The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 294

deglaciations at other large-amplitude upswings? As previously stated, there is a
difference between necessary and sucient.
Spotl et al. (2002) analyzed an alpine speleothem and found the termination
date for the previous ice age to be 135 kybp . Cheng et al. (2009) used d 18 O isotope
records from stalagmites from Sanbao Cave, China, to characterize Asian
Monsoon precipitation through Terminations III and IV. Combining these with
earlier Chinese cave results, Asian Monsoon records were presented for the past
four glacial terminations. Because these records were dated with 230 Th, the
chronology is claimed to be quite accurate. The interpretation of d 18 O isotope
records is complex. It is believed that most of the variability in d 18 O records
derives from changes in the d 18 O of precipitation. Most precipitation in southeast-
ern China is summer monsoon rainfall, with distinctly lower d 18 O than
precipitation during the rest of the year, and thus the value of d 18 O indicates the
relative amount of precipitation during the summer monsoon compared with pre-
cipitation during the rest of the year. These measurements of d 18 O do not measure
temperature. Monsoon rainfall has a distant source whereas other rainfall is
believed to be local. To the extent that the amount of precipitation in the annual
southeastern China summer monsoon depends on the climate in the NH, the
measurements in the Chinese caves might provide a measure of climate fluctua-
tions in the NH over the past several hundred thousand years. The results of
Cheng et al. (2009) for the variation in d 18 O isotope records during the past four
termination periods show remarkable similarity to the peak insolation curves
for 65 N during those periods. This suggests that the variability of monsoon pre-
cipitation levels is controlled by insolation in the far north of the NH. The timing
of the steepest rise in terminations is similar to that from ice cores although the
remaining d 18 O isotope records from caves deviate considerably from those
obtained from ice cores. Cheng et al. (2009) argued that their chronology could be
applied to ice core records at various tie points, but this may not be justifiable.
While the cave data seem to religiously follow peak insolation curves, ice core
data do not. They claimed: ''in all four [terminations], observations are consistent
with a classic Northern Hemisphere summer insolation intensity trigger for an
initial retreat of northern ice sheets.'' The data do not require this claim to be
true, although it may ultimately prove to be.
Severinghaus (2009) wrote a brief commentary on the paper by Cheng et al.
(2009). Severinghaus apparently accepted the claim of Cheng et al. (2009) of being
able to date ice core and ocean sediment records accurately, despite admitting that
''the exact mechanism that links cold winters to weak monsoons is still debated.''
He also emphasized: ''the authors find that the last four meltdowns began when
northern sunshine was intensifying, in accordance with the classical Milankovitch
or astronomical theory of the ice ages.'' However, there were other upswings that
did not produce deglaciation. Why were there no deglaciations at other large-
amplitude upswings?
Severinghaus also accepted the theory that the MOC is involved in the melting
of ice sheets. However, his discussion of this theory seems rather convoluted. He
said: