Principles of terrestrial ecosystem ecology.pdf

36 2. Earth’s Climate System
for a past role of radiatively active gases in
climate change. The unique feature of the
recent anthropogenic increases in these gases
is that they are occurring during an interglacial
period, when Earth’s climate is already relatively
warm. The Vostok record suggests that
the CO2 concentration of the atmosphere is
higher now than at any time in the last 400,000
years. Fine-scale analysis of ice cores from
Greenland suggests that large changes from
glacial to interglacial climate can occur in
decades or less. Such rapid transitions in
the climate system to a new state may be
related to sudden changes in the strength of
the thermohaline circulation that drives
oceanic heat transport from the equator to the
poles.
Tree ring records, obtained from living and
dead trees, provide information about the
climate during the past several thousand years.
The width of tree rings gives a record of temperature
and moisture, and the chemical composition
of wood reflects the characteristics of
the atmosphere at the time the wood was
Time (thousands of years ago)
2.7
3.9
8.6
11.0
Birch
Alder
Fir
Pine
Elm
Oak
0 40 0 0 0 60 0 0 40
Pollen abundance (% of total tree pollen)
formed. Pollen preserved in low-oxygen sediments
of lakes provides a history of plant taxa
and climate over the past 10,000 years or more
(Fig. 2.15). Pollen records from networks
of sites can be used to construct maps of
species distributions at various times in the
past and can provide a history of species migrations
across continents after climatic changes
(COHMAP 1988). Other proxy records provide
measures of temperature (species composition
of Chironomids), precipitation (lake
level), pH, and geochemistry.
The combination of paleoclimate proxies
indicates that climate is inherently variable
over all time scales. Atmospheric, oceanic,
and other environmental changes that are
occurring now due to human activities must be
viewed as overlays on the natural climate variability
that stems from long-term changes in
Earth’s surface characteristics and orbital
geometry.
Earth’s climate is now warmer than at
any time in the last 1000 years (Fig. 2.16) and
perhaps much longer.This warming is most pro-
Drier,
cooler
climate
Moister
climate
Drier
climate
Warming
trend
Figure 2.15. Pollen profile from a bog in
northwestern Minnesota showing changes
in the dominant tree species over the past
11,000 years. (McAndrews 1966).