Geoscience Reference
In-Depth Information
Although during the last years the concept of Milankowich has been acknowl-
edged (but not completely), its individual aspects have been critically analyzed.
This had two reasons:
(1) many new geological data of analysis of the cores of sea bottom rocks and ice
cores have appeared; and
(2) a considerable progress has been achieved in numerical climate modeling.
An analysis of both these sources of information has shown that an adequate
explanation of paleoclimate changes is only possible with regard for not only
variations of the orbital parameters, but also other climate-forming factors
in
particular, variations of the GHGs content in the atmosphere which include carbon
dioxide as most important.
In this connection, Palutikof et al. (1999) performed an analysis of new geo-
logical data on paleoclimate changes, with a higher temporal resolution, in the
context of present ideas of the global climate dynamics. The data of analysis of ice
cores and pollen obtained in 1990 have led to the following two important general
conclusions:
โ€”
(1) part of observational data does not con
rm the glaciation cycles having been
determined by the Milankowich mechanism (this refers especially to data on
สด
18 O in calcite veins Devils Hole in the U.S.S. Nevada, which testify to
opposite phases of the cycles of glaciation and theory of Milankowich); and
(2)
it follows from other data that this mechanism can explain only slow quasi-
periodic variations but not the short-term variability (on time scales from
decades to millennia), with respect to which it has turned out that it had
happened much more often than it was supposed.
This variability of the global mean temperature could reach several degrees
during several decades. In particular, a large-scale sudden climate cooling had
happened in the Ames interglacial period (
122,000 years ago), when climatic
conditions had been very close to the present ones. A typical example of a short-
term climatic variability is the Heinrich and Dansgaard/Oeshger events (Keigwin
and Boyle 2000). Such events can be repeated in the future.
Many uncertainties remain also, concerning the impacts of present changes of
extra-atmospheric insolation on climate. Soon et al. (2000) have demonstrated, for
instance, the presence of hyper-sensitivity of the climate system to changes in UV
insolation, whose effect is intensi
*
ed by feedback due to statistical stability of
clouds, effect of tropical cirrus clouds, and stratospheric ozone (the
โ€œ
ozone-climate
โ€
problem needs a special analysis).
An interactive consideration of the biospheric dynamics as a climate system
'
s
component is of particular interest. The signi
cance of this problem can be exem-
pli
ed by estimates of the climatic impact of deforestation in the tropical Amazon
basin, obtained by Bunyard (1999). Nature in the Amazon basin (
(first of all, wet
tropical forests WTF) performs a number of important functions, still not completely
taken into account of including the energy input from the tropics to higher latitudes,
which, however, is under threat in view of a high rate of WTF destruction.
Search WWH ::




Custom Search