
used are (1) biotic proxies, which are based on changes
in composition of plant and animal groups, and (2)
geological-geochemical proxies, which are measure-
ments of mass movements of materials through the
climate system, either as discrete (physical) particles or
in dissolved (chemical) form.

2-4 Biotic Data
Because no seafloor older than 170 million years exists,
broad-scale reconstructions of earlier oceanic environ- FIGURE 2-12 Past vegetation For older geologic intervals,
ments are not possible. As a result, fossil remains from climate on the continents can be inferred from distinctive
the continents are the main climate proxy for older tec- vegetation. The remains of trees similar to modern palms are
tonic-scale intervals. Most of the organisms that have found in rocks from Wyoming dating to 45 million years ago.
ever existed on Earth are now extinct, and the further Today frigid winters in Wyoming would kill palm trees.
back in time we look, the less recognizable the fossils (Chip Clark.)