Here is one I found that didn't require me to pay to read, concerning the transition between the Pliocene warm period to the cooling.
http://eesc.columbia.edu/courses/w49....etal.2013.pdf
Quote:
None of the currently proposed mechanisms can reproduce all of
the key Pliocene climate features by itself. Some mechanisms address
one aspect of the structural change, but often at the detriment of the
other features; a major problem is to simulate reduced temperature
gradients without raising warm pool SSTs. Increases in greenhouse
gases alone are not sufficient, whereas shifts in oceanic gateways have
much smaller effects than sometimes suggested. The most promising
dynamical mechanisms involve lower extratropical cloud albedo or
enhanced ocean mixing. For these mechanisms to explain the stark
differences between the Pliocene and today’s climates, the corresponding
model has to be modified.
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