With all the focus on human-triggered global warming, it may be hard to imagine that the world is riding a 50-million-year-long cooling trend.

But it is, and blame the trend on a continental-scale collision, say geophysicists Dennis Kent of Rutgers University and Giovanni Muttoni of the University of Milan in Italy.

Researchers say there is strong evidence that increases in atmospheric CO2 contributed to a warm spell 50 million years ago dubbed the Early Eocene climate optimum – the warmest period in 65 million years. But over the following 15 million years, deep sea temperatures fell by about 10.8 degrees F., reflecting a significant cooling at the surface. This cooling ultimately allowed the cycle of ice ages to emerge.

Drs. Kent and Muttoni have mined paleomagnetic and other data and suggest that atmospheric CO2 dropped because India collided with Eurasia, shutting down a productive, natural CO2 factory.

Some 120 million years ago, the subcontinent that is now India was migrating north from Antarctica. As it moved, it shoved the ocean crust that was ahead of it under an existing crustal plate. As long as this zone off the Eurasian coast was under water, bottom muck enriched by carbon from the biologically-rich ocean plunged under the plate. It got recycled as lava in volcanoes along a geological feature dubbed the Kohistan Arc, as well as in a vast lava-oozing formation called the Deccan Traps. The eruptions released the carbon as CO2, which helped warm the climate. But once India collided with Eurasia 50 million years ago, India rode over the top of the zone and shut off the process. This, plus changes in ocean circulation as continents rearranged themselves, contributed to the long chill, the researchers suggest.

The results appear in the current issue of the Proceedings of the National Academy of Sciences.

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