Almost everything I've read says scientists think the alteration of ocean currents, creating among other things the Gulf Stream, and divergence of salinity levels in the Atlantic and Pacific triggered the onset of the glacial cycles, but I can't tell you just how that worked.
It's interesting the opening of the Straits of Gibraltar came just before the Pliocene Warm. Any connection?
There are a whole bunch of things that've happened over the last 10 milliion years. The closing of Panama is probably the biggest, but the rest of South America formed and the various lakes and large inlets were filled in or lifted up.
North America rotated to begin enclosing the Arctic, and when that's done we're back to Snowball Earth where there really isn't any circulation of water currents between the equator and the poles.
The Leftwingtards will undoubtedly object to the use of nuclear energy to "restore circulation" claiming that this will disturb the ecological balance (of that time).