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Abstract:
Interglacials, including the present (Holocene) period, are warm, low
land ice extent (high sea level), end-members of glacial cycles. Based
on a sea level definition, we identify eleven interglacials in the last
800,000years, a result that is robust to alternative definitions. Data
compilations suggest that despite spatial heterogeneity, Marine Isotope
Stages (MIS) 5e (last interglacial) and 11c (similar to 400ka ago) were
globally strong (warm), while MIS 13a (similar to 500ka ago) was cool at
many locations. A step change in strength of interglacials at 450ka is
apparent only in atmospheric CO2 and in Antarctic and deep ocean
temperature. The onset of an interglacial (glacial termination) seems to
require a reducing precession parameter (increasing Northern Hemisphere
summer insolation), but this condition alone is insufficient.
Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and
temperature to external astronomical forcing. The precise timing of
events may be modulated by millennial-scale climate change that can lead
to a contrasting timing of maximum interglacial intensity in each
hemisphere. A variety of temporal trends is observed, such that maxima
in the main records are observed either early or late in different
interglacials. The end of an interglacial (glacial inception) is a
slower process involving a global sequence of changes. Interglacials
have been typically 10-30ka long. The combination of minimal reduction
in northern summer insolation over the next few orbital cycles, owing to
low eccentricity, and high atmospheric greenhouse gas concentrations
implies that the next glacial inception is many tens of millennia in the future.
