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Atmopsheric CO2 levels aren't everything...

Found this interesting...

Jonathan P. LaRiviere, A. Christina Ravelo, Allison Crimmins, Petra S.
Dekens, Heather L. Ford, Mitch Lyle, Michael W. Wara. Late Miocene
decoupling of oceanic warmth and atmospheric carbon dioxide forcing.
Nature, 2012; 486 (7401): 97 DOI: 10.1038/nature11200

Deep-time palaeoclimate studies are vitally important for developing a
complete understanding of climate responses to changes in the
atmospheric carbon dioxide concentration (that is, the atmospheric
partial pressure of CO2, pco2). Although past studies have explored
these responses during portions of the Cenozoic era (the most recent
65.5 million years (Myr) of Earth history), comparatively little is
known about the climate of the late Miocene (~12–5 Myr ago), an
interval with pco2 values of only 200–350 parts per million by volume
but nearly ice-free conditions in the Northern Hemisphere and
warmer-than-modern temperatures on the continents. Here we present
quantitative geochemical sea surface temperature estimates from the
Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth
persisted throughout the interval of low pco2 ~12–5 Myr ago. We also
present new stable isotope measurements from the western equatorial
Pacific that, in conjunction with previously published data, reveal a
long-term trend of thermocline shoaling in the equatorial Pacific
since ~13 Myr ago. We propose that a relatively deep global
thermocline, reductions in low-latitude gradients in sea surface
temperature, and cloud and water vapour feedbacks may help to explain
the warmth of the late Miocene. Additional shoaling of the thermocline
after 5 Myr ago probably explains the stronger coupling between pco2,
sea surface temperatures and climate that is characteristic of the
more recent Pliocene and Pleistocene epochs.

Granted, the conclusion has always been rather intuitive (i.e. the
ocean had something to do with it... major differences in waterways
contributed to the deep thermocline and the warm temperatures of the
late Miocene), but now there's hard data backing it up. Of course, the
study has obvious implications for Mesozoic climates via even more
dramatic ocean differences, and interestingly enough, mostly in the
reverse (cold or cool episodes occurring in a high CO2 world)... which
I've harped on here before.