The full programme book (PDF) - Royal Geographical Society

THEME 7: THE OCEANS
The Oceans, CO 2 , Sea Level and Ice: Four Million Years of Natural Climate
Variability
Maureen E. Raymo
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964
How have the interactions between orbitally-controlled variations in insolation and
geologically and biologically controlled variations in atmospheric CO 2 influenced Earth’s
climate over the last four million years? I will argue that the late Pliocene intensification of
northern hemisphere glaciation was likely driven by a modest decrease in atmospheric
CO 2 levels (Raymo et al., 1988). Various lines of evidence suggest that CO 2 levels during
the mid-Pliocene warm period were between 350-450 ppm and significant effort should be
put into confirming these estimates, especially in light of policy discussions that seek to
determine a “safe” level of atmospheric CO 2 . Subsequent to 3 Ma ago, changes in global
climate and polar ice volume were paced by orbital variations, however, the timing,
physics, and amplitude of the climate response remain uncertain, especially for the early
Pleistocene. How, for instance, do we explain the lack of significant precession variability
in early Pleistocene climate records (e.g. the “41kyr world”)? Proxy evidence suggests
that the last interglacial warm period (the Eemian), Marine Isotope Stage (MIS) 11, and
the mid-Pliocene warm period (MPWP) were progressively warmer than the Holocene.
Each interval also appears to exhibit increasingly higher eustatic sea level (due to polar
ice sheet decay) although the evidence for the mid Pliocene sea level change is less
robust. During MIS11 and MIS5e (the Eemian), our field evidence indicates that the
eustatic rise in sea level occurred at the end of the interglacial warm period, suggesting
that thresholds exist such that the Greenland Ice Sheet and West Antarctic Ice Sheet can
rapidly disintegrate given a long enough interval of sustained warmth. For the mid-
Pliocene warm period more accurate estimates of sea level are necessary if we are to
better constrain Greenland and Antarctic equilibrium ice sheet stability in a slightly warmer
“400ppm CO 2 ” world. This will require better constraints on the influence of glacioisostasy
and dynamic topography on existing (and new) Pliocene shoreline elevation
estimates. Determination of the maximum mid-Pliocene sea level rise will allow climate
modelers to better assess the level at which atmospheric CO 2 concentrations might lead to
significant melting of the Greenland Ice Sheet and West and East Antarctic Ice Sheets.
Keywords: Milankovitch; sea level; ice ages.