Geoscience Reference
In-Depth Information
5.3.7 Paleoclimatic Information
Paleoclimatic information is an important source of data for the comparative
analysis of the present and past climates. Analysis of the data of paleoclimatic
observations reveals large-scale abrupt climate changes in the past, when the cli-
mate system had exceeded certain threshold levels. Though some mechanisms for
such changes have been identi
ed and the existing methods of numerical climate
modeling are being gradually improved, the existing models still do not permit a
reliable reconstruction of past climatic changes. With emphasis on the climatic
implications of the growth of GHGs concentrations in the atmosphere, less effort
has been made to study possible sudden climate changes that may be of natural
origin, though possibly intensi
ed by anthropogenic forcings.
Since such changes lie beyond the problems addressed in the UN FCCC, Alley
et al. (2003) undertook a conceptual evaluation of the problem of large-scale abrupt
climate changes. Though the available long-term stabilizing feedbacks have
determined the existence on the Earth of comparatively persistent global climate for
about 4 billion years, with characteristic time scales from 1 year to 1 million years,
feedbacks prevailing in the climate system had favored an enhancement of forcings
on climate. So, for instance, changes of global average SAT within 5
C during
the glaciation cycles apparently resulted from very weak forcings due to variations
of the orbital parameters.
It is still more surprising that during several decades and in the absence of external
forcings, regional changes have taken place, reaching 30
6
°
-
50 % of those, that had taken
place in the epochs of glaciations. Data from the period of instrumental observations
have revealed abrupt climatic changes, quite often accompanied by serious socio-
economic consequences. So, for instance, the warming in many northern regions in the
20th century took place in two rapid
-
, which enables one to suppose that in this
case there was a superposition of the anthropogenic trend on inter-annual natural
variability. Special attention was paid to the role of the ENSO event. The latter also
refers to a sharp change of the climate system in the Paci
“
steps
”
1977.
Considerable abrupt changes of regional climate in the period of Paleocene were
detected from paleoclimatic reconstructions. They had been manifested as changes
of the frequency of occurrence of hurricanes,
cregionin1976
-
fl
floods, and especially droughts.
Regional SAT changes reaching 8
C had happened in the periods of 10 years
and shorter. Dansgaard-Oeschger (DO) oscillations can serve as an example of
large-scale sudden changes. DO events are rapid climate
16
°
-
fl
fluctuations that occurred
25 times during the last glacial period.
The climatic system involves numerous factors that intensify climatic changes
with minimum forcings. The withering or death of plants, for example, may cause a
decrease of evapotranspiration and hence lead to precipitation attenuation, which
may further increase drought conditions. In the cold-climate regions the snow cover
formation is accompanied by a strong increase of albedo, which favours further
cooling (the so-called
). Substantial climatic feedbacks are associated
with the dynamics of the thermohaline circulation.
“
albedo effect
”
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