About 3 million years ago, vast tracts of land in the Northern Hemisphere frosted over with huge sheets of ice. A report in today's issue of Science  offers a new theory to explain the mystery of how the glaciers began, arguing that shifting tectonic plates combined with shorter summers might have wetted and chilled the North enough to trigger the ice's onset.
Researchers have long known that when the motions of tectonic plates closed the Isthmus of Panama about 4.6 million years ago, they sealed off the waters of the Pacific and Atlantic oceans. This closure shifted Atlantic currents northward and enhanced the Gulf Stream, sending masses of warm, salty water to the high northern latitudes. Because warmer water tends to evaporate more, some researchers proposed that the extra moisture in the atmosphere could have precipitated as snow, if the air was cold enough. But others objected that the warmer water would also have heated the atmosphere--and prevented the glaciers from forming.
Now, geologist Neal Driscoll of the Woods Hole Oceanographic Institute in Massachusetts and paleoceanographer Gerald Haug of the University of Kiel in Germany propose a new idea. They argue that the extra moisture swelled rivers in Siberia that emptied into the Arctic Ocean. By diluting the Arctic Ocean, this fresh water could have slowed the southward currents of deep, cold, and salty water. In turn, less warm water would then have flowed to the north, cooling the land.
Driscoll adds that fresh water is easier to freeze than salt water is, so the Arctic Ocean could have become capped with a layer of ice. This ice insulated the atmosphere from the remaining warmth of the ocean and further lowered temperatures. Moreover, at about the same time, Earth's axis was changing its tilt in such a way that summers in the northern hemisphere tended to become shorter. These combined cooling forces, researchers say, might have just "tipped the scales" in favor of glaciers.
The explanation is the best yet for the rapid glaciation in the northern hemisphere, says Chris Charles of the Scripps Institution of Oceanography in San Diego. Given current concerns that humans may be warming the world, it's critical to understand dramatic climate shifts, he says. And "glaciation is about the most dramatic manifestation of climate change that anybody can think of."