Just had an enlightening dinner with Bill Gray. An hour with Bill is more educational than a lifetime of reading global warming research.
Bill explained tonight that the recent spell of ice ages (~3 million years ago) coincided with the closing of the isthmus of Panama. This isolated the Pacific and Atlantic Oceans, and created the current pattern of regular glacial episodes. The Atlantic now goes through warm episodes of surface evaporation and buildup of dense saline water at the surface, followed by cold periods where the dense saline water sinks and brings up cold water from depth.
Prior to the closing of the isthmus, the Pacific and Atlantic waters mixed and remained a more even and warmer temperature.
Always a pleasure to hear a climate theory which actually seems plausible.