Geoscience Reference
In-Depth Information
geographical changes have had immense paleoclimatic
significance, but of more immediate concern for recent
climate are the radiative forcing agents that affect the
supply and disposition of solar radiation. Solar radiation
changes are external inputs into the atmosphere-
earth-ocean-ice system and they occur over a range of
timescales from tens to hundreds of thousands and
(probably) millions of years. Thus solar radiation is both
a long-term and a short-term external forcing agent.
Astronomical forcings give rise to global temperature
fluctuations of ±2-5°C per 10,000 years. The timing of
orbital forcing is also clearly represented in glacial-
interglacial fluctuations with major glacial cycles
spanning about 100,000 years (or 100 ka). However,
the most striking fact to emerge from analysis of two
recent deep ice cores in central Greenland is the great
rapidity of large changes in atmospheric temperature,
precipitation and aerosol levels, presumably as a result
of major readjustments of atmospheric circulation.
The onset and termination of the Younger Dryas cold
episode 12,900 to 11,600
BP
(before present) (see Figure
13.4), with a switch from glacial to interglacial condi-
tions and back again, apparently occurred within a five-
year time interval for both transitions! The processes
driving such abrupt changes in atmospheric circulation
regime are still unknown.
2 Short-term forcing and feedback
Internal radiative forcing agents mostly involve changes
in atmospheric composition, cloud cover, aerosols and
surface albedo. Although subject to long-term changes,
it is their susceptibility to short-term anthropogenic
influences that makes them of particular climatic
interest. The interactions between short-term external
solar radiative forcing and these internal radiative
forcing agents are central to understanding and
predicting near-term global climate trends. They operate
through a complex
feedback mechanism
which can be
either
positive
(i.e. self-enhancing) or
negative
(i.e. self-
regulating or damping).
Positive feedback mechanisms affecting global
climate appear to be widespread and particularly
effective in response to temperature changes, which is
a matter of especial current concern. Increases in global
temperature lead to increases in atmospheric water
vapour, increases in plant respiration, decreases in CO
2
dissolved in the oceans, and an increase in methane
emissions from wetlands. All of these act to increase the
global concentration of greenhouse gases and, hence, to
increase global temperatures further. Ice and snow cover
is involved in important positive feedback effects
because a more extensive cover creates higher albedo
and lower temperatures that further expand the ice and
snow cover, producing additional cooling. Conversely,
Figure 13.4
The late glacial to inter-
glacial transition (14.7 to 11.6 ka) as
indicated by d
18
O (ppm), electrical
conductivity (micro-amps) and calcium
concentrations (ppb) in the Greenland
Ice Sheet Program (GISP) ice core
from central Greenland.
Source
: Reprinted by permission from
Grootes (1995). Copyright © National
Academy of Sciences. Courtesy of the
US National Academy of Sciences,
Washington, DC.
